Synthesis, structural diversity, dynamics, and acidity of the M(II) and M(IV) complexes [MH3(PR3)4]+ (M = Fe, Ru, Os; R = Me, Et)

Article abstract of DOI:10.1021/ja963692t

The syntheses of complexes MH₂L₄ and their protonated analogues [MH₃L₄]⁺ (M = Fe (1), Ru (2), Os (3); R = Me (a), Et (b)) are described. The structures of 1a, 1b, and 3a were determined in X-ray diffraction studies. The solution structures of complexes 1-3 were established by detailed NMR investigations. 1a, 2b, and 3a form equilibrium mixtures of two isomers in solution. The iron (1a) and ruthenium (2b) complexes isomerize between six-coordinate M(II) dihydrogen cis-[MH(H₂)L₄]⁺ and seven-coordinate M(IV) trihydride [M(H)₃L₄]⁺ molecular geometries: the first is a distorted octahedron and the second can be viewed as a hydride-capped M(PR₃)₄ tetrahedron. Complex 3a is a pentagonal bipyramidal trihydride cis-[Os(H)₃(PMe₃)₄]⁺ in equilibrium with the hydride-capped tetrahedral structural form. Trihydrides 1b and 3b are exclusively represented by the latter structural type. The cationic molecule 2a corresponds to a dihydrogen complex cis-[RuH(H₂)(PMe₃)₄]⁺. The metal fragment [MH(PR₃)₄]⁺ is thus most reactive toward oxidative addition of H₂ for osmium and iron, with a notably lower ability to reduce H₂ for ruthenium. This trend and related properties are due to electronic rather than steric factors. All [MH₃(PR₃)₄]⁺ species (1-3) are fluxional in solution. The intramolecular hydride exchanges and isomerizations were studied between 20 and -140 °C and quantitatively described in terms of their activation parameters. On the basis of these, mechanistic interpretations are provided. Finally the acid/base properties of the [MH₃(PR₃)₄]⁺/M(H)₂(PR₃)₄ systems were established in a series of NMR experiments in THF-d₈. The pK(a) values range from 10.3 to 12.9 units and increase in the following order: 1a (10.3) < 2b (10.7) < 2a (10.9) < 3a (11.2) < 3b (12.9). This series demonstrates a higher acidity than that of the related [MH(H₂)(PP)₂]⁺ molecules with bidentate ligands. The complexes with monodentate phosphine ligands [MH₃(PR₃)₄]⁺ (1-3) represent a new and distinguished family with structural, dynamic, and acid/base properties remarkably different from most of the other known [MH₃L₄]⁺ representatives.

Full text of DOI:10.1021/ja963692t

3716
J. Am. Chem. Soc. 1997, 119, 3716-3731
Synthesis, Structural Diversity, Dynamics, and Acidity of the
M(II) and M(IV) Complexes [MH3(PR3)4]+ (M ) Fe, Ru, Os;
R ) Me, Et)
Dmitry G. Gusev,† Rainer Hu1bener, Peter Burger, Olli Orama,‡ and Heinz Berke*
Contribution from the Anorganisch-Chemisches Institut, UniVersita¨t Zu¨rich,
Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
ReceiVed October 23, 1996X
Abstract: The syntheses of complexes MH2L4 and their protonated analogues [MH3L4]+ (M ) Fe (1), Ru (2), Os
(3); R ) Me (a), Et (b)) are described. The structures of 1a, 1b, and 3a were determined in X-ray diffraction
studies. The solution structures of complexes 1-3 were established by detailed NMR investigations. 1a, 2b, and
3a form equilibrium mixtures of two isomers in solution. The iron (1a) and ruthenium (2b) complexes isomerize
between six-coordinate M(II) dihydrogen cis-[MH(H2)L4]+ and seven-coordinate M(IV) trihydride [M(H)3L4]+
molecular geometries: the first is a distorted octahedron and the second can be viewed as a hydride-capped M(PR3)4
tetrahedron. Complex 3a is a pentagonal bipyramidal trihydride cis-[Os(H)3(PMe3)4]+ in equilibrium with the hydride-
capped tetrahedral structural form. Trihydrides 1b and 3b are exclusively represented by the latter structural type.
The cationic molecule 2a corresponds to a dihydrogen complex cis-[RuH(H2)(PMe3)4]+. The metal fragment [MH-
(PR3)4]+ is thus most reactive toward oxidative addition of H2 for osmium and iron, with a notably lower ability to
reduce H2 for ruthenium. This trend and related properties are due to electronic rather than steric factors. All
[MH3(PR3)4]+ species (1-3) are fluxional in solution. The intramolecular hydride exchanges and isomerizations
were studied between 20 and -140 °C and quantitatively described in terms of their activation parameters. On the
basis of these, mechanistic interpretations are provided. Finally the acid/base properties of the [MH3(PR3)4]+/M(H)2-
(PR3)4 systems were established in a series of NMR experiments in THF-d8. The pKa values range from 10.3 to
12.9 units and increase in the following order: 1a (10.3) < 2b (10.7) < 2a (10.9) < 3a (11.2) < 3b (12.9). This
series demonstrates a higher acidity than that of the related [MH(H2)(PP)2]+ molecules with bidentate ligands. The
complexes with monodentate phosphine ligands [MH3(PR3)4]+ (1-3) represent a new and distinguished family with
structural, dynamic, and acid/base properties remarkably different from most of the other known [MH3L4]+
representatives.
I. Introduction
periodic trends. Thus, more recently, the structure of [MH3L4]+
has additionally become subject of theoretical studies where the
relative stabilities of the six- and seven-coordinate isomers
(Scheme 1) have been determined on the basis of ab initio
calculations.4
In the last 30 years, a number of fundamental developments
in the chemistry of metal hydrides have been associated with
iron, ruthenium, and osmium complexes of the general formula
[MH2L4] and [MH3L4]+ (L4 ) mono- to tetradentate phosphorus
ligands). Among the most significant are the pioneering
dynamic NMR studies of stereochemically nonrigid dihydrides
[MH2L4],1 some of the very first reliably structurally character-
ized examples of coordinated H2,2 and related to the former
studies on the acidity of the “nonclassical” hydrides.3 This list
would be incomplete without mentioning of the rich synthetic
chemistry supported by the structural and spectroscopic
methodologies.1-3 The Fe-Ru-Os triad has always attracted
attention for systematic investigations which intended to evaluate
From the theoretical results it can be concluded4a,c that the
two molecules, [FeH3(PH3)4]+ and [RuH3(PH3)4]+, should prefer
different structures: the cis-H,H2 structural type Cn, (cis,
(3) (a) Baker, M. V.; Field, L. D.; Young, D. J. J. Chem. Soc., Chem.
Commun. 1988, 546. (b) Morris, R. H. Inorg. Chem. 1992, 31, 1471. (c)
Cappellani, E. P.; Drouin, S. D.; Jia, G.; Maltby, P. A.; Morris, R. H.;
Schweitzer, C. T. J. Am. Chem. Soc. 1994, 116, 3375. (d) Field, L. D.;
Hambley, T. W.; Yau, B. C. K. Inorg. Chem. 1994, 33, 2009.
(4) (a) Maseras, F.; Duran, M.; Lledo´s, A.; Bertra´n, J. J. Am. Chem.
Soc. 1991, 113, 2879. (b) Maseras, F.; Duran, M.; Lledo´s, A.; Bertra´n, J.
J. Am. Chem. Soc. 1992, 114, 2922. (c) Maseras, F.; Koga, N.; Morokuma,
K. Organometallics 1994, 13, 4008.
(5) Albertin, G.; Antoniutti, S.; Bordignon, E. J. Am. Chem. Soc. 1989,
111, 2072.
(6) Osakada, K.; Ohshiro, K.; Yamamoto, A. Organometallics 1991, 10,
404.
(7) (a) Amendola, P.; Antoniutti, S.; Albertin, G.; Bordignon, E. Inorg.
Chem. 1990, 29, 318. (b) Albertin, G.; Antoniutti, S.; Baldan, D.; Bordignon,
E. Inorg. Chem. 1995, 34, 6205.
(8) (a) Werner, H.; Gotzig, J. Organometallics 1983, 2, 547. (b)
Desrosiers, P. J.; Shinomoto, R. S.; Deming, M. A.; Flood, T. C.
Organometallics 1989, 8, 2861.
(9) Siedle, A. R.; Newmark, R. A.; Pignolet, L. H. Inorg. Chem. 1986,
25, 3412.
(10) Hills, A.; Hughes, D. L.; Jimenez-Tenorio, M.; Leigh, G. J.; Rowley,
A. T. J. Chem. Soc., Dalton Trans. 1993, 3041.
† Present address: University of Toronto, Department of Chemistry, Lash
Miller Chemical Laboratories, 80 St. George Street, Toronto, Ontario M5S
3H6.
‡ Present address: VTT Chemical Technology, P.O. Box 1401, FIN-
02044 VTT, Finland.
X Abstract published in AdVance ACS Abstracts, March 15, 1997.
(1) (a) Tebbe, F. N.; Meakin, P.; Jesson, J. P.; Muetterties, E. L. J. Am.
Chem. Soc. 1970, 92, 1068. (b) Meakin, P.; Guggenberger, L. J.; Jesson, J.
P.; Gerlach, D. H.; Tebbe, F. N.; Peet, W. G.; Muetterties, E. L. J. Am.
Chem. Soc. 1970, 92, 3482. (c) Meakin, P.; Muetterties, E. L.; Tebbe, F.
N.; Jesson, J. P. J. Am. Chem. Soc. 1971, 93, 4701. (d) Meakin, P.;
Muetterties, E. L.; Jesson, J. P. J. Am. Chem. Soc. 1973, 95, 75.
(2) (a) Morris, R. H.; Sawyer, J. F.; Shiralian, M.; Zubkowski, J. D. J.
Am. Chem. Soc. 1985, 107, 5581. (b) Ricci, J. S.; Koetzle, T. F.; Bautista,
M. T.; Hofstede, T. M.; Morris, R. H.; Sawyer, J. F. J. Am. Chem. Soc.
1989, 111, 8823. (c) Bautista, M. T.; Earl, K. A.; Maltby, P. A.; Morris, R.
H.; Schweitzer, C. T.; Sella, A. J. Am. Chem. Soc. 1988, 110, 7031.
(11) Bautista, M. T.; Cappellani, E. P.; Drouin, S. D.; Morris, R. H.;
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4876.
S0002-7863(96)03692-X CCC: $14.00 © 1997 American Chemical Society

M(II) and M(IV) Complexes [MH3(PR3)4]+
J. Am. Chem. Soc., Vol. 119, No. 16, 1997 3717
Scheme 1
information was provided.6 Two papers reported the preparation
and reactions of the phosphite-substituted [MH(H2)L4]+ com-
plexes (Table 1). These investigations did not, however, lead
to unambigous structural conclusions, although the NMR data
presented evidence for the P structure of most of them.5,7
The analysis of the literature data thus reveals a pronounced
preference for the P structural type. This seemed to be
paralleled by the theoretical results calculated for [RuH3-
(PH3)4]+. The Cn structure derived from the computations on
[FeH3(PH3)4]+ appeared to be in contradiction with the experi-
ment. Challenged by this, we initially pursued the idea that
the small phosphine complexes [FeH3(PMe3)4]+ and [RuH3-
(PMe3)4]+ could indeed display different ground state structures
in solution. The investigations started on this problem were
then naturally extended toward the characterization of [OsH3-
(PMe3)4]+. The choice of monodentate phosphines in these
systems was expected to allow maximum degrees of freedom
for their structural relaxation around the metal center. It was
interesting to see how an increasing steric demand of these
substituents could influence this behavior. Therefore, we
intended to investigate the structural preferences of the PEt3
analogues as well.
We sought to apply detailed variable-temperature (VT) NMR
studies in conjunction with labeling experiments and with single
crystal X-ray investigations in order to get clear insights into
the structural and dynamic behavior of the [MH3L4]+ systems.
This report consists of five sections. After a description of the
synthetic access to these complexes and their M(H)2L4 precursor
compounds, the next two sections deliver and discuss the
experimental structural findings. The fourth section provides
mechanistic studies of the three fluxional processes observed
in the [MH3L4]+ complexes. The final section then deals with
the thermodynamic acidity of the cationic molecules and
compares the determined pKa values to those available in the
literature for the chelating phosphine analogues [MH(H2)-
(dmpe)2]+ and [MH(H2)(depe)2]+.
nonclassical type) and the trans-H,H2 structural type P, respec-
tively (see computed relative energies in Scheme 1). The
symbol P indicates a planar or pseudoplanar arrangement of
four PH3. The classical seven-coordinate alternatives (structural
types with a tetrahedral PH3 arrangment, T, and classical cis-
trihydride, Cc) were found at appreciably higher energies. These
results do not easily match experimental findings.
Most of the known [MH3L4]+ (collected in Table 1) are
preferentially of the P geometry. With some exceptions,17g
tetradentate phosphorus ligands normally induce Cn structures.
A number of molecules (with dcpe, diop, dppf, and PPh3 ligands,
see Table 1) demonstrate a tendency to adopt the T structure
for increasing bulkiness of the phosphines. This is, to a certain
extent, driven by the steric forces.
Most unfortunate for the examination of the theoretical results
is the absence of detailed structural data for [MH3P4]+ with small
monodentate PR3 ligands, for which the PH3 system normally
provides the stereoelectronic model. For instance, [FeH3-
(PMe3)4]+ was previously unknown, and no hint was given in
synthetic reports on the structure of [OsH3(PMe3)4]+.8 For
[RuH3(PMe3)4]+, a short T1 (not minimum) relaxation time
suggested that it could contain coordinated H2, but no further
II. Synthesis of M(H)2L4 and [MH3L4]+ Complexes (M )
Fe, Ru, Os; L ) PMe3, PEt3)
The cis-M(H)2L4 derivatives were in most cases the starting
materials for our investigation on [MH3L4]+ complexes. From
all of the cis-dihydrides of this paper, the syntheses of the PMe3
19d
derivatives8a, and Ru(H)2(PEt3)4 were reported earlier. Due
to low yields in some of these preparations and the complicated
(multistep) nature of the described approach, decisive modifica-
tions of the synthetic procedures had to be developed.
(12) (a) Saburi, M.; Aoyagi, K.; Takahashi, T.; Uchida, Y. Chem. Lett.
1990, 601. (b) Tsukahara, T.; Kawano, H.; Ishii, Y.; Takahashi, T.; Saburi,
M.; Uchida, Y.; Akutagawa, S. Chem. Lett. 1988, 2055. (c) Saburi, M.;
Aoyagi, K.; Takeuchi, H.; Takahashi, T.; Uchida, Y. Chem. Lett. 1990,
991. (d) Saburi, M.; Takeuchi, H.; Ogasawara, M.; Tsukahara, T.; Ishii,
Y.; Ikariya, T.; Takahashi, T.; Uchida, Y. J. Organomet. Chem. 1992, 428,
155. (e) Saburi, M.; Aoyagi, K.; Kodama, T.; Takahashi, T.; Uchida, Y.;
Kozawa, K.; Uchida, T. Chem. Lett. 1990, 1909. (f) Ogasawara, M.; Aoyagi,
K.; Saburi, M. Organometallics 1993, 12, 3393. (g) Ogasawara, M.; Saburi,
M. J. Organomet. Chem. 1994, 482, 7.
(13) Mezzetti, A.; DelZotto, A.; Rigo, P.; Farnetti, E. J. Chem. Soc.,
Dalton Trans. 1991, 1525.
(14) Earl, K. A.; Jia, G.; Maltby, P. A.; Morris, R. H. J. Am. Chem. Soc.
1991, 113, 3027.
In the Fe(H)2L4 series, reduction of FeCl2 was effected in
the presence of excess of L by applying NaBH4 in ethanol (L
) PMe3, yield 65%). The synthesis of Fe(H)2(PMe3)4 described
in a short paper,8a,19a-c however, started from LiAlH4 in THF.
The report, unfortunately, did not provide synthetic details.
The synthesis of Fe(H)2(PEt3)4 was achieved Via the inter-
mediate isolation of Fe(H)2(N2)(PEt3)3 (yield ca. 70%). This
dinitrogen complex was obtained as an oily material slightly
contaminated with the spectroscopically identified Fe(H)2(H2)-
(PEt3)3 complex (4%) and PEt3‚BH3 (< 1%). In a subsequent
reaction in neat PEt3, the N2 and the H2 ligands of these species
could be replaced by the phosphine, which ultimately afforded
(15) Michos, D.; Luo, X.-L.; Crabtree, R. H. Inorg. Chem. 1992, 31,
4245.
(16) Bampos, N.; Field, L. D. Inorg. Chem. 1990, 29, 588.
(17) (a) Bianchini, C.; Peruzzini, M.; Zanobini, F. J. Organomet. Chem.
1988, 354, C19. (b) Eckert, J.; Albinati, A.; White, R. P.; Bianchini, C.;
Peruzzini, M. Inorg. Chem. 1992, 31, 4241. (c) Jia, G.; Drouin, S. D.; Jessop,
P. G.; Lough, A. J.; Morris, R. H. Organometallics 1993, 12, 906. (d)
Bianchini, C.; Perez, P. J.; Peruzzini, M.; Zanobini, F.; Vacca, A. Inorg.
Chem. 1991, 30, 279. (e) Bianchini, C.; Linn, K.; Masi, D.; Peruzzini, M.;
Polo, A.; Vacca, A.; Zanobini, F. Inorg. Chem. 1993, 32, 2366. (f) Bautista,
M. T.; Earl, K. A.; Maltby, P. A.; Morris, R. H. J. Am. Chem. Soc. 1988,
110, 4056. (g) Bautista, M. T.; Earl, K. A.; Maltby, P. A.; Morris, R. H.;
Schweitzer, C. T. Can. J. Chem. 1994, 72, 547.
(19) (a) Klein, H.-F. Angew. Chem., Int. Ed. Engl. 1970, 9, 903. (b)
Dahlenburg, L.; Frosin, K.-M. Polyhedron 1993, 12, 427. (c) Behling, T.,
Girolami, G. S.; Wilkinson, G.; Somerville, R. G.; Hursthouse, M. J. Chem.
Soc., Dalton Trans. 1984, 877. (d) Mitsudo, T.; Nakagawa, Y.; Watanabe,
K.; Hori, Y.; Misawa, H.; Watanabe, H.; Watanabe, Y. J. Org. Chem. 1985,
50, 565. (e) Jones, R. A.; Wilkinson, G.; Colquohoun, I. J.; McFarlane,
W.; Galas, A. M. R.; Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1980,
2480.
(18) Ogasawara, M. Personal communication.

3718 J. Am. Chem. Soc., Vol. 119, No. 16, 1997
GuseV et al.
Table 1. Compilation of the Known [MH3L4]+ Complexes (M ) Fe, Ru, Os)a
L, monodentate
[FeH(H2)(PPh(OEt)2)4]+ (-30, P +Cn?b);5 [FeH(H2)(P(OEt)3)4]+ (N/O,c Cn?b);5 [RuH(H2)(PMe3)4]+ (N/O,c ?b);6
[RuH(H2)(PPh(OEt)2)4]+ (>20, P);7a [RuH(H2)(P(OEt)3)4]+ (>-70, P);7a [RuH(H2)(P(OMe)3)4]+
(>0, P);7a [OsH3(PMe3)4]+ (N/O,c ?b);8 [OsH(H2)(PPh2OR)4]+ R ) Me (>-70, P), Et (10, P), iPr (>-50, P);7b
[OsH(H2)(PPh(OEt)2)4]+ (>-70, P);7a [OsH(H2)(P(OEt)3)4]+ (N/O,c P?);7a [Os(H)3(PPh3)4]+ (N/Oc, T)9
[FeH(H2)(dmpe)2]+ (>10, P);3a,10 [FeH(H2)(depe)2]+ (0, P);3a,11 [FeH(H2)(dppe)2]+ (20, P);2,11 [FeH(H2)(dprpe)2]+
(20, P);3a [RuH(H2)(dmpe)2]+ (>20, P);3d [RuH(H2)(depe)2]+ (20, P);11 [RuH(H2)(dppe)2]+ (>20, P);11
[RuH(H2)(dppp)2]+ (>20, P);12a [RuH(H2)(dppb)2]+ (>-10, P);12a [RuH(H2)(binap)2]+ (>20, P);12b,g
[RuH3(diop)2]+ (>0, P + Td);12c [Ru(H)3(dppf)2]+ (N/O,c T);12e [RuH(H2)(dpbp)2]+ (-10, T);12g
[RuH(H2)(dcpe)2]+ (20, P);13 [OsH(H2)(dppe)2]+ (>0, P);14 [OsH(H2)(depe)2]+ (-20, P);14 [Os(H)3(dcpe)2]+
(N/Oc, T)13
bidentate
tridentate + monodentate [RuH(H2)(triphos)(PMe2Ph)]+ (>0, P);15 [RuH(H2)(triphos)(P(OCH2)3CEt)]+ (-, P)15
tetradentate
[FeH(H2)P(CH2CH2CH2PMe2)3]+ (N/O,c Cn);16 [FeH(H2)P(CH2CH2PPh2)3]+ (10, Cn);17a,b [FeH(H2)P(CH2CH2PCy2)3]+
(-10, Cn);17c [RuH(H2)P(CH2CH2PPh2)3]+ (30, Cn);17d [RuH(H2)P(CH2CH2PCy2)3]+ (0, Cn);17c
[OsH(H2)P(CH2CH2PPh2)3]+ (-80, Cn);17e [Os(H)3(rac-tetraphos)]+ (N/O,c Cc)17f; [Fe,RuH(H2)(meso-tetraphos)]+
(N/O,c P)17g
a Data in parentheses show the temperature in degrees Celcius of the 1H NMR decoalescence in the hydride region and the structural assignments
as given by the authors. b A question mark indicates that the assignment is not reliably established or no structural data are available. c Decoalescence
was not observed. d The second isomer in this mixture was originally thought to be Cn. Subsequent studies proved a trihydride structure, T.18
the desired Fe(H)2(PEt3)4 product. Both complexes, Fe(H)2-
(PMe3)4 and Fe(H)2(PEt3)4, are air sensitive and have to be
handled under an inert gas atmosphere. In addition to this, the
PMe3 derivative appeared to be thermally unstable at room
temperature and slowly changed from light yellow to dark green
in the solid state.
In either case of the reduction with NaBH4, formation of
cationic binuclear side-products {[(PMe3)3Fe]2(µ-H)3}+ and
{[(PEt3)3Fe(H)2]2(µ-BH2)}+ was observed, which were isolated
as their [BPh4]- salts.
The preparation of Os(H)2L4 was described for L ) PMe3
by reduction of OsCl2(PMe3)4 with sodium naphthalide8a or from
easily prepared K2[OsO2(OMe)4] and its reaction with PMe3 in
methanol.19c In our hands this latter approach was successful
with L ) PEt3, but the preparation of Os(H)2(PMe3)4 was
achieved only after a modification of this synthesis.
An initial 31P NMR spectroscopic pursuit of the transforma-
tion of K2[OsO2(OMe)4] and PMe3 in CH2Cl2/CH3OH (4:1)
showed after 1.5 h at room temperature formation of a mixture
of the [Os(CH3)(PMe3)5]+ and [Os(H)(PMe3)5]+ cations in a
7:3 ratio.20 In addition to the resonances of these species, a
signal of OPMe3 was observed, which indicated that PMe3 was
additionally functioning as a reducing agent. All of these
resonances accounted for 90% of the spectrum intensity;
however, no dihydride Os(H)2(PMe3)4 was observed. When this
reaction was attempted in neat ethanol, a similar mixture of
[Os(CH3)(PMe3)5]+ and [Os(H)(PMe3)5]+ was formed contain-
ing ca. 15% of Os(H)2(PMe3)4. In both cases, further stirring
produced more [Os(H)(PMe3)5]+ at the expense of [Os-
(CH3)(PMe3)5]+ (3:2 ratio after 24 h in CH2Cl2/CH3OH).
The original synthetic route was therefore altered to carry
out the reaction in ethanol and with NaBH4 as reducing agent,
which was applied immediately after addition of the solvent.
After 1.5 h at 70 °C the 31P NMR spectrum revealed complete
transformation of [Os(CH3)(PMe3)5]+ to Os(H)2(PMe3)4, while
a small amount of the [Os(H)(PMe3)5]+ cation remained
unreacted. This reduction presumably involves formation of
the neutral complex Os(PMe3)5, which can lose one phosphine
to give Os(H)(CH2PMe2)(PMe3)3.21 The latter complex has
been reported to quantitatively produce the dihydride Os(H)2-
(PMe3)4 in methanol.8a
The yield of {[(PMe3)3Fe]2(µ-H)3}[BPh4] seemed to be
adjustable at the expense of Fe(H)2(PMe3)4, when low stoichio-
metric amounts of NaBH4 were applied. Both binuclear species
were characterized spectroscopically and by single-crystal X-ray
diffraction studies. The latter will be published elsewhere.
{[(PMe3)3Fe]2(µ-H)3}+ possesses a structure of two joint
octahedrons which are face-bridged Via the three hydrogens.
The same structural motif has been seen in a related {[(PMe3)3-
Ru]2(µ-H)3}+ complex.19e The structure of the binuclear PEt3
derivative can be envisaged to consist of two staggered Fe(II)
[(PEt3)3Fe(H)2] units of Cs symmetry which are “held together”
+
by a BH2 cation. In a quite unusual manner, the boron atom
thus establishes contacts to all hydride ligands and the iron
centers and attains apparent “hypercoordination” in a bicapped
octahedral fashion.
Os(H)2(PMe3)4 and [Os(H)(PMe3)5]+ were separated by
extraction of the former complex with hexane. The dihydride
Os(H)2(PMe3)4 obtained in this fashion was still contaminated
with about 4% of OsH4(PMe3)3, but could be used without
further purification for the preparation of [Os(H)3(PMe3)4]+.
The reaction of K2[OsO2(OMe)4] with PEt3 in methanol was
also investigated by 31P NMR spectroscopy. After 1 h the
resulting clear red solution displayed signals for PEt3 (-16.8
ppm), OPEt3 (60.9 ppm), OsH4(PEt3)3 (1.3 ppm), and two lines
at -10.6 and -12.4 ppm corresponding to [Os(H)3(PEt3)4]+ and
some reactive intermediate, respectively. Formation of [Os-
(H)3(PEt3)4]+ was completed after 3.5 h. The solution turned
almost colorless, and at this point a trace amount of Os(H)2-
The preparation of cis-Ru(H)2(PMe3)4 was accomplished
earlier in a three-step synthesis Via RuCl2(PPh3)3 and RuCl2-
(PMe3)4,19b while Ru(H)2(PEt3)4 resulted from the direct conver-
sion of RuCl3‚nH2O with NaBH4 in the presence of PEt3,19d
however in moderate yield. Further explorations on this reaction
showed that by the use of the reducing agent [NBu4]BH4 in
THF it was possible to diminish the amount of undesired side-
products (such as RuH4(PEt3)3) and raise the yield of Ru(H)2-
(PEt3)4 to 76%. This synthetic procedure also allowed a one-
pot preparation of Ru(H)2(PMe3)4 in 70% yield (eq 1).
Ru(H)2(PMe3)4 and Ru(H)2(PEt3)4 were identified according to
their reported NMR spectroscopic properties.19b,d
[NBu4]BH4
(20) (a) OsMe2(PMe3)4 is known.19c (b) [OsH(PMe3)5]OTf has been
prepared and characterized.21
RuCl3‚nH2O + 4L
8 cis-Ru(H)2L4
(1)
THF
(21) Ermer, S. P.; Shinomoto, R. S.; Deming, M. A.; Flood, T. C.
Organometallics 1989, 8, 1377.
L ) PMe3, PEt3

M(II) and M(IV) Complexes [MH3(PR3)4]+
J. Am. Chem. Soc., Vol. 119, No. 16, 1997 3719
Table 2. 1H and 31P NMR Spectroscopic Data for the Complexes [MH3(PR3)4]+ Obtained in CDFCl2/CDF2Cl Solutions
[MH3(PR3)4]+
δ(MH)
2J(H-P)a Hz (mult.)
T1min, ms
δ(31P)a (spin system or mulit.)
2J(P-P), Hz
[FeH(H2)(PMe3)4]+ (1a,Cn)
[Fe(H)3(PMe3)4]+ (1a,T)
exchange averaged datac
[Fe(H)3(PEt3)4]+ (1b,T)d
exchange averaged datac
[RuH(H2)(PMe3)4]+ (2a,Cn)
exchange averaged datac
[RuH(H2)(PEt3)4+ (2b,Cn)d
[Ru(H)3(PEt3)4]+ (2b,T)
exchange averaged datac
[Os(H)3(PMe3)4]+ (3a,Cc)
[Os(H)3(PMe3)4]+ (3a,T)
exchange averaged datac
[Os(H)3(PEt3)4]+ (3b,T)
exchange averaged datac
-11.38 (-105°)
-13.28 (-105°)
-11.68
br. s (90 Hz)b
36 (qi)
20.5 (qi)
13.5 (-120°)
196 (-120°)
19.2, 19.8 (A2B2), (-130°)
16.9(d), 31.6 (q), (-130°)
17.2 (s)
40.1(d), 67.3 (q)
46.2 (s)
-7.2, -8.5 (A2B2), (-50°)
-9.2 (br. s)
16.0 (t), 21.9 (t)
17.0 (d), 65.4 (q)
26.9 (s)
45.5
8.0
-13.98 (-100°)
-14.01
non-first-order
39.4 (qi)
177 (-70°)
8.9
39
-8.03 (-130°)
-7.99
br. s (91 Hz)b
br. s (9 Hz)b
br. s (100 Hz)b
7.9 (qi/-70°)
6.8 (qi)
10.1 (-120°)
-8.84 (-90°)e
-10.33 (-115°)
-10.10
26.1
28.9
102 (-50°)f
99.6 (-115°)
207 (-115°)
-9.68 (-115°)
-8.87 (-115°)
-9.67
-12.21 (-110°)
-12.00
(14.6, -5.1 (tt)
(10.8 (qi)
(5.2 (qi)
15.9 (qi)
15.3 (qi)
-54.5(t), -53.1(t)
18.6
-50.6 (s)
-54.3 (s)
194 (-90°)
23.2, -20.2 (AB3), (-130°)
-11.4 (s)
g
a If not indicated otherwise, the data were obtained at the temperature given in the preceeding column, δ(MH). b Line width for the broadened
single lines. c All in THF-d8, at 20 °C. d In THF-d8. e Under these conditions, the chemical shift of 2b,T is 10.28 ppm. f Underestimated, because
of the exchange with 2b, Cn. g Not resolved.
(CO)(PEt3)3 became detectable also. The cation [Os(H)3-
(PEt3)4]+ could be precipitated from the reaction solution as a
[BPh4]- salt, however impure, since it cocrystallized with
KBPh4. In an alternative treatment evaporation of the reaction
solution effected deprotonation of the trihydride to give Os-
(H)2(PEt3)4, which was extracted with hexane and converted in
a subsequent step into [Os(H)3(PEt3)4]BPh4 by precipitation with
a methanol solution of NaBPh4. For the dihydrides Os(H)2L4,
we were able to obtain the pure compounds only by deproto-
nation of [Os(H)3L4]+ with KOH in THF.
The other dihydride complexes of the M(H)2L4 series could
also be protonated in methanol. These reactions became
complete in a more acidic CH3OH/(CF3)2CHOH mixture. The
cationic complexes [MH3L4]+ are readily precipitated as [BPh4]-
salts by the addition of NaBPh4.16 For X-ray structural
characterization the [Fe(H)3(PEt3)4]+ cation has also been
obtained as a [B(C6H3(CF3)2)4]- salt .
relaxation and appreciable broadness are predicted to be
characteristic of the Cn isomers.
Other insights can be provided by dynamic NMR. On the
basis of coalescence/decoalescence phenomena, it is in many
cases possible to evaluate and compare the rates of intramo-
lecular fluxional processes. This work treats the dynamic NMR
data as a very important structurally related piece of informa-
tion. When a satisfactory geometry is assigned on the basis of
chemical shifts and coupling constants, the structure should as
well provide an explanation for the dynamic properties of the
molecule.
Facile fluxionality of polyhydride complexes may typically
involve significant motions within the H-ligand framework and
concomitant minor positional adjustment of the positions of the
other hydrides or/and the heavier ligands. It should be
recognized that this may lead to averaging of chemical shifts
of the heavy ligands as well and therefore pretention of their
physical exchange. In other cases these rearrangements are
associated with real physical motions of non-hydrogen atoms
and molecular subunits. Then, presumably, not too strong
topological dispositions are involved. The structural types C,
P, and T of Scheme 1 nicely illustrate this concept. It should,
however, be pointed out that it may be quite deceptive to assume
that a small topological rearrangement of the heavy ligand
framework can be concluded from only minor changes in the
VT NMR (e.g., 31P NMR) spectra. The fact that no change is
observed may simply mean that the chemical shift difference
of the averaging nuclei is very small or very large.
III. Stuctural Results: Characterization of the [MH3-
(PMe3)4]+ and [MH3(PEt3)4]+ Complexes by NMR and
X-ray Complexes by NMR and X-ray
The [MH3L4]+ complexes investigated in this paper are
numbered as given below and are eventually additionally
denoted to indicate the specific structural isomers (T, Cc, Cn,
P; see Scheme 1).
[MH3L4]+
PMe3
Fe
1a
1b
Ru
2a
2b
Os
3a
3b
Fast phosphorus positional exchange is not feasible in the
OsP4 skeleton of the Cn and Cc structures. The known
dihydrides cis-M(H)2(PR3)4 demonstrate slow or no exchange
in their 31P NMR spectra.1 For instance, Fe(H)2(PMe3)4 and
Ru(H)2(PMe3)4 are not fluxional at room temperature.19a,e The
cis-M(H)3 and cis-MH(H2) substructures, however, typically
show fast hydride or H/H2 scrambling in the reported complexes.
This has also been found for all C-type isomers in the present
work.
PEt3
Table 2 lists the most prominent NMR data of the nine
[MH3L4]+ isomers traced in our studies. Three solid-state
structures (X-ray) will also be reported in this section. However,
some of the isomers of [MH3(PR3)4]+ exist only in solution and
are not amenable to X-ray determinations. Their structures were
solely derived by NMR techniques.
In the [MH3(PR3)4]+ molecules the metal-bound nuclei are
magnetically active, and this often permitted unambigous
structural assignment based on the interpretation of the 1H and
31P NMR chemical shifts and couplings. Clearly, the T and C
geometries can be well distinguished by 31P NMR through the
appearance of AB3 Vs A2B2 spin systems in T Vs C structures,
respectively. The Cc structures are expected to exhibit long
relaxation times for the hydrides and hence should display a
The trans-H and -H2 ligands of the P structural type are
separated by the ML4 plane. This does not permit fast H/H2
scrambling, and the complexes of this structure quite often
display decoalesced H and H2 resonances at ambient temper-
ature. In this case, the ML4 skeleton ought to show a single
31P chemical shift, if the phosphorus ligands are indeed located
in one plane. Otherwise, fast 31P fluxionality is expected, since
the deviation from the ideal plane should be small and no
1
1
well-resolved H NMR pattern in the hydride region. Fast H

3720 J. Am. Chem. Soc., Vol. 119, No. 16, 1997
GuseV et al.
substantial motion is required for the exchange between
inequivalent positions.
Finally, the T type has a single 1H chemical shift and a highly
fluxional AB3 spin system in the 31P NMR. It shall be discussed
later in a separate section, how the 31P chemical shift exchange
is actually established by hydride motion according to the so-
called tetrahedral jump mechanism.1 Following these com-
ments, a detailed characterization of the [MH3L4]+ complexes
is presented starting with 3b.
a. Structure of [OsH3(PEt3)4]+ (3b,T). The characterization
of the osmium complex 3b is based on a comparison with the
X-ray and NMR data reported for [Os(H)3(PPh3)4]+ (3c,T).9 In
the solid state 3c shows a slightly angularly distorted tetrahedral
arrangement of four PPh3 around the metal (P-Os-P angles,
105° to 114°). Three of the Os-P distances are appreciably
longer than the fourth (average 2.473 Vs 2.290 Å) clearly
affected by the trans-hydrides (not located) which are presum-
ably all disposed on three of the four tetrahedral faces. The
31P NMR of 3c shows a “fingerprint” of the T structure, two
1:3 resonances (AB3 spin system). The molecule is fluxional
and the two chemical shifts coalesce at -40 °C. At -80 °C,
the resonances are still broad and do not exhibit any coupling.
Figure 1. Structure of the cation in [Os(H)3(PMe3)4]BPh4 with selected
angles (deg) and distances (Å) determined by X-ray diffraction.
Hydrogen and carbon atoms of the PMe3 ligands have been omitted
for clarity. Additional structural information: Os-H1 1.57(9); Os-H2
1.73(11); Os-H3 1.93(12); H1-Os-H2 147(5); H1-Os-H3 99(5); H2-
Os-H3 48(5); H1-Os-P1 67(3); H2-Os-P3 55(4); P1-Os-P4 96.12-
(5); P2-Os-P3 98.91(5).
1
A single H NMR resonance of the hydrides persists over the
whole temperature range studied (22 to -80 °C).
The structure of [Os(H)3(PEt3)4]+ (3b) can now easily be
derived, since the NMR behavior of this molecule is very similar
to that of 3c. The only substantial difference is that 3b (with
smaller phosphines) is more fluxional. The AB3 31P NMR
pattern coalesces at -110 °C (the fluxional behavior of all
complexes 1-3 will be interpreted following the presentation
of the structural results). Measurements in a CDFCl2/CDF2Cl
mixture extended the temperature range to -140 °C, at which
end the phosphorus lines did not sharpen enough to reveal any
coupling. The hydride resonance of 3b is a quintet at and above
-110 °C. It loses the fine structure below -110 °C, but the
expected ABB′B′′XX′X′′ pattern is not yet resolved in the slow
exchange regime at -140 °C. The relaxation time T1min is long
(194 ms) as expected in the absence of H-H bonding interac-
tions.
Figure 2. Variable-temperature NMR spectra of [Os(H)3(PMe3)4]BPh4
(3a) in CDFCl2/CDF2Cl2.
prevents a direct structural assignment even at -140 °C (3a,Cc
might have shown an XY2 hydride pattern in the slow-exchange
regime). T1 relaxation time measurements provide an indirect
approach.
Long T1min times have been found at -115 °C, well below
the decoalescence temperature: 100 and 207 ms for the major
and minor isomer, respectively. The 100 ms value can be
interpreted in terms of a small contribution to the relaxation
from the PMe3 ligands and either (a) two equal H‚‚‚H separa-
tions, or (b) one short and one long H‚‚‚H distances.22 The
phosphine contribution has been estimated experimentally in
some monohydride octahedral complexes of tungsten and
rhenium. The known maximum values are 0.87 s-1 in WH-
(CO)2(NO)(PMe3)2 at -115 °C,23a 0.75 s-1 in ReH(CO)2(PMe3)3
at -106 °C, 1.02 s-1 in cis-ReH(CO)(PMe3)4 at -108 °C, and
1.21 s-1 in trans-ReH(CO)(PMe3)4 at -101 °C.23b For the
present calculations, the 1.02 s-1 value provided by cis-ReH-
(CO)(PMe3)4 seems more appropriate, since this complex adopts
a geometry of the phosphorus framework very close to that
found in solid 3a. The case a allows us to estimate that the
H‚‚‚H separation ought to be 1.64(1) Å ( H-Os-H angle ca.
59°, assuming r(Os-H) )1.65 Å). The model b leads to a short
contact of 1.46(1) Å ( H-Os-H ) 52°).
The H-D coupling is not resolved at room temperature in
the quintet pattern of OsH2D of the [OsH2D(PEt3)4]+ isoto-
pomer. This monodeuterated derivative of 3b formed exlusively
in CH3OD solution within 10-15 min preceeding the precipita-
-
tion as a BPh4 salt. The 2.3 Hz 2J(P-D) coupling was
resolvable in the exchange-averaged 31P{1H} NMR at 20 °C,
which permitted the assignment to the [OsD(H)2(PEt3)4]+
structure.
All of these data represent a firm ground for the unambigous
formulation of [Os(H)3(PEt3)4]+ as a classical trihydride (3b,T).
b. Structure of [OsH3(PMe3)4]+ (3a,T and 3a,Cc). The
solid-state structure of [Os(H)3(PMe3)4]+ is shown in Figure 1
along with selected angles and distances. Different from the
PEt3 analogue, this molecule adopts the C geometry. The
hydrides were located and refined, but given the high standard
deviations in their positions, this does not provide reliable
structural information. Solution 1H and 31P NMR experiments
were carried out to get more data on the bonding in the OsH3
fragment.
Other Os(IV) trihydrides have a comparable geometry of the
Os(H)3 fragment (as calculated from the T1min times): Os(H)3-
Cl(PiPr3)2 ( H-Os-H ) ca. 58°),22 Os(H)3(η2-BH4)(PiPr3)2
The solution NMR spectra of [Os(H)3(PMe3)4]+ (Figure 2)
show an equilibrium mixture of two isomers. The major
component of the mixture (ca. 88% at -115 °C) is one
exclusively present in the solid state. In the low-temperature
31P spectra, a characteristic A2B2 (A ) P1, P3; B ) P2, P4
according to the numbering of Figure 1) spin system is observed.
Fast exchange between the hydride ligands in each isomer
(22) For an example of these calculations reported for a related trihydride
Os(H)3Cl(PiPr3)2 see: Gusev, D. G.; Kuhlman, R.; Sini, G.; Eisenstein, O.;
Caulton, K. G. J. Am. Chem. Soc. 1994, 116, 2685.
(23) (a) Shubina, E. S.; Belkova, N. V.; Krylov, A. N.; Vorontsov, E.
V.; Epstein, L.M.; Gusev, D. G.; Niedermann, M.; Berke, H. J. Am. Chem.
Soc. 1996, 118, 1105. (b) Gusev, D. G.; Nietlispach, D.; Vymenits, A. B.;
Bakhmutov, V. I.; Berke, H. Inorg. Chem. 1993, 32, 3270.
(24) Esteruelas, M. A.; Jean, Y.; Lledo´s, A.; Oro, L. A.; Ruitz, N.
Volatron, F. Inorg. Chem. 1994, 33, 3609.

M(II) and M(IV) Complexes [MH3(PR3)4]+
J. Am. Chem. Soc., Vol. 119, No. 16, 1997 3721
Figure 4. 1H{31P} spectra in the hydride region of the isotopomers of
[Ru(H,D)3(PMe3)4]BPh4 in THF-d8 at 0 °C. The triplet H2D and quintet
HD2 are shaded.
spectrum shows line broadening at -115 °C, but decoalescence
of the resonance was not observed as low as -140 °C. The
T1min value of 207 ms is longer than the value of 100 ms in
3a,Cc, presumably because of the longer hydride-hydride
distances in 3a,T. Fluxionality is an intrinsic feature of this
structural type, and the 31P NMR decoalescence temperature
decreases in the order of decreasing bulkiness of the PR3: -40
°C (PPh3) > -110 °C (PEt3) > ca. -140 °C (PMe3).
Figure 3. 1H{31P} NMR spectra of the [Os(H,D)3(PMe3)4]BPh4
isotopomers at 20 °C. The OsH3 (singlet), OsH2D (triplet), and OsHD2
(quintet) resonances are shaded. The extent of deuteration increases
on going from A to B.
( H-Os-H ) ca. 60°),24 and [Os(H)3(NCMe)2(PiPr3)2]+
(
H-Os-H ) ca. 57°).25 All of these complexes show T1min
values shorter than in 3a (72, 82, and 65.5 ms (300 MHz))
because of the larger contribution from the PiPr3 ligands
(estimated as ca. 3.85 s-1). Only Os(H)3Cl(PiPr3)2 revealed
1
The variable-temperature H NMR data in Figure 2 (-140
to +23 °C) indicate very low enthalpy difference ∆H between
the isomers. The equilibrium constant K ) [3a,Cc]/[3a,T] shows
virtually no temperature dependence below decoalescence (7.4
(-100 °C), 6.9 (-115 °C), 7.3 (-130 °C), and 7.7 (-140 °C)).
The averaged 1H chemical shift at 23 °C (-9.565 ppm in
CDFCl2/CDF2Cl) predicts K ) 6.2 assuming no temperature
dependence for the chemical shifts of the isomers (-8.869, 3a,T
and -9.676, 3a,Cc). Molar fractions a and b and their ratio K
) a/b can be estimated using the H-P couplings at the slow
and fast exchange limits. The average coupling is 5.22 Hz at
23 °C. At -115 °C the two isomers show H-P couplings of
( 10.80 Hz (quintet of 3a,T) and (14.62 per multiplet with a
coupling constant of -5.09 Hz (triplet of triplets of 3a,Cc). This
estimates K as 12.0 at 23 °C. The signs of the couplings follow
from the averaging scheme.
1
decoalescence in the hydride region of the H NMR spectra;
the other two examples demonstrated more facile fluxionality.
This consideration indicates that the major isomer is better
described as a trihydride with a pentagonal bipyramidal
geometry (3a,Cc). A helpful comparison is provided by the
structure of [Os(H)5(PMe2Ph)3]+.26 The neutron diffraction
study of this cation shows a dodecahedral pentahydride that is
reminiscent of the molecule in Figure 1 and can formally be
generated from 3a,Cc by replacement of one phosphine by two
hydrogens. These new two hydrides would be in the position
of either P1 or P3 and split up and down the P1-Os-P3 plane.
Finding a coordinated dihydrogen in the presence of four good
donors, the PMe3 in 3a would have been rather inexplicable.
Figure 3 shows the isotopically shifted hydride resonances of
the [Os(H,D)3(PMe3)4]+ isotopomers, which were obtained from
the reaction of Os(H)2(PMe3)4 with CH3OD. Phosphorus
decoupling clearly reveals a 3.8 Hz H-D coupling in [OsH2D-
(PMe3)4]+ and [OsHD2(PMe3)4]+, which is the largest among
the other known two-bond couplings in classical hydrides: 3.7
Hz in [CpIr(H)3(AsPh3)]+,27a 3.3 Hz in Tp*Ir(H)4,27b 2.8 Hz in
[Os(H)3(NCMe)2(PiPr3)2]+,25 and 2.4 Hz in [(dippp)Pd]2(µ-
H)2.27c
These equilibrium data disclose that ∆H is most probably
between +0.2 and 0 kcal/mol, while the corresponding ∆S limits
are +5.6 eu and +3.9 eu (the latter is R ln K, and K ) 7.3, if
∆H ) 0). The thermodynamics of the isomerization [3a,T] to
[3a,Cc] is thus almost exclusively governed by the higher
entropy of 3a,Cc.
c. Structure of [RuH(H2)(PMe3)4]+ (2a,Cn). Some 1H and
31P NMR parameters have already been reported for the complex
[RuH(H2)(PMe3)4]+ (2a).6 The present low-temperature mesure-
ments were performed to -140 °C. Neither H/H2 decoalescence
nor appreciable broadening was observed in the hydride region
(singlet at δ -8.0, line width 91 Hz at -130 °C). The T1min
time (10.1 ms, -120 °C) represents an average value for the
three metal-bound hydrogens, which is characteristic of a
dihydrogen complex.
The H-D couplings in Tp*Ir(H)4, [Os(H)3(NCMe)2(PiPr3)2]+,
and [Os(H)3(PMe3)4]+ are exchange-averaged avJ(H-D), so that
their interpretation is model-dependent. For a related example
the one-bond H-D coupling of a coordinated H-D ligand in
MH(HD)Ln is three times of avJ(H-D).14 Alternatively, the
isotopomers of classical M(H,D)3Ln systems must show an H-D
coupling between the cis-H,D ligands 1.5 times avJ(H-D), given
that the second H-D coupling in this configuration is negligible.
From this it is derived to be 5.7 Hz for [Os(H)3(PMe3)4]+, if
we also neglect that one of the two isomers (e.g., 3a,T) which
may not contribute to the avJ(H-D).
Further information on the structure of 2a was obtained from
deuterium labeling studies. A mixture of the isotopomers of
2a was prepared from the reaction of Ru(H)2(PMe3)4 with CH3-
1
OD. The H{31P} NMR pattern in the hydride region (Figure
4) shows splittings by H-D couplings. The magnitude of
avJ(H-D) is 10.9(1) Hz in the H2D isotopomer Vs 10.2(1) Hz
in the HD2. This indicates a nonstatistical distribution of the
isotopes and preferential coordination of the remaining hydrogen
as a hydride, while deuterium is enriched in the molecular D2
ligand.
From the avJ(H-D) values a one-bond H-D coupling of 30.6
-32.7 Hz can be calculated in the H-D ligand of 2a. No
decoalescence was found in the low-temperature 1H NMR
spectra of the deuterated 2a to -100 °C in THF-d8. In addition,
The minor isomer of [Os(H)3(PMe3)4]+ exhibits a single
resonance in the 1H and 31P NMR (Figure 2). The phosphorus
(25) Smith, K.-T.; Tilset, M.; Kuhlman, R.; Caulton, K. G. J. Am. Chem.
Soc. 1995, 117, 9473.
(26) Johnson, T. J.; Albinati, A.; Koetzle, T. F.; Ricci, J.; Eisenstein,
O.; Huffman, J. C.; Caulton, K. G. Inorg. Chem. 1994, 33, 4966.
(27) (a) Computed from the reported 2J(H-T) coupling. See: Heinekey,
D. M.; Payne, N. G.; Schulte, G. K. J. Am. Chem. Soc. 1988, 110, 2303.
(b) Paneque, M.; Poveda, M. L.; Taboada, S. J. Am. Chem. Soc. 1994, 116,
4519. (c) Fryzuk, M. D.; Lloyd, B. R.; Clentsmith, G. K. B.; Rettig, S. J.
J. Am. Chem. Soc. 1994, 116, 3804.

3722 J. Am. Chem. Soc., Vol. 119, No. 16, 1997
GuseV et al.
1
Figure 6. Variable-temperature H and 31P NMR spectra of [Fe(H)3-
(PEt3)4][B(C6H3(CF3)2)4] in THF-d8.
Figure 5. 1H and 31P NMR spectra of [RuH3(PEt3)4]BPh4 (2b) in
acetone-d6 (-90 °C) and a CDF2Cl/CDFCl2 mixture. The larger signals
belong to 2b,T. Arrows indicate the two low-intensity triplets of the
2b,Cn isomer.
there was no unusual broadening that could signify an isotope
effect on the rate of the H/H2 scrambling.
The 31P NMR spectra of [RuH(H2)(PMe3)4]+ show decoa-
lescence at -20 °C. The slow-exchange spectrum displays a
well-resolved A2B2 pattern at -50 °C. On the basis of all of
these NMR observations [RuH(H2)(PMe3)4]+ (2a) must be
assigned a Cn structure. Characteristic of the structural types
Cn and Cc is a fast exchange between the metal-bound
hydrogens, which, however, takes place in a fairly rigid M(PR3)4
skeleton.
d. Structure of [RuH3(PEt3)4]+ (2b,T and 2b,Cn). [RuH3-
(PEt3)4]+ (2b) is another example of this work, which establishes
an equilibrium of two isomers in solution. The major isomer
2b,T identifies itself by the “fingerprint” NMR data (Figure 5).
It shows a well-resolved AB3 31P spin system (2J(P-P) ) 29
Hz) at -115 °C. This coalesces at -70 °C and is notably less
fluxional than that of [Os(H)3(PEt3)4]+. The second isomer of
2b is present in a very low amount and can be detected in the
31P NMR spectra as an A2B2 spin system (two triplets at -90
°C, Figure 5).
Analyzing the hydride region of the variable-temperature 1H
NMR spectra, one observes a quintet at -10.10 ppm (2J(H-P)
) 6.8 Hz) at 20 °C in THF-d8. It broadens and loses the fine
structure below -10 °C. The maximum broadness (51 Hz) is
reached at -50 °C, and subsequent decoalscence brings about
two lines at -8.90 and -10.29 ppm at -70 °C. The more
intense high-field resonance of 2b,T appears as a quintet (2J(H-
P) ) 7.9 Hz) at -70 °C in CDFCl2/CDF2Cl. In neither of the
three solvents employed here (THF, acetone, and the CDFCl2/
CDF2Cl mixture) the low-intensity resonance at -8.9 showed
further decoalescence. In addition to this, the line was
persistently broad and did not show any coupling to the 31P
nuclei. These spectroscopic properties suggest a Cn structure
for the minor isomer. It is assumed to be a dihydrogen complex
[RuH(H2)(PEt3)4]+ by analogy with the PMe3 derivative. The
direct observation of an expected large J(H-D) coupling or a
short T1min time was not possible for 2b,Cn due to the
unfavorable equilibrium situation.
Figure 7. The structure of [Fe(H)3(PEt3)4][B(C6H3(CF3)2)4] (1b,T) with
selected angles (deg) and distances (Å) determined by X-ray diffraction.
Hydrogen and carbon atoms of the PEt3 ligands have been omitted for
clarity. Additional data: Fe-H (av) ) 1.44(7) Å, P3-Fe-H (av) )
67°(3), P1,2,4-Fe-H (av) ) 77°(3), 173°(3).
contribution of 2b,T, since the second isomer is present in a
very low amount only. It is noteworthy, that the equilibrium
constant [2b,T]/[2b,Cn] increases when the solvent polaritiy
decreases: 22 (acetone-d6) < 32 (THF-d8) < ca. 55 (CDFCl2/
CDF2Cl) at -90 °C.
The pair of ruthenium complexes 2a and 2b corroborate the
trend already shown by the osmium molecules 3a and 3b: for
each metal the heavier PEt3 congeners prefer the arrangement
which allows maximum relief of steric strain. This is the T
type structure, where the phosphorus ligands are pseudotetra-
hedrally arranged around the metals. The ruthenium system is
indeed driven by the sterics to adopt the high +IV formal
oxidation state in 2b,T, which otherwise is only rarely found
within the realm of ruthenium hydrides.28
e. Structure of [Fe(H)3(PEt3)4]+ (1b,T). This molecule is
the most representative example of the T structural type
characterized within this work. Species 1b,T is the only isomer
present. It shows the fingerprint of an AB3 31P spin system,
which is moderately fluxional and sharpens nicely at -100 °C,
the lowest temperature attainable in THF-d8 (Figure 6). The
decoalescence temperature is about -40 °C and reveals another
trend for the T geometry: the lighter metal congeners are more
rigid in the Fe-Ru-Os triad.
A high-temperature quintet resonance of Fe(H)3 exhibits
changes on lowering the temperature to give a complicated
pattern of an ABB′B′′XX′X′′ spin system in the slow-exchange
regime. The relaxation time T1min ) 177 ms rules out any
bonding interaction between the hydrides.
The molecular structure of 1b,T is shown in Figure 7. The
coordination around the iron atom can be envisaged as a
distorted FeP4 tetrahedron capped by the three terminal hydrogen
atoms on three of the trigonal faces. Alternatively this molecule
can be assembled from an octahedral fac-[Fe(H)3(PEt3)3]+
At -70 °C the hydride resonance of 2b,T transforms from
an exchange-averaged quintet into a complicated pattern of an
ABB′B′′XX′X′′ spin system at -115 °C (Figure 5). A T1min
relaxation time of 102 ms was measured for the hydride ligands
at -50 °C. At this temperature the equilibrium is fast on the
NMR time scale, which causes the relaxation time to be
averaged for the two isomers. It is mainly determined by the
(28) (a) Chinn, M. S.; Heinekey, D. M. J. Am. Chem. Soc. 1990, 112,
5166. (b) Jia, G.; Morris, R. H. J. Am. Chem. Soc. 1991, 113, 875. (c) Jia,
G.; Lough, A. J.; Morris, R. H. Organometallics 1992, 11, 161.

M(II) and M(IV) Complexes [MH3(PR3)4]+
J. Am. Chem. Soc., Vol. 119, No. 16, 1997 3723
Figure 9. Low-temperature 31P NMR spectra of the two isomers of
[FeH3(PMe3)4]BPh4 in a CDFCl2/CDF2Cl mixture.
hydrogen bonds. They are shorter in the experimental structure
(Fe-H 1.48 Å, Fe(H2) 1.53 and 1.72 Å) and are in a better
agreement with the neutron diffraction data available for the
related Fe(H)2(H2)(PEtPh2)3 molecule.30 The latter shows two
Fe-H distances of 1.514 Å (trans to PEtPh2) and 1.538 Å (trans
to coordinated dihydrogen) and two distances in the Fe(H2)
fragment (1.576 and 1.607 Å). There is a remarkable asym-
metry of bonding in the Fe(H2) demonstrated by all three
(calculated and experimental) structures. This was explained
invoking a weak attractive interaction between the cis-H and
-H2 ligands (between H1 and H2 in Figure 8). The theoretical
report on [FeH(H2)(PH3)4]+ also mentions that those hydrogens
in the positions of H3 and H2 look as if they retained “memory”
on their chemical origin, i.e., that one of H3 appears as a proton
coordinating onto a classical hydride-iron bond.4a This
structural feature however still remains speculative without
neutron diffraction data for [FeH(H2)(PMe3)4]+.
Figure 8. Comparison of the structural data provided by an ab initio
calculation4a on [FeH(H2)(PH3)4]+ (top) and the X-ray diffraction of
[FeH(H2)(PMe3)4]BPh4 (two views below). Other selected distances (Å)
and angles (deg): H2-H3 0.84(4), Fe-P1 2.230(1), Fe-P2 2.231(1),
Fe-P3 2.260(1), Fe-P4 2.212(1), H2-Fe-H3 32(2), P1-Fe-P2 152.51-
(3), P1-Fe-P4 95.59(3), P2-Fe-P4 98.05(3).
fragment and a PEt3 cap on the face of the three hydrogens.
This fourth phosphorus ligand is indeed located on an idealized
3-fold axis of symmetry. Since there is no trans-ligand for that
phosphorus moiety, the Fe-P bond is rather short, 2.170(2) Å,
while the other three Fe-P distances are elongated to 2.268-
2.288(2) Å because of the strong trans-influence of the hydrides.
Similar features can be seen in the other two solid-state
structures of the T type, [Ru(H)3(dppf)2]+ (2.325 Vs av 2.416
Å)12d and [Os(H)3(PPh3)4]+ (2.290 Vs av 2.473 Å).9 Quite
naturally, the Fe-P bonds are 5-8% shorter than the corre-
sponding Ru-P and Os-P ones and the ligand arrangement is
more compact (compressed) in [Fe(H)3(PEt3)4]+.
In solution, a fast equilibrium is established between the two
isomers of 1a (Figure 9). The rate of equilibration becomes
slow on the NMR time scale below -80 °C. Resonances for
the solid-state isomer 1a,Cn can then be identified. In the 31P
NMR, they give rise to a characteristic A2B2 pattern. The
second isomer appears as an AB3 spin system, which confirms
an 1a,T structural assignment (Figure 9). The 1a,T is more
fluxional than 1b,T bearing larger phosphines. The former
displays resolved 31P-31P coupling at -137 °C, while the latter
complex shows this at -100 °C.
The formal +IV oxidation state in [Fe(H)3(PEt3)4]+ is unusual
for iron hydrides. The other reliably established Fe(IV) hydride
systems are the cationic [Cp*Fe(dppe)(H)2]+29a and the neutral
(η6-arene)Fe(H)2(SiCl3)2 complexes.29b For the hydrido silyl
and stannyl complexes FeH3(PPh2R′)3ER3 (ER3 ) SnPh3 (R′
n
n
) Bu), SiMePh2 (R′ ) Bu), SnPh3 (R′ ) Et)), the same
structure is assumed on spectroscopic grounds, however, definite
structural conclusions were not given.29c The C3 symmetric
heavy atom skeleton of FeH3(PPh2Et)3(SnPh3) is reminescent
of the Fe(PEt3)4 fragment (Fe-P: 2.239 Å (average) and P-Fe-
Sn: 113.7°,114.0°, 118.7°). The metal-bound hydrogens were
not located. The reported spectroscopic data indicated the
possibility of an additional E-H secondary type bonding.
f. Structure of [FeH3(PMe3)4]+ (1a,T and 1a, Cn). The
X-ray structure analysis of [FeH3(PMe3)4]+ reveals a distorted
octahedral geometry 1a,Cn around the iron center (Figure 8).
The three metal-bound hydrogens were located as a cis-disposed
hydride and a dihydrogen ligand. The two equatorial phosphines
P3 and P4 show different distances to iron: longer (2.260 Å,
P3) when trans to H and shorter (2.212 Å, P4) when trans to
dihydrogen. Both bond lengths are in the range of Fe-P
distances observed in the [Fe(H)3(PEt3)4]+ cation: (2.28 Å (trans
to H, av) to 2.17 Å (trans to an empty coordination site).
The geometries of the experimental and calculated structures
of Figure 8 compare well. They demonstrate a reasonable
agreement between most of the angles and distances. Some
substantial disparity in Figure 8 is evident only in the iron to
As expected from the 31P NMR investigations, the hydride
1
region of the H NMR spectrum reveals resonances for 1a,Cn
and 1a,T below the decoalescence temperature of -80 °C. A
single broad line of the metal-bound hydrogens of 1a,Cn, present
even at -140 °C, indicates a typical very fast H/H2 scrambling.
The hydride resonance of 1a,T is a quintet (avJ(H-P) ) 36 Hz)
at -105 °C. When the temperature is lowered, it shows a
transformation quite similar to that given by [Fe(H)3(PEt3)4]+
in Figure 6. The central lines of the quintet broaden, and then
the quintet decoalesces into a multiplet, which may remind one
of a triplet of doublets at -130 °C. The doublet feature (38
Hz) proved to be the 2J(H-PA) coupling by selective decoupling
at the B3 31P chemical shift.
The T1min relaxation times were reached at -120 °C. They
are characteristically short (13.5 ms) for the averaged FeH(H2)
line of 1a,Cn and long (196 ms) for the classical hydride
resonance of 1a,T.
A deuterated derivative of 1a was prepared by the reaction
of Fe(H)2(PMe3)4 with CH3OD precipitated as a BPh4- salt. A
freshly prepared (1 h after isolation of the solid) and strongly
D-enriched sample of 1a shows a residual HD2 quintet resonance
(29) (a) Hamon, P.; Toupet, L.; Hamon, J.-R.; Lapinte, C. Organome-
tallics 1992, 11, 1429. (b) Yao, Z.; Klabunde, K. J.; Asirvatham, A. S.
Inorg. Chem. 1995, 34, 5289. (c) Schubert, U.; Gilbert, S.; Mock, S. Chem.
Ber. 1992, 125, 835.
(30) Van Der Sluys, L. S.; Eckert, J.; Eisenstein, O.; Hall, J. H.; Huffman,
J. C.; Jackson, S. A.; Koetzle, T. F.; Kubas, G. J.; Vergamini, P. J.; Caulton,
K. G. J. Am. Chem. Soc. 1990, 112, 4831.
1
at δ -11.651 in the H{31P} NMR (avJ(H-D) ) 8.1(1) Hz at
20 °C in CD2Cl2). This solid material changed and revealed a
decreased amount of metal-bound deuterium after 6 h of storage
at 20 °C. The 1H{31P} NMR spectrum then showed a mixture

3724 J. Am. Chem. Soc., Vol. 119, No. 16, 1997
GuseV et al.
Scheme 2
position of this ligand which is “surrounded” by all three
hydrides in [Fe(H)3(PMe3)4]+.
In the spectra of Figures 9 and 10 it can be seen that 1a,Cn
is the major isomer in solution. The equilibrium constant K )
[1a,Cn]/[1a,T] is solvent-dependent and increases with increasing
polarity: 2.7 (CDFCl2/CDF2Cl) < 4.0 (CD2Cl2) < 6.7 (acetone-
d6) (all between -100 and -110 °C). In the CD2Cl2 case, it
appears that K ) 4 does not change appreciably between -100
and +20 °C. However, fast exchange does not permit a direct
measurement of K above -90 °C; it can be estimated on the
basis of avJ(H-D) ) 8.1 (in HD2) to 8.6 Hz (in H2D) at 20 °C.
A related complex of the Cn structure [FeH(H2)P(CH2CH2CH2-
PMe2)3]+ shows avJ(H-D) ) 10 (in HD2) to 11 Hz (in H2D).16
Assuming a negligible H-D coupling in the classical isomer
1a,T, allows to estimate its relative contribution between 19-
22% and a K of 3.6-4.3 at 20 °C. In agreement with this, the
averaged chemical shift of FeH3 does not show any temperature
dependence between 20 and -60 °C.
Most probably the two isomers 1a,Cn and 1a,T have the same
enthalpy and ∆H° ) 0. The thermodynamic situation in CD2-
Cl2 is then governed only by the entropy difference which
obviously is ∆S° ) R ln K ) ca. 3 eu. This small entropy
change is indeed associated with the thermodynamics of a
classical dihydride to dihydrogen complex transformation. It
is explained by the loss of the spinning degree of freedom of
the dihydrogen ligand.32
Figure 10. 1H{31P} NMR spectra of the isotopomers of [Fe(H,D)3-
(PMe3)4]BPh4 recorded in a CDFCl2/CDF2Cl mixture at -130 °C. The
spectra A, B, and C differ in the deuterium content. They were obtained
by the isomerization of a fully deuterated solid sample of 1a (A after
1 h, B after 12 h, and C after 24 h).
of the HD2, H2D isotopomers (triplet at δ -11.730, avJ(H-D)
) 8.6(1) Hz) and the perprotio complex (singlet at δ -11.758).
Finally, after 24 h at 20 °C, the crystalline material turned out
2
to be mostly FeH3. The H NMR of this solid dissolved in
CH3CN demonstrated that methyl groups of the PMe3 ligands
were randomly deuterated: three 2:1:1 resonances of a newly
formed and stereochemically rigid [FeH(CH3CN)(P(Me-d)3)4]+
molecule could be resolved.
This remarkable intramolecular H/D exchange, which takes
place in the solid state, is unprecedented among dihydrogen
complexes. It appears that mechanistically it requires replace-
ment of the D2 (or HD) ligand by an agostic C-H bond,
activation of the C-H bond and ultimately Fe-D/C-H
scrambling. If this happens relatively fast and in a reversible
fashion, the D2 trapped in the lattice is expected to regenerate
the starting material. An agostic interaction of this type was
reported in the 16 electron complexes [RuH(P-P)2]PF6 (biden-
tate phosphine P-P ) dppb and diop) where the agostic
hydrogen was in exchange with the terminal hydride.12f
A
related example of unusual H/D exchange is known for the
Kubas complex W(H2)(CO)3(PiPr3)2, where W(D2)(CO)3(PiPr3)2
is formed under D2 also in solid state. However, the mechanism
of this process is not yet fully established.31
IV. Discussion of the Structural Results for the
Complexes 1-3
The [C a T] isomerization illustrated in Scheme 2 is the
major structural theme, one is persistently encountering, review-
ing the structure of the [MH3(PR3)4]+ cations 1-3. Both types
of isomers are formed in solutions of [FeH3(PMe3)4]+, [RuH3-
(PEt3)4]+, and [OsH3(PMe3)4]+. Two complexes are completely
on the right side of the [C a T] equilibrium: [Fe(H)3(PEt3)4]+
and [Os(H)3(PEt3)4]+. [RuH(H2)(PMe3)4]+ exclusively exist as
Cn isomers.
The different steric bulk of PMe3 and PEt3 is an apparent
structural characteristic that significantly influences the [C a
T] equilibrium (the electronic properties of these ligands are
similar). The C type is destabilized by more bulky PEt3 ligands,
which prefer a tetrahedral arrangement around the metal.
Another important circumstance is that the [C a T] transforma-
tion changes the formal oxidation state of M(II) to M(IV). This
represents a crucial factor for ruthenium and iron, which are
difficult to oxidize and strongly favor the oxidation state +II.
[OsH3(PMe3)4]+ shows little preference for 3a,Cc over 3a,T,
and both are classical hydrides. [RuH(H2)(PMe3)4]+ can only
exist as a dihydrogen complex 2a,Cn. Note, that the M-P bonds
are actually shorter in ruthenium complexes and make the
interligand repulsions stronger than in the isostructural osmium
molecules. Even the additional steric bulk introduced by PEt3
The isotopomers of 1a,Cn demonstrate peculiar isotope effects
at low temperature. The three 1H{31P} NMR spectra in Figure
10 differ in the relative D/H isotopomeric contents. The sample
of 1a,Cn with a maximum degree of deuteration (spectrum A)
almost exclusively consists of the HD2 isotopomer. Its 1H{31P}
NMR resonance is significantly shifted down-field with respect
to both the H2D and H3 signals. Because of this difference in
the chemical shifts, two resolved resonances of approximately
1:2 intensity can be observed for a less D-enriched sample of
1a,Cn (spectrum B) and thus can mistakenly be interpreted as
an H/H2 decoalescence. The H3 resonance of spectrum C in
Figure 10 is broad, apparently because of the strong H-H dipole
interactions. It sharpens upon deuteration, but the expected
H-D coupling of 10-11 Hz could still not be resolved.
Another peculiar isotope effect is observed in the low-
temperature 31P NMR spectra of the deuterated classical
complex 1a,T. The axial PMe3 ligand, which appears strongly
shifted low-field (Figure 9, δ 31.68 at -130 °C), shows
pronounced isotope shifts: δ 32.25 (DH2), 32.82 (D2H), and
33.38 (D3). The total change ∆δ(31P) between the Fe(H)3 and
Fe(D)3 isotopomers is unusually large for a secondary isotope
effect: 1.7 ppm. This can be explained by the unique structural
(31) Kubas, G. J. Acc. Chem. Res. 1988, 21, 129.
(32) Jessop, P. G.; Morris, R. H. Coord. Chem. ReV. 1992, 121, 155.

M(II) and M(IV) Complexes [MH3(PR3)4]+
J. Am. Chem. Soc., Vol. 119, No. 16, 1997 3725
in [RuH3(PEt3)4]+ did not impose a strong enough driving force
to shift the [C a T] equilibrium completely to the right.
The equilibrium behavior of both the PMe3 and PEt3 iron
molecules, i.e., a relatively high stability of 1a,T and 1b,T, is
unexpectedly more reminiscent of that shown by the osmium
rather than ruthenium systems. This is indeed counterintuitive
and violates the normal periodic trend. Some contraction of
the M-P bond distances on going from ruthenium to iron could,
in principle, cause some destabilization of 1a,Cn and 1b,Cn. A
comparison provided by the available X-ray data of cis-[FeH-
(H2)(PMe3)4]+ and cis-Ru(H)2(PMe3)4,19b however, does not
reveal a drastic difference: 2.260 (Fe-P) Vs 2.306 Å (Ru-P)
(trans P-M-H) and 2.230 (Fe-P) Vs 2.276 and 2.289 Å (Ru-
P) (trans P-M-P).
In searching for an explanation of this phenomenon, some
other chemical and spectroscopic properties of complexes 1a
and 2a are expected to be relevant here. The first is that the
T1min relaxation time is longer in [FeH(H2)(PMe3)4]+ (13.5 ms)
than in [RuH(H2)(PMe3)4]+ (10.1 ms). This indicates a longer
H-H bond (i.e., more activated dihydrogen ligand) in 1a,Cn.
Coordinated H2 appears to be more labile in [RuH(H2)(PMe3)4]+.
It loses H2 reversibly, and both dissolved H2 and RuH3
resonances are noticeably broad at 20 °C. The reactivity of
[RuH(PMe3)4]+ has been reported,33,34 and the five-coordinate
complexes of bidentate dppe, dppp,12a binap,12b dppb,12a,f and
diop12c,d,f ligands, [RuH(P-P)2]+, have been characterized.
The corresponding iron molecular fragment [FeH(PMe3)4]+
presumably cannot be isolated. It is this intermediate which
should be responsible for the C-H bond activation and the H/D
scrambling observed in the deuterated solid 1a. The deuterated
ruthenium complex 2a does not exchange methyl hydrogens
for the metal-bound deuterons, i.e., the ruthenium fragment
[RuH(PMe3)4]+ is less reactive toward C-H oxidative addition.
We believe that the higher tendency of 1a and 1b (in
comparison with the ruthenium representatives 2a and 2b) to
adopt a trihydride T structure is driven by electronic rather than
steric factors. Previous reports on trans-[MH(H2)(P-P)2]+ with
chelating phosphines already indicated that3c “...the dihydrogen
ligand is activated toward homolytic cleavage the most in the
osmium complexes and the least in the ruthenium complexes,
with the iron complexes being intermediate in nature”. The
present findings in the isomeric cis-[MH(H2)(PMe3)4]+ systems
corroborate this trend.
both steric and electronic properties of the ligands L. The
maximum number of different isomers four is represented in
the osmium system, while the ruthenium and iron molecules
show three isomeric types (Table 1). For any computational
work dealing with such metal complexes containing three or
four phosphorus ligands, it is therefore desirable to consider
all conceivable structural possibilities. Since the structural
preference of these complex systems is highly dependent on
the stereoelectronic properties of the phosphine ligands, one
should be prepared to find modified and even altered relative
energies of the isomers, when the bulkiness and donicity of the
PR3 ligands significantly differ from those of the widely used
theoretical model ligand PH3.
V. Mechanism of Isomerization and Ligand Exchange in
[MH3(PR3)4]+
a. Very Fast H/H2 Scrambling. Three dynamic processes
influence the NMR spectra of the [MH3(PR3)4]+ complexes.
One is the H/H2 scrambling in 1a,Cn, 2a,Cn, and 2b,Cn of the
type which has been thoroughly analyzed theoretically.4b
A
reasonable mechanism for this exchange has been named “open
direct transfer”. It requires elongation of the H-H bond with
concomitant shortening of the separation between the hydride
and the contiguous hydrogen of the H2. Both H‚‚‚H distances
become equal and relatively short in the transition state. Fast
H2 spinning in the ground-state structure completes the site
exchange.
The examples of this work and others from the literature show
that cis H and H2 ligands are as a rule engaged in very fast
exchange when decoalescence cannot be reached in low-
temperature 1H NMR experiments. The only notable exception
is represented by the complexes [MH(H2)L4]+ with tetradentate
phosphorus ligands L4 ) PP3 (Table 1), which have H/H2
decoalescence temperatures as high as 30 °C. This exceptional
behavior has never been discussed and is far from being
understood. This group, however, may have a structure of a
capped trigonal bipyramid, where the hydride resides on a
P-P-P face and is thus separated from the H2 ligand by a P-P
edge.
cis-Trihydrides exchange metal-bound hydrogens by a pair-
wise “replacement” mechanism, which was extensively analyzed
by computational methods.35 On the way to the transition
structure, two neighboring hydrides swing up and down, off
the ground-state M(H)3 plane. The H‚‚‚H vector shortens and
becomes perpendicular to the plane in the transition structure.
Exchange of this type has been studied experimentally in a
number of trihydrides, which all showed facile fluxionality. The
trihydride [Os(H)3(PMe3)4]+ (3a,Cc) did not reveal decoales-
cence in the hydride region even at -140 °C. In addition to
this, no deuterium isotope effect was noticeable for this process
in the deuterated complex of 3a. Apparently, in the presence
of small phosphine ligands the height of the barrier for the site
exchange in [Os(H)3(PMe3)4]+ can be relatively low.
b. “Tetrahedral Jump” Hydride Reorientation in the T
Structure. The T type complexes [Fe(H)3(PMe3)4]+, [Fe(H)3-
(PEt3)4]+, [Ru(H)3(PEt3)4]+, [Os(H)3(PMe3)4]+, and [Os(H)3-
(PEt3)4]+ are nonrigid, and the four phosphorus atoms appear
magnetically equivalent at 20 °C. When the temperature is
lowered, the characteristic AB3 spectra could be observed in
the 31P NMR. The rate of the fluxional process responsible for
averaging of the 31P chemical shifts shows pronounced depen-
dence on the size of the phosphines and metal. It increases in
In a different series of the type [Cp*M(P-P)H2]+, the only
known iron complex (P-P ) dppe) is a Fe(IV) dihydride in
solution.29a The ruthenium analogue has not been reported, but
the related [CpRu(dppe)H2]+,28b [Cp*Ru(dmdppe)H2]+,28a and
[Cp*Ru(dppm)H2]+28b,c complexes show equilibrium mixtures
of the dihydride and dihydrogen isomers. [Cp*Ru(dppp)H2]+
is a dihydride in solution at 20 °C.28b This comparison also
shows that the ability of iron toward oxidative additon of H2 is
(at least) not lower than that of ruthenium.
It has already been mentioned in the introduction (Table 1)
that the phosphite and phosphonite complexes [MH(H2)P4]+ (M
) Fe, P ) PPh(OEt)2; M ) Ru, P ) PPh(OEt)2, P(OEt)3,
P(OMe)3) display spectroscopic properties consistent with a P
structure.5,7 The metal centers of these molecules are certainly
less electron-rich than those of the phosphine complexes in this
work. For this reason formation of any phosphite trihydride is
expected to be electronically less favorable.
The results of this work demonstrate that a reliable theoretical
evaluation of the [MH3L4]+ complexes should take into account
(33) (a) Rappert, T.; Yamamoto, A. Chem. Lett. 1994, 2211. (b) Burn,
M. J.; Bergman, R. G. J. Organomet. Chem. 1994, 472, 43.
(34) Rappert, T.; Yamamoto, A. Organometallics 1994, 13, 4984.
(35) (a) Jarid, A.; Moreno, M.; Lledo´s, A.; Lluch, J. M.; Bertra´n, J. J.
Am. Chem. Soc. 1995, 117, 1069. (b) Clot, E.; Leforestier, C.; Eisenstein,
O.; Pe´lissier, M. J. Am. Chem. Soc. 1995, 117, 1797.

3726 J. Am. Chem. Soc., Vol. 119, No. 16, 1997
GuseV et al.
A number of factors would determine the energy of the
transition state. An obvious one is the crowding in the P-M-P
edge region. There is a clear correlation in the experimental
data: the larger the steric bulk of PR3, the higher the barrier.
An explanation for the metal dependence can be provided as
well: the shorter the M-P distances (Os-P > Ru-P > Fe-
P), the higher the congestion in the coordination sphere that
would hinder the hydride motion. Electronic contributions can
also be very important, although it is difficult to evaluate
qualitatively relative energies of the structures in Scheme 3 with
different PR3 and M centers.
The idea, that intramolecular hydride traversing of the
L-M-L edges is responsible for the fluxional NMR behavior
of many MHnLm complexes, originates from the early 1970s.1
Interesting for the present study is that the first complexes
analyzed in terms of the tetrahedral jump model were dihydrides
of iron and ruthenium, M(H)2(PR3)4, closely related to the
neutral precursors of the cationic complexes of this work.
An essential structural feature of the dihydrides is that they
are quite distorted octahedrons. Their [MP4] skeletons tend to
adopt pseudotetrahedral arrangement around the metal, although
some of the P-M-P angles remain relatively small (98-99°
in Fe(H)2(PhP(OEt)2)4), while the trans P-M-P angle is quite
large (136.7° in this example).36
Given that the same mechanism, tetrahedral jump, is operative
in both the dihydrides and the cationic trihydride molecules of
the T structure, these two systems demonstrate distinctly
different dynamic behavior. Contrary to the [M(H)3(PR3)4]+
system, the dihydrides are more fluxional with lighter M and
bulkier PR3. For example, Fe(H)2(PEt3)4 shows a broad
exchanging 31P A2B2 pattern at 20 °C, while well-resolved
resonances are observed at this temperature for Fe(H)2(PMe3)4
and Ru(H)2(PEt3)4.
Figure 11. A schematic representation of the “tetrahedral jump”
hydride reorientation in the T type structure and Eyring plot of the rate
constants of this process in [Fe(H)3(PEt3)4]+, [Ru(H)3(PEt3)4]+, and [Os-
(H)3(PEt3)4]+ derived from the variable-temperature 31P NMR spectra
using the DNMR5 program. Determined activation enthalpies and
entropies are given in the plot.
Scheme 3
It has been suggested1c that in M(H)2(PR3)4 (a) “...the
contribution of the phosphorus skeletal rearrangement to the
barrier should decrease with bulkier ligands...” and (b) “...the
increase in barrier on going from iron to ruthenium as central
atom is consistent with decreased steric push toward a regular
tetrahedron due to increased metal covalent radius”.
It might be as well that the nature of the trend is mostly
electronic instead. By analogy with the transformations shown
in Scheme 3, the transition geometry of M(H)2(PR3)4 can be a
distorted octahedron with trans-hydrides. The relative stability
of the cis- and trans-dihydride isomers with different metals
would then determine the fluxional properties. Theoretical
calculations might evaluate the periodic trends and provide better
understanding for the relative contribution of the steric and
electronic factors.37
c. Isomerization between Cn and T Isomers. The third
and rather slow dynamic process observed in this work is the
[C a T] isomerization of [FeH3(PMe3)4]+, [RuH3(PEt3)4]+, and
[OsH3(PMe3)4]+. In none of these cases was it feasible to isolate
and provide detailed (X-ray) structural characterization for both
types of isomers. The two solid-state structures of [FeH(H2)-
(PMe3)4]+ and [Fe(H)3(PEt3)4]+ can be recalled here to envisage
an isomerization mechanism that takes place on the NMR time
scale in 1a.
the order: Fe < Ru < Os (Figure 11) and PPh3 < PEt3 < PMe3.
[Os(H)3(PMe3)4]+ shows the fastest rate, and the slow exchange
regime could not be attained even at -140 °C. A barrier for
the rearrangement in this molecule must be lower than 6.4 kcal/
mol determined for [Os(H)3(PEt3)4]+ by analysis of the variable
temperature 31P NMR data with the DNMR5 program (Figure
11). For [Fe(H)3(PMe3)4]+ a rough estimate for the barrier is
∆Gq ) 7.8(1) kcal/mol between -100 and -120 °C.
A mechanism assuming hydride migration as a key step
(principal motion) provides a reasonable explanation for the very
fast fluxionality and the experimental observations. Given in
Figure 11, it involves the passage of one hydride from its face
to a vacant one Via the tetrahedral edge of the MP4 skeleton
(tetrahedral jump). In the 31P NMR the chemical shifts are
altered in a pairwise fashion. Figure 11 shows how the
environments of two phosphorus atoms (shaded before and after
the jump) are interchanged in a single event of exchange. The
rate constants determined in Figure 11 from the 31P NMR spectra
thus describe the rate of the hydride face-to-face migration. More
strictly, they represent the lifetime of the empty site of the
tetrahedron.
Structure 1a,Cn (Figure 12) shows a specific view of this
molecule with the three phosphines numbered P2, P3, and P4
confined to the plane of the figure. An analogous representation
can be achieved (not shown) viewing from the P4 side and
confining P1, P2, and P3 to one plane. The second experimental
geometry of 1b,T is meant to be a structural model for 1a,T.
Certain secondary topological modifications of the skeleton
are required to lower the barrier for the hydride migration.
Indicated in Scheme 3: both P1-M-P2 and P3-M-P4 angles
should open up - one to permit the passage of the hydride, the
other to accomodate the hydrogens H2 and H3, which are moving
toward each other and away from P1 and P2. The transition
geometry presumably has a shape of a distorted pentagonal
bipyramid.
(36) Guggenberger, L. J. Inorg. Chem. 1973, 12, 1317.
(37) Berke, H.; Jacobsen H. Chem. Eur. J. 1997, in press.

M(II) and M(IV) Complexes [MH3(PR3)4]+
J. Am. Chem. Soc., Vol. 119, No. 16, 1997 3727
An approximate pKa (pseudoaqueous value) can be obtained
by NMR determination of the equilibrium constant Keq for the
reaction between a suitable acid (BH+) of known pKa and the
conjugate hydride precursor (MHnLm) of the protonated complex
[MHn+1Lm]+:3c,38
MHnLm + BH+ a [MHn+1Lm]+ + B
pKa([MHn+1Lm]+) ) pKa(BH+) + log(Keq)
Thermodynamic acidities have been reported for complexes
of the trans-[MH(H2)(P-P)2]+ (P-P ) bidentate phosphine)3
and [Ru(C5R5)(H2)(PR3)]+ families.28 The pKa values range
from 4 to 16 for these molecules with increasing basicity of
the phosphine resulting in decreasing acidity of the [MHn+1Lm]+.
All pseudoaqueous pKa([MHn+1Lm]+) values available in the
literature have been referenced to the phosphonium salts, BH+
) HPR3+, of different acidity.3c,38 This approach is certainly
based on two assumptions (which, however, have never been
addressed experimentally) that (a) the difference pKa(BH+) -
pKa([MHn+1Lm]+) should be the same in THF and water and
(b) the difference pKa(HPR′3+) - pKa(HPR′′3+) in THF is
unchanged when compared to that in water.
The complexes [MH3(PR3)4]+ are not amenable to any acidity
determination by NMR in H2O because of the low water
solubility of the neutral dihydrides. All M(H)2(PMe3)4 are well-
soluble in methanol, which among the organic solvents has acid/
base properties closest to water. The acidity of a number of
hydride complexes (HB) has already been determined by
measuring the rates k1 and k-1 of the reaction:
Figure 12. Proposed mechanism of the isomerization of [FeH3-
(PMe3)4]+ in solution, reconstructed using the solid-state structures of
1a,Cn and 1b,T. Characteristic distances and angles are given in the
figure.
Many features of the skeleton of these structures 1a,Cn and 1b,T
are quite similar and therefore excessive heavy ligand motion
is not required during the isomerization process.
HB + CH3O- a B- + CH3OH
A transition structure suggested in the center of Figure 12
emphasizes two essential coordinates for the [C a T]
rearrangement: bending of the Fe-P1 bond and hydrogen jump
over the P1-P2 edge. The P1-Fe-P2 (102.1°) angle in 1a,Cn
compares well to the P-Fe-P angles in the T structure and
therefore the rise in energy associated with the edge crossing
might be comparable in both molecules. On going from Cn to
T the hydrogen motion is accompanied by the Fe-P1 bending.
An alternative for this is a jump over the P2-P4 edge, i.e.,
motion that would require bending of the Fe-P4 vector. What
is difficult to show in the transition state structure of Figure 12
is that the P2 and P3 ligands should slightly drift toward the
departing phosphine (P1 in Figure 12) to complete the isomer-
ization.
A barrier for the 1a,T to 1a,Cn isomerization can be estimated
from the exchange-broadened 1H{31P} NMR spectra simulated
with the DNMR5 program. An activation energy ∆Gq of 8.8(2)
kcal/mol is found in the temperature range from -70 to -90
°C, where the two resonances coalesce in the hydride region.
The isomerization of 3a,T to 3a,Cn has a comparable barrier of
9.1(2) kcal/mol at -80 °C (the decoalescence in this system is
observed between -60 and -70 °C). Both are slightly higher
than the barriers for the tetrahedral jumps of the 1a,T and 3a,T
structures estimated as 7.8(1) kcal/mol (-100 to -120 °C, Fe)
and <6.4 kcal/mol (Os).
The pKa values have been calculated from the difference pKs
- log(k1/k-1), where Ks is the ion product of methanol, pKs )
16.7.39
The strategy employed in these experiments involved (a)
determination of the equilibrium constant Keq for the protonation
of M(H)2(PMe3)4 by methanol (in methanol), (b) calculation of
pKa([MH3(PMe3)4]+) ) pKa(CH3OH) + log(Keq), and (c)
determination of the acid/base equilibria between [OsH3-
(PMe3)4]+ and M(H)2(PR3)4 in THF-d8. The results of these
experiments are collected in Table 3.
The complexes M(H)2(PMe3)4 are reversibly protonated by
methanol (pKa(CH3OH) ) 15.5), and the equilibrium favors the
dihydrides. The equilibrium constants given in Table 3 (nos.
3, 8, and 9) estimate very similar pKa of [RuH(H2)(PMe3)4]+
and [Os(H)3(PMe3)4]+ as 11.3 and 11.5 (the error is at least (
0.1). The second value is an apparent acidity, pKa(3a), of the
complex which actually exists in two isomeric forms in solution.
The apparent pKa must be greater that the actual pKa of the
isomers.38 For the present case, pKa(3a,Cc) ) pKa(3a) - log-
(1+1/K1) and pKa(3a,T) ) pKa(3a,Cc) - log(K1), K1 ) [3a,Cc]/
[3a,T].
With the equilibrium constant K1 ) ca. 7 (see discussion in
section IIIb), the values pKa(3a,Cc) ) ca. 11.4 and pKa(3a,T)
) ca. 10.6 result. The pKa of the more abundant isomer is
always close to the apparent pKa; the difference is maximal (0.3
pK units) when K1 ) 1.38 The value for the less abundant
isomer can be determined less reliably.
VI. Acidity of the [MH3(PR3)4]+ Complexes (1-3)
Weak Bronsted acidity is an intrinsic chemical property of
the cationic hydrides [MH3(PR3)4]+. In this section, we attempt
the determination of the pKa values for complexes 1-3 by NMR
spectroscopy.
A cross-experiment of protonation of Ru(H)2(PMe3)4 by [Os-
(H)3(PMe3)4]+ in THF-d8 (Table 3, no. 2) established an
(39) (a) Walker, H. W.; Kresge, C. T.; Ford, P. C.; Person, R. G. J. Am.
Chem. Soc. 1979, 101, 7428. (b) Walker, H. W.; Pearson, R. G.; Ford, P.
C.; J. Am. Chem. Soc. 1983, 105, 1179. (c) Pearson, R. G. Chem. ReV.
1985, 85, 41.
(38) (a) Kristjansdottir, S. S.; Norton, J. R. In Transition Metal Hydrides:
Recent AdVances in Theory and Experiment; Dedieu, A., Ed.; VCH, New
York, 1991; Chapter 10.

3728 J. Am. Chem. Soc., Vol. 119, No. 16, 1997
GuseV et al.
Table 3. Acid/Base Equilibria for the Complexes [MH3(PR3)4]+/MH2(PR3)4 at 20 °C
B1 + B2H+ a B1H+ + B2
N
[B1], mol/L
[B2H]+, mol/L
solvent
Keq, (1H)/(31P)a
∆pKab, (1H)/(31P)a
pKac, [MH3(PMe3)4]+
1
2
3
4
5
6
7
8
FeH2(PMe3)4, 0.064
RuH2(PMe3)4,, 028
RuH2(PMe3)4, 0.049
RuH2(PEt3)4, 0.017
OsH2(PMe3)4, 0.037
RuH2(PEt3)4, 0.026
RuH2(PEt3)4, 0.027
OsH2(PMe3)4, 0.043
OsH2(PMe3)4, 0.054
OsH2(PEt3)4, 0.033
OsH2(PEt3)4, 0.018
OsH2(PEt3)4, 0.033
PCy3, 0.071
[OsH3(PMe3)4]+, 0.028
[OsH3(PMe3)4]+, 0.024
CH3OH, 24.686
THF-d8
THF-d8
CH3OH
THF-d8
THF-d8
THF-d8
THF-d8
CH3OH
CH3OH
THF-d8
C2H5OH
C2H5OH
THF-d8
THF-d8
0.117/0.120
0.478/-d
-0.93/-0.92
-0.32/-d
-/-4.2
10.6, [FeH3(PMe3)4]+
11.2, [RuH3(PMe3)4]+
11.3, [RuH3(PMe3)4]+
11.0, [RuH3(PEt3)4]+
11.0, [RuH3(PEt3)4]+
>12.5,?f [RuH3(PEt3)4]+
>14.2,?g [RuH3(PEt3)4]+
11.6, [OsH3(PMe3)4]+
11.4, [OsH3(PMe3)4]+
13.2, [OsH3(PEt3)4]+
11.8, [OsH3(PEt3)4]+
11.8, [OsH3(PEt3)4]+
11.440
-/0.000060
-e/0.0058
3.14/3.25
-/>600
[OsH3(PEt3)4]+, 0.025
[RuH3(PEt3)4]+, 0.033
HPCy3+, 0.031
-e/-2.2
0.50/0.51
-/>2.8
HPtBu3+, 0.031
CH3OH, 24.686
-/>700
-/>2.8
-/0.000124
-/0.000085
56.2/50.2
-/0.000073
-/0.000082
-/0.14
-/-3.91
-/-4.07
1.75/1.70
-/-4.14
9
CH3OH, 24.686
10
11
12
13
14
[OsH3(PMe3)4]+, 0.037
C2H5OH, 17.387
C2H5OH, 17.387
HPtBu3+, 0.034
-/-4.09
-/-0.85h
-/-0.82i
PnBu3, 0.071
HPCy3+, 0.034
-/0.15
9.741
a 1H and 31P NMR data, respectively. b Determined as log(Keq) ) pKa(B1H+) - pKa(B2H+). c Referenced to the pKa of [Os(H)3(PMe3)4]+ determined
in methanol as 11.5, see Discussion. d The 31P NMR resonance of [RuH3(PMe3)4]+ is broad at 20 °C, this makes accurate integration difficult.e Not
determined, because of the overlapping RuH2 and OsH3 chemical shifts. f Referenced to pKa(HPCy3+) ) 9.7. g Referenced to pKa(HPtBu3+) ) 11.4.
h The difference ∆pKa (pseudoaqueous scale) was determined by titration in CH3NO2 as 1.75.40 i The difference ∆pKa (pseudoaqueous scale) was
determined by titration in CH3NO2 as 1.27.41
equilibrium with log(Keq) ) ∆pKa ) -0.3, in agreement with
the result of the two independent determinations in methanol.
The differences in solvation energies of the bulky M(H)2(PR3)4
and [MH3(PR3)4]+ cations appear similar in methanol and THF,
and thus, the relatiVe acidity of complexes 1-3 can be measured
in THF on the pseudowater scale.
The pKa of [Os(H)3(PMe3)4]+ (11.5) was chosen as an internal
reference in this work mostly because of its high stability. This
trihydride does not dissociate PMe3 during the time (0.5 to 3
h), which is necessary for the establishment of the acid/base
equilibria. Within this time, the other examples, complexes 1
and 2, show PMe3/PEt3 scrambling when mixed, which prevents
any reliable measurements.
(P-P)2]+ (P-P ) dmpe, depe). Qualitatively this is established
by the different behavior in methanol: Ru(H)2(PMe3)4 and Os-
(H)2(PMe3)4 are protonated to give less than 25% of the cations
in solution. On the contrary, all M(H)2(P-P)2 are completely
protonated by CH3OH and establish equlibria in C2H5OH which
strongly favor the protonation product.3,11 The equlibrium
concentrations have been reported for Fe(H)2(dmpe)2/[FeH(H2)-
(dmpe)2]+ (1:5 at -6 °C, total concentration of 67 mM)3a and
Ru(H)2(dmpe)2/[RuH(H2)(dmpe)2]+ (3:5 at 12 °C, total con-
centration of 41 mM).3d These lead to estimated pKa values of
14.1 (Fe) and 13.3 (Ru) in ethanol at the respective temperatures,
determined as 15.9 + log(Keq) with the equilibrium constants
Keq ) [dihydrogen complex][EtO-]/([dihydride][EtOH]).
The pKa values of [MH3(PR3)4]+ show the following order:
3b (13.2) > 3a (11.5) > 2a (11.2) > 2b (11.0) > 1a (10.6).
The pKa determination was impossible with 1b, because of a
fast reaction between [Fe(H)3(PEt3)4]+ and its conjugated base
Fe(H)2(PEt3)4 that produced stable Fe(H)2(H2)(PEt3)3. The
apparent value of pKa(2b) should be taken as acidity of the
classical trihydride 2b,T. This is very close to the pKa of the
dihydrogen complex [RuH(H2)(PMe3)4]+ (2a,Cn). The less
abundant dihydrogen isomer 2b,Cn must be more acidic. This
is opposite to 1a where the dihydrogen complex 1a,Cn (pKa )
ca. 10.5) is more basic, since the trihydride isomer 1a,T is
thermodynamically less stable.
Only the most basic of the dihydrides, Os(H)2(PEt3)4, was
detectably protonated by ethanol, which is less acidic than
methanol ((pKa(C2H5OH) ) 15.9). Two determinations (Table
3, nos. 11 and 12) both have found a higher acidity of [Os-
(H)3(PEt3)4]+ in C2H5OH than in CH3OH, pKa ) 11.8 Vs 13.2,
respectively. These and the others pKa values in Table 3 are
only pseudoaqueous values and to a certain extent are influenced
by the solute/solvent interactions in the solvent of determination.
All PEt3 derivatives of M(H)2L4 are notably less soluble than
those of PMe3 in alcohols, especially in methanol. The
thermodynamic acidity might be increased if the protonated [Os-
(H)3(PEt3)4]+ is then even less soluble, i.e., thermodynamically
destabilized by the interaction with the solvent. In this respect,
the pKa values permit a reliable comparison of relatiVe acidity,
if measured in one and the same solvent (e.g., THF in this series)
in which compounds to be compared are all well-soluble.
Except for [Os(H)3(PEt3)4]+, all complexes of this work are
markedly more acidic than the corresponding trans-[MH(H2)-
In addition to this, the temperature dependence of Keq was
determined, which led to ∆H ) -8.8 kcal/mol and ∆S ) -45.9
eu for Fe(H)2(dmpe)2/[FeH(H2)(dmpe)2]+ and to ∆H ) -6.0
kcal/mol and ∆S ) -19.9 eu for Ru(H)2(dmpe)2/[RuH(H2)-
(dmpe)2]+.3a,d In both cases the equilibrium constants at 20 °C
should be smaller than those reported at -6 and 12 °C. There
is some ambiguity in the original publications concerning the
definition of this equilibrium constant. It appeared as if it was
measured as the ratio M(H)2(dmpe)2/[MH(H2)(dmpe)2]+ in both
cases. This predicts the Ru(H)2(dmpe)2/[RuH(H2)(dmpe)2]+
ratio of 3:4 at 20 °C and pKa ) 13.2. We cannot however
securely interpret the thermodynamic parameters for Fe(H)2-
(dmpe)2/[FeH(H2)(dmpe)2]+, since there is no apparent way to
reproduce the ratio (1:5) reported at -6 °C. In a recent review,38
the pKa of [FeH(H2)(dmpe)2]+ was estimated from the men-
tioned equilibrium data as ∼12. It results from the sum 15.9
+ log(Keq) with the Keq value formally calculated from the ∆H
and ∆S at 293 K.
The observation of higher acidity of [MH3(PR3)4]+ Vs [MH-
(H2)(P-P)2]+ (R ) Me, Et, and P-P ) dmpe, depe) is
presumably determined more by the different thermodynamic
stability of the conjugate bases M(H)2(PR3)4 and M(H)2(P-P)2
rather than by any electronic difference between the ligands.38
All M(H)2(PR3)4 of this work are exclusively cis-dihydrides in
solution, while the dihydrides M(H)2(P-P)2 form equilibrium
mixtures of the cis- and trans-isomers when dissolved,3a,d,11
which indicates that the lower energy cis-structure can be
destabilized by the chelating phosphorus ligands.
The pKa difference between [MH3(PR3)4]+ and the standards
of previous determinations, HPCy3+and HPtBu3+, cannot be

M(II) and M(IV) Complexes [MH3(PR3)4]+
J. Am. Chem. Soc., Vol. 119, No. 16, 1997 3729
standard 5-mm NMR tube filled with the deuterated solvent. 1H T1
measurements were performed at 300 MHz using the inversion recovery
method. A long repetition time of 20 s was employed for the
established experimentally. Even one of the least basic
complexes Ru(H)2(PEt3)4 is completely protonated by both of
these acids in THF-d8 (Table 3, nos. 6 and 7). The equilibrium
constants in this case must be more than 600-700 based on
the experimental [PR3]/[HPR3+] ratios (ca. 6-7, see concentra-
tions in Table 3) and the assumption that the ratio [RuH3-
(PEt3)4+]/[RuH2(PEt3)4] must be more than 100, if for the second
1
equilibrium constant determinations by H and 31P NMR (Table 3).
This was typically more than 3T1 relaxation times of the integrated
lines. In the experiments in CH3OH and for the protonation of PR3 by
HPR′3,+ the acids and conjugated bases were in fast exchange on the
relaxation time scale at 20 °C according to the T1 time estimates.
Therefore, the ratios [B]/[BH+] and the reported Keq (Table 3) are not
affected by any relaxation effects.
Rate constants for the intramolecular exchange (hydride jump) in
all T isomers were determined by simulation of the exchange-broadened
31P{1H} NMR spectra using the DNMR5 program.43 The model
assumed a mutual exchange (MU ) 1) between four nuclear configura-
tions of four nuclei (1234, 2134, 3214, and 4231). The isomerization
[C a T] was modeled as a two-site exchange. Experimental T1 times
were used for the simulations of all 1H{31P} NMR spectra, while
reasonable line widths (2-3 Hz) were employed for the 31P spectra.
During the simulation the rate constants were iterated to reach a close
agreement between the calculated and experimental line widths and
intensities.
1
species the H and 31P NMR detection limit is reached. The
+
pKa of [RuH3(PEt3)4]+ then must be greater than pKa(HPtBu3
)
+ log(Keq) ) 11.440 + 2.8, i.e., >14.2. Consequently, the pKa
of our standard complex 3a should be greater than 14.7 on a
+
scale referenced to HPtBu3 (Vs 11.5 determined here in
methanol). This we are reluctant to accept, since at this acidity
the protonation of all our complexes 1-3 should have been
practically complete in both CH3OH and C2H5OH. We also
found no detectable formation of HPtBu3+ in a 0.12 M solution
of PtBu3 in CH3OH, indicating that the pKa of HPtBu3+ is less
than 11.4 in this alcohol.
Table 3 also shows results of two additional experiments:
+
protonation of PCy3 by HPtBu3 and protonation of PnBu3 by
The following reagents were from commercial suppliers: [NBu4]BH4,
NaBH4, NaBPh4, (CF3)2CHOH (Aldrich), anhydrous FeCl2 (Fluka),
RuCl3‚nH2O (assay 42.12%, Johnson Matthey Co.), OsO4 (Johnson
Matthey Co.). K2[OsO2(OMe)4] was prepared by the method of
Criegee.44
HPCy3+ (nos. 13 and 14) in THF-d8. The measured difference
+
+
∆pKa is 0.85(5) between HPtBu3 and HPCy3 and 0.82(5)
between HPCy3+ and HPnBu3 . These differences (pseudoaque-
+
ous scale) are known as 1.7540 and 1.27,41 respectively, from
the potentiometric titration experiments in CH3NO2. Thus, it
appears, that even the phosphines of a close basicity range show
a significantly contracted pKa scale in THF (total 1.67 pKa units
in THF Vs 3.02 in the literature data). Any determination
Preparation of the Dihydrides M(H)2(PR3)4. Dihydridotetrakis-
(trimethylphosphino)iron(II), Fe(H)2(PMe3)4. PMe3 (3.9 mL, 37.7
mmol) was added to a suspension of FeCl2 (0.6 g, 4.7 mmol) in 30 mL
of ethanol. The mixture was stirred for 1 h. To the resulting clear,
almost colorless solution was added NaBH4 (0.36 g, 9.5 mmol), and
the dark lilac reaction mixture was left stirring for 2.5 h. The solvent
was then removed completely in Vacuo, and the residue was extracted
with hexane. The solvent was removed again, and the resulting solid
was sublimed (80 °C, 4 × 10-2 Torr) to afford a light yellow product:
yield 0.53 g (31%). The yield of Fe(H)2(PMe3)4 depended on the
relative amount of NaBH4 and PMe3. In a reaction of FeCl2 (0.61 g,
4.8 mmol) with 3 mL PMe3 (29.9 mmol) and 0.55 g NaBH4 (14.5
mmol), the sublimation afforded 1.14 g (65%) of Fe(H)2(PMe3)4. Fe-
(H)2(PMe3)4 is extremely air-sensitive and is thermally unstable at 20
°C. When kept under N2 in a drybox in a tightly closed bulb, it slowly
(days) changes the color from light yellow to green. 1H NMR (THF-
d8): δ 1.27 (s, PCH3), -14.46 (m, FeH2). 31P{1H} NMR (THF-d8):
an A2B2 pattern centered at δ 24.68, δA 24.32, δB 25.03 (2J(A-B)) )
29.7 Hz).
+
referenced with HPR3 spanning a larger range can therefore
be endangered by significant errors both in the absolute (on
the pseudoaqueous scale) and relative acidity of [MHn+1Lm]+.
More work is necessary to verify if ∆pKa([MHn+1Lm]+
-
HPR3+) is comparable in different solvents like THF and H2O.
This would require preparation of a series of complexes with
at least one representative of known pKa in H2O (or CH3OH)
and one which is reversibly protonated by HPtBu3+, with the
mutual acid/base properties also established in THF.
VII. Experimental Section
When not mentioned otherwise, all operations were performed under
an atmosphere of N2 using standard Schlenk and glovebox techniques.
All solvents were rigorously dried over an appropriate drying agent,
followed by distillation under N2. Deuterated solvents used in the NMR
experiments were dried over sodium (C6D6, THF-d8) or P2O5 (CD2-
Cl2) and vacuum transferred for storage in Schlenk flasks fitted with
Teflon stopcocks. The CDFCl2/CDF2Cl mixture was prepared by a
reported procedure.42 Anhydrous CH3OH (Aldrich) was vacuum-
distilled before use in the acidity measurements.
For the NMR experiments typically a solid [MH3(PR3)4]+ sample
was weighed in a 5-mm NMR tube in the glovebox. The charged tube
was tightly fitted into a small apparatus closed with Teflon stopcock
that preserved the inner space from passage of air during subsequent
manipulations. This apparatus was removed from the box, attached to
a vacuum line, and evacuated, and then the solvent was vacuum-
transferred into the tube. The tube could be flame-sealed under 850
Torr of Ar, H2, or under vacuum. This provided solutions of [MH3-
(PR3)4]+ prepared between -80 and -100 °C. The NMR experiments
were started below -90 °C. This was important for complexes 1 and
2 in CDFCl2/CDF2Cl which slowly decomposed in this solvent at room
temperature.
The residue which remained in the first preparation after extraction
with hexane as described above was additionally extracted with CH2-
Cl2, the solvent was removed in Vacuo, and the resulting solid was
redissolved in 10 mL of methanol. Addition of 0.82 g of NaBPh4 (2.4
mmol) in 5 mL of methanol to this solution precipitated violet [(PMe3)3-
Fe(µ-H)3Fe(PMe3)3]BPh4 (reported in detail elsewhere): yield 0.67 g,
32%. 1H NMR (CD2Cl2): δ 1.44 (m, PCH3), -22.40 (sept, Fe(µ-H)3-
2
Fe, J(H-P) ) 8.6 Hz). 31P{1H} NMR (CD2Cl2): δ 35.1 (s). Anal.
Calcd. for C42H77BFe2P6: C, 56.65; H, 8.72. Found: C, 56.53; H,
8.81.
Dihydridotetrakis(triethylphosphino)iron(II), Fe(H)2(PEt3)4. A
mixture of FeCl2 (0.5 g, 3.94 mmol) and PEt3 (6 mL, 40.63 mmol)
was stirred for 0.2 h in 40 mL of ethanol to form a clean colorless
solution. NaBH4 (0.45 g, 11.90 mmol) was added, and after 24 h of
stirring, a white precipitate was filtered from the orange reaction
solution. The solvent was then removed in Vacuo. The residue was
extracted with 3 × 20 mL of hexane affording a solution of Fe(H)2-
(N2)(PEt3)4 and a yellow solid of BH2[(PEt2)3Fe(H)2]2OEt (0.43g
(25%)). This complex could be dissolved in 10 mL of methanol and
+
reprecipitated as a BPh4 salt by the addition of NaBPh4 in methanol
All NMR experiments were carried out on a Varian Gemini 300
spectrometer. 1H NMR spectra were referenced to the residual proton
resonance of the deuterated solvent. 31P chemical shifts were externally
referenced to 85% H3PO4 sealed in a capillary and inserted into a
(reported in detail elsewhere).
Fe(H)2(N2)(PEt3)3. The above hexane extract was evaporated, and
the oily residue was dried under vacuum for 3 h. Addition of 15 mL
of pentane caused precipitation of PEt3‚BH3, and the mixture was left
(40) Allman, T.; Goel, R. G. Can. J. Chem. 1982, 60, 716.
(41) Streuli, C. A. Anal. Chem. 1960, 32, 985.
(42) Siegel, J. S.; Anet, F. A. L. J. Org. Chem. 1988, 53, 2629.
(43) Available from the QCPE at Indiana University (QCMP 059)
(44) Criegee, R. Ann. 1942, 550, 99.

3730 J. Am. Chem. Soc., Vol. 119, No. 16, 1997
GuseV et al.
overnight at -30 °C. The cold mother liquor was quickly removed
from the precipitate via a cannula and dried under vacuum to give about
1.2 g of oily Fe(H)2(N2)(PEt3)3: yield, ca. 70%. This oil showed at
least 95% of Fe(H)2(N2)(PEt3)3 in the 1H and 31P NMR. It also
contained about 4% of Fe(H)2(H2)(PEt3)4 and less than 1% of remaining
PEt3‚BH3.
[Os(H)3(PMe3)4]BPh4. Pure Os(H)2(PMe3)4 could only be recovered
from this salt as described below for Os(H)2(PEt3)4.
From the residue of the extraction, a small amount of [OsH-
(PMe3)5]OEt was extracted with CH2Cl2, redissolved in methanol and
precipitated as a BPh4- salt by addition of NaBPh4 in methanol. Data
for [Os(CH3)(PMe3)5]+. 1H NMR (CD2Cl2): δ -0.28 (doublet of
3
quintets, OsCH3, J(H-P) ) 3.5, 10.9 Hz). 31P{1H} NMR (CD2Cl2):
The preparation of Fe(H)2(PEt3)4 was continued by dissolving 0.8 g
of Fe(H)2(N2)(PEt3)3 in 5 mL of PEt3 in a Schlenk flask fitted with
Teflon closure. The flask was heated to 45 °C under vacuum, stirred
for 2 days, and repeatedly degassed from evolving N2. The formed
Fe(H)2(PEt3)4 was kept in the PEt3 solution under Ar to prevent thermal
decomposition. A typical isolation of solid Fe(H)2(PEt3)4 included the
following: PEt3 was removed from ca. 0.7 mL of the Fe(H)2(PEt3)4
solution in PEt3, and the yellow solid was dried under vacuum, cooled
to -70 °C, and washed under Ar with 3 × 2 mL of cold (-70 °C)
methanol, which could be conveniently removed from the solid with a
pipet. The afforded light yellow Fe(H)2(PEt3)4 was dried under
vacuum: yield, 80-90 mg, 60-67%. This solid complex can be
handled under N2 for a short time (1 h) at room temperature without
apparent decomposition. Data for BH2[(PEt3)3Fe(H)2]2BPh4. 1H NMR
(CD2Cl2): δ 1.68 (m, PCH2), 1.15 (m, PCH2CH3), -14.24 (br. m,
FeH3). 31P{1H} NMR (CD2Cl2): δ 49.1 (s). 11B{1H} NMR (CD2-
Cl2): δ 0.0 (s, BPh4-), 66.2 (br. s, ∆ ) 370 Hz, Fe-B-Fe). Anal.
Calcd. for C60H116B2Fe2P6: C, 62.30; H, 10.11. Found: C, 62.45; H,
9.92. Data for Fe(H)2(N2)(PEt3)3. 1H NMR (C6D6): δ 1.56 (br. m,
2
δ -54.9 (d, J(P-P) ) 14.2, becomes a doublet of quartets in the
selectively PMe3-decoupled 31P spectrum), -64.3 (qi). Data for [OsH-
(PMe3)5]+. 1H NMR (CD2Cl2): δ -12.27 (doublet of quintets, OsH,
2J(H-P) ) 54.6, 21.9 Hz). 31P{1H} NMR (CD2Cl2): δ -55.7 (d, 2J(P-
P) ) 18.0, becomes a doublet of doublets in the selectively PMe3-
decoupled 31P spectrum), -60.7 (qi). Data for Os(H)2(PMe3)4. 1H
NMR (THF-d8): δ -11.26 (m, OsH2). 31P{1H} NMR (THF-d8): δ
2
1
-52.8 (t, J(P-P) ) 18.0 Hz), -47.2 (t). Data for OsH4(PMe3)3. H
NMR (THF-d8): δ -10.10 (q, OsH4, J(H-P) ) 10.5 Hz). 31P{1H}
2
NMR (THF-d8): δ -48.8 (s).
Dihydridotetrakis(triethylphosphino)osmium(II), Os(H)2(PEt3)4.
A mixture of [Os(H)3(PEt3)4]BPh4 (0.147 g, 0.15 mmol) and KOH (0.05
g, 0.89 mmol) in 5 mL of THF was stirred for 2 h. This resulted in
clean formation of Os(H)2(PEt3)4 (31P NMR observation). The solution
was filtered and evaporated. The residue was dried for 2 h under
vacuum at 75 °C to give white crystalline Os(H)2(PEt3)4‚1/2THF: 0.094
g, 95%. 1H NMR (THF-d8): δ -13.08 (m, OsH2). 31P{1H} NMR
(THF-d8): δ -18.8 (t, 2J(P-P) ) 13.4 Hz), -11.7 (t). Anal. Calcd.
for C24H62P4Os‚0.5THF: C, 44.55; H, 9.49. Found: C, 44.60; H, 9.53.
2
PCH2), 1.08 (m, PCH2CH3), -17.21 (ddt, FeH, J(H-H) ) 17.6 Hz,
2
2J(H-P) ) 50.1, 61.9 Hz), -12.89 (ddt, FeH, J(H-P) ) 28.2, 76.6
Preparation of the Cationic Complexes [MH3(PR3)4]BPh4, 1-3.
[FeH3(PMe3)4]BPh4 (1a), [RuH(H2)(PMe3)4]BPh4 (2a), [RuH3(PEt3)4]-
BPh4 (2b), [Os(H)3(PMe3)4]BPh4 (3a). A typical preparation of these
complexes consisted of the following: 0.05 g of the corresponding
dihydride was dissolved in 3 mL of CH3OH/(CF3)2CHOH (2:1) and
mixed with a solution of 0.05 g of NaBPh4 in CH3OH to afford a
precipitate of [MH3(PMe3)4]BPh4 which was filtered and washed with
3 × 2 mL of methanol and 3 × 2 mL of hexane or ether: typical
yield, ca. 80%. In a similar manner, the preparation of the deuterium-
substituted complexes 1a-3a was achieved in CH3OD without (CF3)2-
CHOH in a lower yield of about 50%. In the preparation of 1a, it is
recommended to carry out all manipulations under argon. Otherwise
it will be contaminated with [FeH(N2)(PMe3)4]BPh4. Anal. Calcd. for
C36H59BFeP4 (1a): C, 63.36; H, 8.71. Found: C, 63.67; H, 8.53. Anal.
Calcd. for C48H83BP4Ru (2b): C, 64.34; H, 9.34. Found: C, 64.10;
H, 9.29.
Hz). 31P{1H} NMR (C6D6): δ 56.5 (d, 2J(P-P) ) 18.1 Hz), 49.7 (t).
Data for Fe(H)2(H2)(PEt3)3. 1H NMR (C6D6) δ -12.20 (q, FeH4, 2J(H-
P) ) 28.4 Hz). 31P NMR (C6D6): δ 65.5 (s), quintet in the hydride-
coupled spectrum. Data for Fe(H)2(PEt3)4. 1H NMR (C6D6, 21 °C):
δ 1.59 (m, PCH2), 1.06 (m, PCH2CH3), -15.94 (multiplet with three
broad inner and two sharp outer lines, FeH2, the separation between
the outer sharp lines is 183.9 Hz). 31P NMR (C6D6, 21 °C): δ 46.5
(br. s, ∆ ) 133 Hz), 51.3 (br. s, ∆ ) 131 Hz).
Dihydridotetrakis(trimethylphosphino)ruthenium(II), Ru(H)2-
(PMe3)4. PMe3 (2 mL, 19.3 mmol) was dissolved in 15 mL of THF
and added into a very dark mixture of RuCl3‚nH2O (0.377, 1.57 mmol)
in 15 mL of THF. This immediately afforded a brown precipitate.
After 1 h of stirring, [NBu4]BH4 (1.22 g, 4.74 mmol) was added, and
the stirring was continued for an additional 2 h to give a dark yellow
solution with some precipitate of [NBu4]Cl. This was filtered, and the
THF was removed completely. The residue was extracted with 5 ×
30 mL of pentane. The solvent was then removed again to give a
solid, which was dried in Vacuo for 6 h to afford spectroscopically
pure Ru(H)2(PMe3)4: yield, 0.45 g, 70%. The purity of this material
was sufficient for the subsequent preparation of 2a. The complex could
[Fe(H)3(PEt3)4][B(C6H3(CF3)2)4], (1b). This preparation was carried
out under an atmosphere of argon using argon-saturated solvents. A
mixture of Fe(H)2(PEt3)4 (0.062 g, 0.117 mmol) and [Et2O‚H]B(C6H3-
(CF3)2)4 (0.1 g, 0.099 mmol) was dissolved in 8 mL of ether at -70
°C. Addition of 10 mL of cold (-70 °C) hexane caused precipitation
of a light yellow solid. The solid was filtered, washed with cold hexane,
and dried under vacuum for 0.5 h: yield, 0.137 g, 96%. Complex 1b
is thermally unstable in solution at 20 °C. Slow decomposition (weeks)
was observed in THF-d8 in an NMR tube sealed under argon.
Apparently this reaction proceeded via hydrogen loss. Compound 1b
immediately reacts in solution with Fe(H)2(PEt3)4 to form stable Fe-
(H)2(H2)(PEt3)3, PEt3, and some unidentified product.
be additionally purified by sublimation.31 1H NMR (THF-d8):
δ
2
-10.12 (m, RuH2). 31P NMR (THF-d8): δ -6.9 (t, J(P-P) ) 26.4
Hz), 0.7 (t).
Dihydridotetrakis(triethylphosphino)ruthenium(II), Ru(H)2(PEt3)4.
This dihydride was prepared by the above method using 0.30 g (1.04
mmol) of RuCl3‚nH2O, 1.5 mL (10.2 mmol) of PEt3, and 1 g (3.9 mmol)
of [NBu4]BH4 in 40 mL of THF. The pentane extract was, however,
treated differently. The volume of the pentane solution was reduced
to about 20 mL (when precipitation of PEt3‚BH3 started) and left at
-30 °C overnight. The cold mother liquor was quickly removed from
the solid via a cannula, and the solvent was then evaporated. The
residue was washed with 3 × 5 mL of cold (-70 °C) methanol and
dried for 1 h in Vacuo at 80 °C to afford spectroscopically pure Ru-
(H)2(PEt3)4: yield, 0.55 g, 76%. 1H NMR (THF-d8): δ -11.79 (m,
Trihydridotetrakis(trimethylphosphino)osmium(IV) Tetraphen-
ylborate, [Os(H)3(PEt3)4]BPh4 (3b). A mixture of K2[OsO2(OMe)4]
(0.17 g, 0.4 mmol) and PEt3 (0.4mL, 2.71 mmol) in 8 mL of methanol
was stirred at 20 °C for 1 h. The 31P NMR spectrum of the resulting
clean red solution showed resonances of PEt3 (δ -16.8), OPEt3 (δ 60.9),
OsH4(PEt3)3 (δ 1.3), and two lines at δ -10.6 and -12.4 of [OsH3-
(PEt3)4]+ and some reactive intermediate, respectively. Formation of
[OsH3(PEt3)4]+ was completed after 3.5 h of stirring when the solution
turned almost colorless. A small amount of OsH4(PEt3)3 (5%) was
also present. This methanol reaction solution was evaporated, and the
residue was extracted with 3 × 5 mL of hexane. The solvent was
removed under vacuum, and the residue was redissolved in 10 mL of
methanol. Addition of a solution of NaBPh4 (0.2 g, 0.58 mmol) in 5
mL of methanol afforded a precipitate that was washed with 2 × 5
mL of methanol and 2 × 5 mL of hexane to give [Os(H)3(PEt3)4]BPh4:
yield, 0.2 g, 51%. Anal. Calcd. for C48H83BP4Os: C, 58.52; H, 8.49.
Found: C, 58.26; H, 8.45.
2
RuH2). 31P NMR (THF-d8): δ 21.7 (t, J(P-P) ) 20.7 Hz), 32.1 (t).
Dihydridotetrakis(trimethylphosphino)osmium(II), Os(H)2(PMe3)4.
A mixture of K2[OsO2(OMe)4] (0.187 g, 0.44 mmol) and PMe3 (0.4
mL, 3.86 mmol) was stirred in 8 mL of ethanol for about 5 min until
a clear solution was formed. NaBH4 (66 mg, 1.75 mmol) was added,
and the stirring was continued for 1.5 h at 70 °C. After removal of
the solvent in Vacuo, the dihydride Os(H)2(PMe3)4 was extracted with
hexane to afford a crude product of approximately 90% purity according
to the 1H and 31P NMR data (a detectable impurity was OsH4(PMe3)3,
ca. 4%): yield, 0.16 g, 73%. Subsequent protonation of this material
by a CH3OH/(CF3)2CHOH mixture (see below) afforded 177 mg of

M(II) and M(IV) Complexes [MH3(PR3)4]+
J. Am. Chem. Soc., Vol. 119, No. 16, 1997 3731
Preparation of [HPR3]BPh4 (R ) Cy, tBu). PtBu3 (0.125 g, 0.62
mmol) was dissolved in 2 mL of (CF3)2CHOH, and with stirring, a
slight excess of NaBPh4 (0.26 g, 0.67 mmol) in 3 mL of (CF3)2CHOH/
CH3OH (2:1) was added. This immediately produced a white
precipitate which was filtered, washed with 2 × 5 mL of methanol
and 2 × 5 mL of hexane, and dried under vacuum to yield 0.27 g
(77%) of [HPtBu3]BPh4. [HPCy3]BPh4 was prepared with a similar
yield by this above method. 31P{1H} NMR (THF-d8): δ 63.70 (s,
PtBu3), 57.60 (s, HPtBuF3+), 29.95 (s, HPCy3+), 10.94 (s, PCy3). Other
31P chemical shifts from the acidity measurements in THF-d8: -31.39
(s, PnBu3) and 11.87 (s, HPnBu3+).
X-ray Structure Determination of [FeH(H2)(PMe3)4]BPh4 (1a,Cn),
[Fe(H)3(PEt3)4]B(C6H3(CF3)2)4 (1b,T), and [Os(H)3(PMe3)4]BPh4
(3a,Cc). Crystals of 1a,Cn, 1b,T, and 3a,Cc were prepared by slow
diffusion of hexane into solutions of ca. 10 mg of the complexes in ca.
0.4 mL of THF. The instability of 1b,T in solution required the
preparation at -30 °C under Ar.
Intensity data were collected on a Nicolet R3 diffractometer for 1a
and 3a and on a Siemens P3 diffractometer for 1b using graphite-
monochromated Mo KR radiation (λ ) 0.710 73 Å). Crystal data, data
collection, and least-squares parameters are listed in Table 4.
1a. A yellow crystal of 1a (0.5 × 0.45 × 0.4 mm3) was mounted
on a glass fiber with polybutene. Intensity measurements were made
with 4° < 2θ < 54°. A total of 8456 reflections were collected of
which 8159 were unique (Rint ) 0.0205). The structure was solved by
direct methods (SHELXTL-PLUS)45a and refined by full-matrix least-
squares on F2 (SHELXL-93).45b All atoms were refined with
anisotropic displacement coefficients. The refinement converged to
R(F) ) 0.048, wR(F2) ) 0.1114, and S ) 1.102 for 7105 reflections
with F > 4σ(Fo) and 615 variables.
1b. A yellow crystal with approximate dimensions of 0.5 × 0.2 ×
0.15 mm3 was mounted on a glass fiber with polybutene. The
orientation matrix and cell parameters were determined from 36
machine-centered reflections with 8° < 2θ < 26°. Axial photographs
were used to verify the unit cell choice. A total of 14 414 reflections
were collected of which 13754 were unique (Rint ) 0.052). The
structure was solved by direct methods (SHELXTL-PLUS)45a and
refined by full-matrix least-squares on F2 (SHELXL-93).45b All non-
hydrogen atoms were refined anisotropically, and hydrogen atoms were
included with a model and isotropic displacement coefficients (u(H)
) 0.08). The hydrogen atoms H1, H2, and H3 were located from a
difference Fourier map and refined as isotropic atoms (u(H) ) 0.08).
The refinement converged to R(F) ) 0.0729, wR(F2) ) 0.1275, and S
) 1.022 for 7189 reflections with F > 4σ(Fo) and 784 variables.
Table 4. Summary of Crystal Data, Details of Intensity Collection,
and Least-Squares Refinement Parameters of the Complexes 1a,Cn,
1b,T, and 3a,Cc.
1a,Cn
1b,T
3a,Cc
chem. formula
fw
a, Å
b, Å
c, Å
â, deg
V, Å3
Z
C36H59BFeP4
682.37
15.899(3)
12.702(2)
20.169(4)
110.39(1)
3817.9(12)
4
C56H75BF24FeP4
1394.7
C36H59BOsP4
816.72
12.136(2)
14.205(3)
37.117(6)
98.93(1)
6321(2)
4
15.901(2)
12.815(3)
20.300(3)
110.62(1)
3871.6(12)
4
space group
T, K
P21/c
153(2)
0.710 73
1.187
0.585
P21/c
173(2)
0.710 73
1.466
0.447
P21/c
173(2)
0.710 73
1.401
3.482
λ, Å
F
calcd, g cm-3
µ, mm-1
trans. coeff
(max, min)
R, %b
a
0.955, 0.9
0.997, 0.612
4.80
7.29
4.88
Rw, %
11.14
12.75
11.79
a No absorption correction. b R ) ∑(||Fo| - |Fc||)/∑|Fo|, Rw
)
∑w1/2(||Fo| - |Fc||)/∑w1/2|Fo|.
3a. A white transparent crystal of 3a measuring 0.7 × 0.5 × 0.4
mm3 was mounted on a glass fiber. Intensity measurements were made
with 4° < 2θ < 54°. A total of 9105 reflections were measured of
which 8797 were unique (Rint ) 0.0463). The structure was solved by
the “heavy atom” method (SHELXTL-PLUS)45a and refined by full-
matrix least-squares on F2 (SHELXL-93).45b All non-hydrogen atoms
were refined with anisotropic displacement coefficients. The hydrogen
atoms H1, H2, and H3 were located from a difference Fourier map and
refined as independent isotropic atoms. The refinement converged to
R(F) ) 0.0488, wR(F2) ) 0.1179, and S ) 1.038 for 6973 reflections
with F > 4σ(Fo) and 391 variables.
Full details are given in the Supporting Information.
Acknowledgment. We thank the Swiss National Science
Foundation for the financial support, and we are grateful to
Johnson Matthey Co. for a loan of OsO4.
Supporting Information Available: Listings of crystal data,
structure determination summary, atomic coordinates, and
thermal displacement parameters of the X-ray diffraction studies
of 1a, 1b, and 3a (28 pages). See any current masthead page
for ordering and Internet access instructions.
(45) (a) Sheldrich, G. M. SHELXTL-PLUS, Release 4.21; Siemens
Analytical X-ray Instruments: Madison, Wisconsin, 1990. (b) Sheldrick,
G. M. SHELXL-93. Program for the Refinement of Crystal Structures;
University of Go¨ttingen, 1993.
JA963692T