Synthesis and structural characterization of [κ3-B,S,S- B(mimR)3]Ir(CO)(PPh3)H (R = But, Ph) and [κ4-B(mimBut)3]M(PPh3)Cl (M = Rh, Ir): Analysis of the bonding in metal borane compounds

Article abstract of DOI:10.1021/ic051975t

A series of iridium and rhodium complexes that feature M→B dative bonds, namely [κ³-B,S,S-B(mim^R)₃]Ir(CO)- (PPh₃)H (R = Bu^t, Ph) and [κ⁴-B(mim ^But)₃]M(PPh₃)CI (M = Rh, Ir), has been synthesized via (i) the reactions of Ir(PPh₃)₂(CO)Cl with [Tm^But]TI and [Tm^Ph]Li and (ii) the reactions of (COD)M(PPh₃)Cl with [Tm^But]K. The complexes have been structurally characterized by X-ray diffraction, thereby demonstrating the presence of a M→B dative bond in each complex. The nature of the M→B interaction in these complexes has been addressed by computational methods which indicate that the metal centers possess a d⁶ configuration. The d⁶ configuration is in accord with the value predicted by using a method that employs the valence to determine dⁿ, but is not in accord with the d⁸ configuration that is predicted using the oxidation number. Thus, even though B(mim^R)₃ may be regarded as a neutral closed-shell ligand, coordination to a dⁿ transition metal via the boron results in the formation of a complex in which the metal center possesses a d^n-2 configuration.

Full text of DOI:10.1021/ic051975t

Inorg. Chem. 2006, 45, 2588−2597
Synthesis and Structural Characterization of
[
[
K
K
3-B,S,S-B(mimR)3]Ir(CO)(PPh3)H (R
)
But, Ph) and
t
4-B(mimBu )3]M(PPh3)Cl (M
) Rh, Ir): Analysis of the Bonding in Metal
Borane Compounds
Victoria K. Landry, Jonathan G. Melnick, Daniela Buccella, Keliang Pang, Joseph C. Ulichny, and
Gerard Parkin*
Department of Chemistry, Columbia UniVersity, New York, New York 10027
Received November 15, 2005
A series of iridium and rhodium complexes that feature M
fB dative bonds, namely [κ
3-B,S,S-B(mimR)3]Ir(CO)-
t
(PPh3)H (R
)
But, Ph) and [
κ
4-B(mimBu )3]M(PPh3)Cl (M
)
Rh, Ir), has been synthesized via (i) the reactions of
t
t
Ir(PPh3)2(CO)Cl with [TmBu ]Tl and [TmPh]Li and (ii) the reactions of (COD)M(PPh3)Cl with [TmBu ]K. The complexes
have been structurally characterized by X-ray diffraction, thereby demonstrating the presence of a M
fB dative
bond in each complex. The nature of the M B interaction in these complexes has been addressed by computational
f
methods which indicate that the metal centers possess a d6 configuration. The d6 configuration is in accord with
the value predicted by using a method that employs the valence to determine dn, but is not in accord with the d8
configuration that is predicted using the oxidation number. Thus, even though B(mimR)3 may be regarded as a
neutral closed-shell ligand, coordination to a dn transition metal via the boron results in the formation of a complex
-2
in which the metal center possesses a dn configuration.
Introduction
Tris(2-mercapto-1-R-imidazolyl)hydroborato
ligands,
[HB(mimR)3] ) [TmR] (Figure 1),1 which may be regarded
as the sulfur counterparts to the well-known tris(pyrazolyl)-
hydroborato ligands, [TpRR′],2 have recently found widespread
applications in coordination chemistry. For example, tris(2-
mercapto-1-R-imidazolyl)hydroborato ligands have been
employed in areas as diverse as bioinorganic chemistry and
organometallic chemistry. In the vast majority of cases, the
[TmR] ligand coordinates in a tridentate manner via the three
sulfur atoms (Figure 1a), although more complex coordina-
Figure 1. C3-symmetric coordination modes for [TmR] and [B(mimR)3]
ligands.
(1) For example, derivatives with R ) Me,a-c Ph,d Mes,d Cum,e But,e
CH2Ph,f and p-Tolf are known. See: (a) Garner, M.; Reglinski, J.;
Cassidy, I.; Spicer, M. D.; Kennedy, A. R. J. Chem. Soc., Chem.
Commun. 1996, 1975-1976. (b) Reglinski, J.; Garner, M.; Cassidy,
I. D.; Slavin, P. A.; Spicer, M. D.; Armstrong, D. R. J. Chem. Soc.,
Dalton Trans. 1999, 2119-2126. (c) Santini, C.; Lobbia, G. G.;
Pettinari, C.; Pellei, M.; Valle, G.; Calogero, S. Inorg. Chem. 1998,
37, 890-900. (d) Kimblin, C.; Bridgewater, B. M.; Churchill, D. G.;
Parkin, G. Chem. Commun. 1999, 2301-2302. (e) Tesmer, M.; Shu,
M.; Vahrenkamp, H. Inorg. Chem. 2001, 40, 4022-4029. (f) Bakbak,
S.; Bhatia, V. K.; Incarvito, C. D.; Rheingold, A. L.; Rabinovich, D.
Polyhedron 2001, 20, 3343-3348.
tion modes have also been observed. A particularly interest-
ing example is provided by the lead complex [TmPh]2Pb in
which one of [TmPh] ligands coordinates with an “inverted”
κ4 configuration (Figure 1b).3 Thus, in addition to interacting
with the three sulfur donors, the lead center also participates
in a Pb‚‚‚H-B interaction along the 3-fold axis (Figure 1b).
Further illustrations of the departure from simple κ3 coor-
dination via the three sulfur donors are provided by (i) [κ3-
(2) (a) Trofimenko, S. ScorpionatessThe Coordination Chemistry of
Polypyrazolylborate Ligands; Imperial College Press: London, 1999.
(b) Trofimenko, S. Polyhedron 2004, 23, 197-203.
(3) Bridgewater, B. M.; Parkin, G. Inorg. Chem. Commun. 2000, 3, 534-
536.
2588 Inorganic Chemistry, Vol. 45, No. 6, 2006
10.1021/ic051975t CCC: $33.50
© 2006 American Chemical Society
Published on Web 02/18/2006

Analysis of the Bonding in Metal Borane Compounds
H,S,S-TmMe]Ru(CO)(PPh3)H,4 [κ3-H,S,S-Tmp-Tol]Ni(dppe)-
Scheme 1
t
Cl,5 and [TmBu ]2Co,6 in which the B-H group coordinates
to the metal via a 3-center-2-electron interaction, and (ii)
t
7
{[TmPh]Tl}2 and {[TmBu ]2Co2Br}[PF6],6 in which the [TmR]
ligands bridge two metal centers.
An important recent development in the application of
[TmR] ligands is concerned with the discovery by Hill that
the B-H entity is reactive and may be cleaved by a metal
center to generate so-called metallaboratrane8 complexes
which feature the [B(mimR)3] ligand (Figure 1c). Examples
of such complexes include [κ4-B(mimMe)3]Ru(CO)(PPh3),4,9
(CO)(PPh3)H necessarily involves a reaction sequence that
comprises metathesis of Cl by [TmR] and cleavage of the
B-H bond,16 details of the mechanism are otherwise
unknown. The analogue [κ3-B,S,S-B(mimMe)3]Ir(CO)(PPh3)H
has been previously synthesized, but efforts to obtain crystals
suitable for X-ray diffraction were unsuccessful and the
existence of an Ir-B bond was inferred from the observation
of a broad 31P{1H} NMR signal.13 It is, therefore, noteworthy
that we have been able to determine the molecular structures
t
[κ4-B(mimMe)3]Os(CO)(PPh3),10 {[κ4-B(mimBu )3]Co(PPh3)}-
[BPh4],6 [κ4-B(mimMe)3]Rh(PPh3)Cl,11 {[κ4-B(mimMe)3]Rh-
(PPh3)(CNR)}+,11 {[κ4-B(mimMe)3]Rh(PMe3)2}+,11,12 [κ3-
B,S,S-B(mimMe)3]Ir(CO)(PPh3)H,13 {[κ4-B(mimMe)3]Pt(PPh3)-
H}Cl,14 and [κ4-B(mimMe)3]Pt(PPh3).14 Examination of this
series of [B(mimR)3] complexes indicates that they are largely
restricted to those of the parent [B(mimMe)3] derivative,15 with
only one report of a structurally characterized complex that
incorporates bulky substituents.6 In this paper, we (i) describe
the synthesis and structural characterization of rhodium and
t
of both [κ3-B,S,S-B(mimBu )3]Ir(CO)(PPh3)H and [κ3-B,S,S-
B(mimPh)3]Ir(CO)(PPh3)H by X-ray diffraction, as illustrated
in Figures 2 and 3, which clearly indicate the κ3-B,S,S
coordination mode of the ligand. In accord with these
t
iridium complexes of [B(mimBu )3] and [B(mimPh)3] ligands
which feature MfB dative bonds and (ii) provide a
theoretical analysis of the nature of the bonding in these
derivatives.
1
structures, H NMR spectroscopy indicates that the three
mimR groups are chemically inequivalent, as illustrated by
t
the 1H NMR spectrum of [κ3-B,S,S-B(mimBu )3]Ir(CO)-
Results and Discussion
(PPh3)H (Figure 4).
t
1. Syntheses and Structures of [κ3-B,S,S-B(mimBu )3]Ir-
(CO)(PPh3)H and [κ3-B,S,S-B(mimPh)3]Ir(CO)(PPh3)H.
t
The iridium borane complexes [κ3-B,S,S-B(mimBu )3]Ir(CO)-
(PPh3)H and [κ3-B,S,S-B(mimPh)3]Ir(CO)(PPh3)H may be
obtained via reaction of Vaska’s complex Ir(PPh3)2(CO)Cl
t
with [TmBu ]M (M ) K, Tl) and [TmPh]Li, respectively
(Scheme 1). While the formation of [κ3-B,S,S-B(mimR)3]Ir-
(4) Foreman, M. R. St.-J.; Hill, A. F.; Owen, G. R.; White, A. J. P.;
Williams, D. J. Organometallics 2003, 22, 4446-4450.
(5) Alvarez, H. M.; Tanski, J. M.; Rabinovich, D. Polyhedron 2004, 23,
395-403.
(6) Mihalcik, D. J.; White, J. L.; Tanski, J. M.; Zakharov, L. N.; Yap, G.
P. A.; Incarvito, C. D.; Rheingold, A. L.; Rabinovich, D. Dalton
Trans.2004, 1626-1634.
(7) Kimblin, C.; Bridgewater, B. M.; Hascall, T.; Parkin, G. J. Chem.
Soc., Dalton Trans. 2000, 1267-1274.
(8) The term “atrane” is traditionally used to describe molecules in which
two bridgehead atoms are bridged by three three-atom moieties.a-c
As such, it is not completely appropriate for [κ3-B,S,S-B(mimR)3]
derivatives in which the third ligand arm does not coordinate. (a)
Verkade, J. G. Coord. Chem. ReV. 1994, 137, 233-295. (b) Verkade,
J. G. Acc. Chem. Res. 1993, 26, 483-489. (c) Woronkow, M. G.;
Seltschan, G. I.; Lapsina, A.; Pestunowitschich, W. A. Z. Chem. 1968,
8, 214-217.
t
Figure 2. Molecular structure of [κ3-B,S,S-B(mimBu )3]Ir(CO)(PPh3)H.
t
2. Syntheses and Structures of [κ4-B(mimBu )3]Ir(PPh3)-
t
Cl and [κ4-B(mimBu )3]Rh(PPh3)Cl. Whereas the carbonyl
compounds [κ3-B,S,S-B(mimR)3]Ir(CO)(PPh3)H exhibit tri-
dentate coordination of the [B(mimR)3] ligand, tetradentate
coordination is observed in the carbonyl-free complexes [κ4-
(9) Hill, A. F.; Owen, G. R.; White, A. J. P.; Williams, D. J. Angew.
Chem., Int. Ed. 1999, 38, 2759-2761.
(10) Foreman, M. R. St.-J.; Hill, A. F.; White, A. J. P.; Williams, D. J.
Organometallics 2004, 23, 913-916.
t
t
B(mimBu )3]Ir(PPh3)Cl and [κ4-B(mimBu )3]Rh(PPh3)Cl, which
(11) (a) Crossley, I. R.; Foreman, M. R. St.-J.; Hill, A. F.; White, A. J. P.;
Williams, D. J. Chem. Commun. 2005, 221-223. (b) Crossley, I. R.;
Hill, A. F.; Willis, A. C. Organometallics 2006, 25, 289-299.
(12) For a binuclear {[κ4-B(mimMe)3]Rh} derivative, see: Crossley, I. R.;
Hill, A. F.; Humphrey, E. R.; Willis, A. C. Organometallics 2005,
24, 4083-4086.
are obtained via the reactions of (COD)M(PPh3)Cl with
t
[TmBu ]K (Scheme 2). The molecular structures of [κ4-
t
t
B(mimBu )3]Ir(PPh3)Cl and [κ4-B(mimBu )3]Rh(PPh3)Cl have
been determined by X-ray diffraction, as illustrated in Figures
(13) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2005, 24,
1062-1064
(16) For other examples of B-H addition to Ir(PR3)2(CO)X derivatives,
see: (a) Westcott, S. A.; Marder, T. B.; Baker, R. T.; Calabrese, J. C.
Can. J. Chem. 1993, 71, 930-936. (b) Cleary, B. P.; Eisenberg, R.
Organometallics 1995, 14, 4525-4534.
(14) Crossley, I. R.; Hill, A. F. Organometallics 2004, 23, 5656-5658.
(15) B(mimMe)3 is often abbreviated as B(mt)3 in view of the trivial name
of methimazolyl for mimMe
.
Inorganic Chemistry, Vol. 45, No. 6, 2006 2589

Landry et al.
Figure 3. Molecular structure of [κ3-B,S,S-B(mimPh)3]Ir(CO)(PPh3)H.
t
Figure 6. Molecular structure of [κ4-B(mimBu )3]Rh(PPh3)Cl.
t
Figure 4. 1H NMR spectrum of [κ3-B,S,S-B(mimBu )3]Ir(CO)(PPh3)H
showing the chemical inequivalence of the three tert-butyl groups of the
t
B(mimBu )3 ligand.
Scheme 2
Figure 7. 1H and 1H{31P} NMR spectra of the hydride signal of [κ4-
t
t
B(mimBu )3]Rh(PPh3)H in C6D6 (the signal is observed at -16.2 in CD3CN).
5 and 6. The formation of [κ4-B(mimBu )3]M(PPh3)Cl is
undoubtedly complex, and the nature of the byproducts is
not known. In the case of the rhodium system, however, an
intermediate which is tentatively characterized as the hydride
exhibits coupling to both 103Rh (1JRh-H ) 17 Hz) and 31P
(2JP-H ) 6 Hz), with the latter value being consistent with
a cis disposition of hydride and PPh3 ligands.18 Further
t
derivative [κ4-B(mimBu )3]Rh(PPh3)H has been observed in
t
support for the identification of [κ4-B(mimBu )3]Rh(PPh3)H
t
solution prior to crystallization of B(mimBu )3]Rh(PPh3)Cl.17
t
Evidence for the formulation of [κ4-B(mimBu )3]Rh(PPh3)H
as the intermediate is provided by the fact that the same
1
hydride signal in the H NMR spectrum is also observed
is provided by a signal attributable to the hydride ligand at
t
when [κ4-B(mimBu )3]Rh(PPh3)Cl is treated with LiBH4.
1
-15.05 ppm in the H NMR spectrum (Figure 7), which
3. Nature of the Metal-Boron Bonding in [κ3-B,S,S-
t
B(mimR)3]Ir(CO)(PPh3)H and [κ4-B(mimBu )3]M(PPh3)Cl
(M ) Rh, Ir). The most important feature of the molecular
t
structures of [κ3-B,S,S-B(mimBu )3]Ir(CO)(PPh3)H, [κ3-B,S,S-
t
B(mimPh)3]Ir(CO)(PPh3)H, [κ4-B(mimBu )3]Ir(PPh3)Cl, and
t
[κ4-B(mimBu )3]Rh(PPh3)Cl, together with the aforementioned
metallaboratrane compounds,4,6,9-14 pertains to the presence
of a MfBR3 datiVe covalent bond (also referred to as
coordinate covalent bond or donor-acceptor bond).19 While
(17) A similar observation has been made pertaining to the conversion of
[κ4-B(mimMe)3]Rh(PPh3)H to [κ4-B(mimMe)3]Rh(PPh3)Cl. See ref 11b.
2
(18) For representative values of cis and trans JP-H coupling constants,
see: (a) Okazaki, M.; Ohshitanai, S.; Tobita, H.; Ogino, H. J. Chem.
Soc., Dalton Trans. 2002, 2061-2068. (b) Paneque, M.; Sirol, S.;
Trujillo, M.; Gutie´rrez-Puebla, E.; Monge, M. A.; Carmona, E. Angew.
Chem., Int. Ed. 2000, 39, 218-221. (c) Circu, V.; Fernandes, M. A.;
Carlton, L. Polyhedron 2003, 22, 3293-3298.
t
Figure 5. Molecular structure of [κ4-B(mimBu )3]Ir(PPh3)Cl.
(19) Haaland, A. Angew. Chem., Int. Ed. Engl. 1989, 28, 992-1007.
2590 Inorganic Chemistry, Vol. 45, No. 6, 2006

Analysis of the Bonding in Metal Borane Compounds
dative bonding is a common feature of transition metals, the
metal is normally the electron pair acceptor, rather than the
donor. There are, nevertheless, notable exceptions in which
a metal center serves as an electron pair donor, which include
situations in which (i) there is a M‚‚‚H-X hydrogen bond20,21
and (ii) a metal-ligand bond is supplemented by back-
bonding (e.g., carbonyl, olefin, and boryl compounds).22
With respect to the dative covalent nature of the MfB
investigations pertaining to the reactivity of transition metal
compounds with BR3 derivatives were initiated in the
1960s,29a but the results of many of these early investigations
have been called into question because of the lack of
structural verification.31 For example, the first transition metal
complex proposed to have a MfB bond was Cp2WH2(BF3),
obtained via treatment of Cp2WH2 with BF3,32 but the product
of this reaction has subsequently been shown to be [Cp2-
WH3][BF4].31,33 It is, therefore, evident that the ability to
t
bond, the Ir-B bond lengths in [κ3-B,S,S-B(mimBu )3]Ir(CO)-
t
(PPh3)H [2.179(4) Å], [κ3-B,S,S-B(mimPh)3]Ir(CO)(PPh3)H
isolate complexes such as [κ3-B,S,S-B(mimBu )3]Ir(CO)-
t
t
[2.186(3) Å], and [κ4-B(mimBu )3]Ir(PPh3)Cl [2.15(2) and
(PPh3)H,[κ3-B,S,S-B(mimPh)3]Ir(CO)(PPh3)H,[κ4-B(mimBu )3]-
t
2.18(2) Å for two crystallographically independent mol-
ecules] are comparable to that of [κ3-B,S,S-B(mimMe)2(H)]-
Ir(CO)(PPh3)H, [2.21 Å].13 Furthermore, although these bond
lengths are longer than those of the Ir-B normal covalent
bonds in iridium-boryl complexes such as Ir(PMe3)3(9-
BBN)H2 (2.09 Å),23 Ir(PPh3)2(CO)(Cl)(Bcat)H (2.05 Å),16a
and Ir(PMe3)2(CO)(B5H8)Br2 (2.07 Å),24,25 it is evident that
the difference is sufficiently small that the IrfB dative
covalent interaction must be considered to be significant.
Ir(PPh3)Cl, and [κ4-B(mimBu )3]Rh(PPh3)Cl which feature
MfB dative bonds is a consequence of the chelating nature
of the [B(mimR)3] borane ligand.
Of particular interest, therefore, is the nature of the bonding
in {[B(mimR)3]M} complexes and the electronic impact that
the borane ligand exerts on a metal center. In this regard,
two important factors pertaining to the coordination of any
ligand to a metal center are the effects that it has on (i) the
electron count and (ii) the dn configuration, both of which
play an important role in evaluating the stability and
reactivity of a molecule.
t
Likewise, the Rh-B bond length in [κ4-B(mimBu )3]Rh(PPh3)-
Cl [2.095(3) Å] is comparable to the Rh-B bond lengths in
[κ4-B(mimMe)3]Rh(PPh3)Cl [2.13 Å]11 and rhodium-boryl
compounds, such as RhHCl(PPri3)2(Bcat) [1.97 Å],26
Rh(PMe3)4(Bcat) [2.05 Å],27 and Rh(PMe3)3Cl2(Bcat) [2.10
Å].28
In view of the fact that BR3 (R ) H, halogen, alkyl, aryl)
derivatives are well-known Lewis acids and that electron rich
metal centers are basic,29 it is perhaps surprising that
complexes with MfBR3 interactions have not been structur-
ally characterized for simple monodentate boranes.30 Indeed,
With respect to the electron count of the metal center, the
MfB dative bond contributes no electrons since the boron
atom serves the role of an electron acceptor. As such, the
number of electrons contributed by the [B(mimR)3] ligand
to the electron count of a metal center depends only on the
number of sulfur atoms coordinated, i.e., it is a 6-electron
donor if coordinated through three sulfur atoms and a 4-
electron donor if coordinated through two sulfur atoms. Thus,
t
[κ3-B,S,S-B(mimBu )3]Ir(CO)(PPh3)H, [κ3-B,S,S-B(mimPh)3]Ir-
(20) For lead references, see: (a) Brammer, L. Dalton 2003, 3145-3157.
(b) Martin, A. J. Chem. Educ. 1999, 76, 578-583. (c) Crabtree, R.
H.; Eisenstein, O.; Sini, G.; Peris, E. J. Organomet. Chem. 1998, 567,
7-11. (d) Shubina, E. S.; Belkova, N. V.; Epstein, L. M. J. Organomet.
Chem. 1997, 536, 17-29. (e) Brammer, L.; Zhao, D.; Ladipo, F. T.;
Braddock-Wilking, J. Acta Crystallogr., Sect. B: Struct. Commun.
1995, 51, 632-640. (f) Brammer, L. In Implications of Molecular
and Materials Structure for New Technologies; Howard, J. A. K., Ed.;
Kluwer Academic Publishers: Netherlands 1999; pp 197-210. (g)
Mareque Rivas, J. C.; Brammer, L. Coord. Chem. ReV. 1999, 183,
43-80. (h) Braga, D.; Grepioni, F.; Tedesco, E.; Biradha, K.; Desiraju,
G. R. Organometallics 1997, 16, 1846-1856. (i) Epshtein, L. M.;
Shubina, E. S. Organomet. Chem. U.S.S.R. 1992, 5, 31-38.
(21) A simple illustration of an intramolecular Mo‚‚‚H-O hydrogen bond
is provided by [η6-C6H5C6H3(Ph)OH]Mo(PMe3)3. See: Hascall, T.;
Baik, M.-H.; Bridgewater, B. M.; Shin, J. H.; Churchill, D. G.;
Friesner, R. A.; Parkin, G. J. Chem. Soc., Chem. Commun. 2002,
2644-2645.
(22) Frenking, G.; Fro¨hlich, N. Chem. ReV. 2000, 100, 717-774.
(23) Baker, R. T.; Ovenall, D. W.; Calabrese, J. C.; Westcott, S. A.; Taylor,
N. J.; Williams, I. D.; Marder, T. B. J. Am. Chem. Soc. 1990, 112,
9399-9400.
(24) Churchill, M. R.; Hackbarth, J. J. Inorg. Chem. 1975, 14, 2047-2051.
(25) For reviews that feature metal-boryl complexes, see: (a) Irvine, G.
J.; Lesley, M. J. G.; Marder, T. B.; Norman, N. C.; Rice, C. R.; Robins,
E. G.; Roper, W. R.; Whittell, G.; Wright, L. J. Chem. ReV. 1998, 98,
2685-2722. (b) Aldrige, S.; Coombs, D. L. Coord. Chem. ReV. 2004,
248, 535-559.
(29) (a) Shriver, D. F. Acc. Chem. Res. 1970, 3, 231-238. (b) Werner, H.
Angew. Chem., Int. Ed. Engl. 1983, 22, 927-949. (c) Angelici, R. J.
Acc. Chem. Res. 1995, 28, 51-60. (d) Pearson, R. G.; Ford, P. C.
Comm. Inorg. Chem. 1982, 1, 279-291. (e) Kotz, J. C.; Pedrotty,
D. G. Organomet. Chem. ReV., Sect. A 1969, 4, 479-547. (f)
Kristja´nsdo´ttir, S. S.; Norton, J. R. In Transition Metal Hydrides:
Recent AdVances in Theory and Experiment; Dedieu, A., Ed.; VCH:
New York, 1991; pp 309-359.
(30) Although simple borane adducts of transition metals are unknown,
other Lewis acids are known to form adducts, as illustrated by
CpCo(CO)2[HgCl2],a-c CpCo(PMe3)2[HgCl2],d CpCo(PMe3)2[ZnCl2-
(PMe3)],d CpRh(PMe3)2[AlR3],e [H2C(C5H4)2]Rh(CO)2[HgX2] (X )
Cl, Br, I),f Cp*Ir(CO)2[MCl2] (M ) Zn, Cd, Hg),g Cp2MoH2[ZnBr2-
(DMF)],h CpM(CO)(NO)(PPh3)[HgCl2] (M ) Mo, W),i {(η6-C6H3-
Me3)Mo(CO)3[(HgCl2)2]}2,j,k and [CpFe(CO)2(AlPh3)]-. (a) Nowell,
I. N.; Russell, D. R. Chem. Commun. 1967, 817. (b) Cook, D. J.; Daws,
J. L.; Kemmitt, R. D. W. J. Chem. Soc. (A) 1967, 1547-1551. (c)
Nowell, I. N.; Russell, D. R. J. Chem. Soc., Dalton Trans. 1972, 2393-
2395. (d) Dey, K.; Werner, H. J. Organomet. Chem. 1977, 137, C28-
C30. (e) Mayer, J. M.; Calabrese, J. C. Organometallics 1984, 3,
1292-1298. (f) Cox, M. G.; Manning, A. R.; McCabe, A. J.
Organomet. Chem. 1990, 384, 217-221. (g) Einstein, F. W. B.; Yan,
X.; Zhang, X.; Sutton, D. J. Organomet. Chem. 1992, 439, 221-230.
(h) Crotty, D. E.; Anderson, T. J.; Glick, M. D.; Oliver, J. P. Inorg.
Chem. 1977, 16, 2346-2350. (i) Ginzburg, A. G.; Aleksandrov, G.
G.; Struchkov, Yu. T.; Setkina, V. N.; Kursanov, D. N. J. Organomet.
Chem. 1980, 199, 229-242. (j) Ciplys, A. M.; Geue, R. J.; Snow, M.
R. J. Chem. Soc., Dalton Trans. 1976, 35-37. (k) Edgar, K.; Johnson,
B. F. G.; Lewis, J.; Wild, S. B. J. Chem. Soc. A 1968, 2851-2855.
(l) Burlitch, J. M.; Leonowicz, M. E.; Petersen, R. B.; Hughes, R. E.
Inorg. Chem. 1979, 18, 1097-1105.
(26) (a) Lam, W. H.; Shimada, S.; Batsanov, A. S.; Lin, Z.; Marder, T. B.;
Cowan, J. A.; Howard, J. A. K.; Mason, S. A.; McIntyre, G. J.
Organometallics 2003, 22, 4557-4568. (b) Westcott, S. A.; Taylor,
N. J.; Marder, T. B.; Baker, R. T.; Jones, N. J.; Calabrese, J. C. J.
Chem. Soc., Chem. Commun. 1991, 304-305.
(27) Dai, C.; Stringer, G.; Marder, T. B.; Scott, A. J.; Clegg. W.; Norman,
N. C. Inorg. Chem. 1997, 36, 272-273.
(28) Souza, F. E. S.; Nguyen, P.; Marder, T. B.; Scott, A. J.; Clegg, W.
Inorg. Chim. Acta 2005, 358, 1501-1509.
(31) (a) Braunschweig, H. Angew. Chem., Int. Ed. 1998, 37, 1786-1801.
(b) Braunschweig, H.; Colling, M. Coord. Chem. ReV. 2001, 223,
1-51.
(32) (a) Shriver, D. F. J. Am. Chem. Soc. 1963, 85, 3509-3510. (b) Johnson,
M. P.; Shriver, D. F. J. Am. Chem. Soc. 1966, 88, 301-304.
(33) Braunschweig, H.; Wagner, T. Z. Naturforsch. 1996, B51, 1618-1620.
Inorganic Chemistry, Vol. 45, No. 6, 2006 2591

Landry et al.
t
t
(CO)(PPh3)H, [κ4-B(mimBu )3]Ir(PPh3)Cl, and [κ4-B(mimBu )3]-
Rh(PPh3)Cl possess 18-electron configurations.
+1 oxidation number (which is most uncommon for mo-
lybdenum),40 the molybdenum centers of Cp(CO)3Mo-
Mo(CO)3Cp are divalent because each molybdenum must
use one of its electrons in forming the Mo-Mo bond; as
such, the molybdenum centers possess a d4 configuration,
rather than the d5 configuration that would be implied by
the +1 oxidation number (eq 1).
The dn configuration, i.e., the number of transition metal
d electrons that are not involved in the formation of the
metal-ligand bonds,34 is an important quantity since it
indicates whether a metal center possesses sufficient electrons
for further reactivity, such as an oxidative addition reaction
which requires a dg2 configuration; furthermore, the dn
configuration plays an important role in determining the
magnetic properties and electronic spectroscopy of a mol-
ecule. In a molecular orbital sense, the dn configuration refers
to the number of electrons that occupy nonbonding metal
and metal-ligand antibonding orbitals with d character.34-37
Since coordination of a [B(mimR)3] ligand requires two
electrons from the metal center to provide the MfB dative
bond, it is evident that coordination to a dn metal center
results in a molecule that necessarily possesses a dn-2 center.
On this basis, each of the complexes [κ3-B,S,S-B(mimBu )3]-
Ir(CO)(PPh3)H, [κ3-B,S,S-B(mimPh)3]Ir(CO)(PPh3)H, [κ4-
B(mimBu )3]Ir(PPh3)Cl, and [κ4-B(mimBu )3]Rh(PPh3)Cl are
described as octahedral 18-electron d6 molecules. However,
this assignment is not in accord with the d8 configuration
that has been assigned to the related rhodium compound,
[κ4-B(mimMe)3]Rh(PPh3)Cl.11
An illustration of the impact of an inappropriate ligand
charge on the dn configuration is provided by the cyclohepta-
trienyl ligand which is generally considered to be a closed-
shell cation, (C7H7)+, for the purpose of oxidation number
assignments.41-43 On this basis, molecules such as CpTi(η7-
C7H7)44 and a series of (η7-C7H7)TiL2X complexes45 are
classified as Ti(0) with a d4 configuration.42 However, a large
body of theoretical studies indicates that the assignment of
a +1 charge to the cycloheptatrienyl ligand in such com-
plexes is misleading because the metal is required to
contribute three electrons to the M-(η7-C7H7) interaction.45-47
The titanium centers in CpTi(η7-C7H7) and (η7-C7H7)TiL2X,
therefore, adopt a d0 configuration, which is a much more
reasonable assignment for titanium compounds than is a d4
configuration.
t
t
t
The dn configuration is, therefore, more appropriately
determined by the expression:35
The origin of this discrepancy is a consequence of
following the often assumed relationship between dn con-
figuration and oxidation number:38
n ) number of valence electrons in neutral atom - valence
(2)
The discrepancy with the assignment of the dn configuration
n ) number of valence electrons in neutral atom -
(40) Examples of Mo(I) complexes include [(η6-C6H6)2Mo]+, (dppe)2Mo-
(N2)Cl, and CpMo(η6-C6H6). See: Cotton, F. A.; Wilkinson, G.;
Murillo, C. A.; Bochmann, M. AdVanced Inorganic Chemistry, 6th
Ed.; Wiley: New York, 1999; p 921.
(41) Elschenbroich, Ch.; Salzer, A. Organometallics; 2nd ed.; VCH: New
York, 1992; p 358.
oxidation number (1)
While this relationship provides the correct dn values in many
cases, it fails in situations where the oxidation number is
not coincidentally the same as the valence (i.e., the number
of electrons that an atom uses in bonding).39 For example,
common situations that result in departures of oxidation
number from the valence are (i) the molecule possesses a
metal-metal bond and (ii) the closed-shell charge associated
with a ligand does not accurately reflect the bonding
situation. As an illustration of the influence of a metal-
metal bond, the molybdenum atoms in Cp(CO)3Mo-
Mo(CO)3Cp possess an oxidation number of +1 because,
by definition, homonuclear element-element bonds do not
contribute to oxidation number. Despite the assignment of a
(42) For specific examples, see: (a) Andre´a, R. R.; Terpstra, A.; Oskam,
A.; Bruin, P.; Teuben, J. H. J. Organomet. Chem. 1986, 307, 307-
317. (b) Gourier, D.; Samuel, E. Inorg. Chem. 1988, 27, 3018-3024.
(c) Vogler, A.; Kunkely, H. Coord. Chem. ReV. 2004, 248, 273-278.
(43) In addition to the monocation, the cycloheptatrienyl ligand has also
been considered to be an anion, [C7H7]-, for oxidation number
assignments,a,b but since the monoanion is not closed-shell, it is not
particularly appropriate for the determination of oxidation numbers.
The trianion [C7H7]3- is a closed-shell species and has also been
proposed for the assignment of oxidation numbers.c However, the
oxidation number should be determined by assigning the charge which
is associated with the ligand in its most stable uncoordinated form,d
and on this basis, the trianion [C7H7]3- should not be used for oxidation
number assignments (although it is appropriate for the assignment of
valence). (a) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R.
G. Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987; p 26. (b) Reference
38b, p 150. (c) Janiak, C. Klapo¨tke, T. M.; Hans-Ju¨rgen Meyer, H. J.;
Reidel, E. Moderne Anorganische Chemie; Gruyter: Berlin, 2003. (d)
Reference 41, p 309.
(34) Jean, Y. Molecular Orbitals of Transition Metal Complexes; Oxford
University Press: New York 2005; pp 12, 31.
(35) Green, M. L. H. J. Organomet. Chem. 1995, 500, 127-148.
(36) Albright, T. A.; Burdett, J. K.; Whangbo, M. H. Orbital Interactions
in Chemistry; Wiley: New York, 1985; pp 299, 301.
(37) In this regard, it is pertinent to note that π and δ back-bonding
interactions are not traditionally considered to change the dn config-
uration, presumably because these are not the sole component of the
metal-ligand bond. In the case of a MfBR3 bond, however, the
“back-bonding” interaction is of σ symmetry and is the only
component. Thus, it is essential to consider this interaction in
evaluating the dn configuration.
(38) (a) Encyclopedia of Inorganic Chemistry; King, R. B., Ed.; Wiley:
New York 1994; Vol. 2, p 961. (b) Crabtree, R. H. The Organometallic
Chemistry of the Transition Metals, 4th ed.; Wiley: New York, 2005;
p 43. (c) Mingos, D. M. P. Essential Trends in Inorganic Chemistry;
Oxford University Press: New York, 1998; p 299.
(44) (a) van Oven H. O.; de Liefde Meijer, H. J. J. Organomet. Chem.
1970, 23, 159-163. (b) Zeinstra, J. D.; de Boer, J. L. J. Organomet.
Chem. 1973, 54, 207-211.
(45) Davies, C. E.; Gardiner, I. M.; Green, J. C.; Green, M. L. H.; Hazel,
N. J.; Grebenik, P. D.; Mtetwa, V. S. B.; Prout, K. J. Chem. Soc.,
Dalton Trans. 1985, 669-683.
(46) (a) Menconi, G.; Kaltsoyannis, N. Organometallics 2005, 24, 1189-
1197. (b) Kaltsoyannis, N. J. Chem. Soc., Dalton Trans. 1995, 3727-
3730. (c) Green, J. C.; Green, M. L. H.; Kaltsoyannis, N.; Mountford,
P.; Scott, P.; Simpson, S. J. Organometallics 1992, 11, 3353-3361.
(d) Green, J. C.; Kaltsoyannis, N.; Sze, K. H.; MacDonald, M. J. Am.
Chem. Soc. 1994, 116, 1994-2004. (e) Tamm, M.; Kunst, A.;
Bannenberg, T.; Herdtweck, E.; Schmid, R. Organometallics 2005,
24, 3163-3171.
(39) (a) Sidgwick, N. V. The Electronic Theory of Valency; The Clarendon
Press: Oxford, 1927. (b) Parkin, G. J. Chem. Educ. 2006, in press.
(47) Green, M. L. H.; Ng, D. K. P. Chem. ReV. 1995, 95, 439-473.
2592 Inorganic Chemistry, Vol. 45, No. 6, 2006

Analysis of the Bonding in Metal Borane Compounds
Figure 8. Geometry-optimized structures of [κ4-B(mimH)3]Ir(PH3)Cl and [κ3-B(mimH)3]Ir(PH3)Cl, with the latter constrained to having a planar geometry
and a B‚‚‚Ir separation of 3.0 Å.
for {[B(mimR)3]M} derivatives is, therefore, a result of the
fact that, although the [B(mimR)3] ligand may be considered
to be neutral in its closed-shell form,48,49 coordination of the
boron to a metal requires the metal to provide two electrons
and thereby reduces the dn configuration by two units.
To address the nature of the bonding in more detail, and
thereby establish that the use of the valence provides a better
method of evaluating the electronic nature of a molecule,
we have performed DFT calculations on the simplified
counterpart [κ4-B(mimH)3]Ir(PH3)Cl. The geometry-optimized
structure of [κ4-B(mimH)3]Ir(PH3)Cl is illustrated in Figure
8a, and a simplified molecular orbital diagram based on
Fenske-Hall calculations is presented in Figure 9. The most
important aspect of the calculation is that it clearly indicates
that the metal center has a d6 configuration and not a d8
configuration. Specifically, with respect to the metal-based
(48) The term “closed shell” refers to molecules for which all occupied
orbitals are fully occupied, i.e., there are no unpaired electrons and
there are no partially occupied degenerate orbitals. It is important to
note that a closed-shell configuration does not require an inert gas
configuration. Thus, the neutral B(imR)3 molecule is a closed-shell
molecule. See, for example: (a) Kutzelnigg, W.; Smith, V. H., Jr.
Figure 9. Qualitative molecular orbital diagram for [κ4-B(mimH)3]Ir-
Int. J. Quantum Chem. 1968, 2, 531-552. (b) Daudel, R.; Kozmutza,
(PH3)Cl.
C.; Goddard, J. D.; Csizmadia, I. G. J. Mol. Struct. 1978, 50, 363-
369.
(49) It is pertinent to comment on the charge assigned to a BR3 ligand for
the purpose of oxidation number assignments. Specifically, boron has
a Pauling electronegativity (2.0) that is intermediate between the values
for many transition metals, e.g., Co (1.8) and Rh (2.2),a and thus, the
charge assigned to a BR3 ligand (0 or -2) could, in principle, vary
for two otherwise closely related compounds. For this reason, it is
pertinent to consider other factors to establish which charge should
be assigned to a BR3 ligand. Two different criteria that are used to
determine the charge are (i) the charge assigned to the ligand should
be such that the donor atom has an octet configurationb and (ii) the
charge assigned to the ligand should correspond to the ligand in its
stable closed-shell configuration,c a criterion that is in keeping with
the notion that the oxidation number of an atom was originally derived
by removing the ligand in the form that it is commonly encountered
in an uncoordinated state (e.g., Cl-, NH3, and OH2). The former
criterion would assign a charge of -2 to the ligand, whereas the latter
would assign a charge of zero. Of these, a charge of zero is preferred
for oxidation number assignments because neutral BR3 derivatives are
ubiquitous, whereas the dianion [BR3]2- is an unusual example of a
compound with boron in the +1 oxidation state and is unknown. For
this reason, the [B(mimR)3] ligand has been assigned a charge of zero
in the assignment of oxidation numbers (for example, see ref 14). (a)
Pauling, L. The Nature of The Chemical Bond, 3rd ed.; Cornell
University Press: Ithaca, New York, 1960; p 93. (b) Reference 34, p
13. (c) Reference 43d.
d orbitals, there are only 6 electrons housed in formally
nonbonding orbitals, i.e., the dxy, dxz, and dyz orbitals of the
“t2g” set in octahedral symmetry.50 All other electrons
involved in the [B(mimH)3]-Ir interaction are located in
bonding orbitals. The molecular orbital analysis, therefore,
clearly indicates that the dn configuration predicted by
consideration of the valence (eq 2) is more appropriate than
that derived by consideration of the oxidation number (eq
1). In support of the proposed d6 configuration for [κ3-B,S,S-
t
B(mimBu )3]Ir(CO)(PPh3)H, ν(CO) (1989 cm-1) is distinctly
greater than those of monovalent d8 iridium complexes such
as Ir(PR3)2(CO)Cl (1929-1961 cm-1),51 an observation that
is consistent with reduced back-bonding for the higher-valent
complex. For further comparison, ν(CO) for [κ3-B,S,S-
(50) Using Green’s “Covalent Bond Classification”, the complexes are
classified as trivalent ML3X3. See ref 35.
(51) Shin, J. H.; Bridgewater, B. M.; Churchill, D. G.; Parkin, G. Inorg.
Chem. 2001, 40, 5626-5635.
Inorganic Chemistry, Vol. 45, No. 6, 2006 2593

Landry et al.
An illustration of the manner in which the orbitals involved
in the 3-center-4-electron interaction transform upon ap-
proach of the boron is provided by comparison of the
molecular orbitals of [κ4-B(mimH)3]Ir(PH3)Cl with those of
the geometry-optimized structure of [κ3-S,S,S-B(mimH)3] in
which the [B(mimH)3] fragment is constrained to having a
planar geometry at boron with an Ir‚‚‚B distance of 3 Å
(Figure 8). The Ir-Cl σ-bonding interaction in [κ3-S,S,S-
B(mimH)3] is largely composed of overlap between the Ir
2
dz and Cl pz orbital, as illustrated by MO 52 in Figure
11,53 while MO 60 corresponds largely to the Ir-Cl
σ-antibonding orbital. However, it is evident that boron pz
character is already mixing into this orbital and that the Ir-B
interaction is starting to be developed at an Ir‚‚‚B separation
of 3.0 Å; as a consequence, MO 61 does not correspond
to a pure boron pz orbital but has a significant amount of Ir
2
dz character. Despite this mixing, the structure does,
nevertheless, provide a useful means to indicate how the
orbitals transform upon coordination of the boron. Compari-
son with the three corresponding orbitals of [κ4-B(mimH)3]-
Ir(PH3)Cl indicates that the most dramatic change is observed
for the Ir-Cl σ-bonding orbital 52 of [κ3-S,S,S-B(mimH)3],
which takes on substantial Ir-B bonding character in MO
49 of [κ4-B(mimH)3]Ir(PH3)Cl. A distinct change is also
observed for the Ir-Cl σ-antibonding MO 60 of [κ3-S,S,S-
B(mimH)3], which becomes the B-Cl ligand-based MO 56
of [κ4-B(mimH)3]Ir(PH3)Cl, with significant Cl pz character
2
Figure 10. Orbital correlation diagram for coordination of a BR3 moiety
to Ir in a position that is trans to a Cl ligand.
and negligible iridium dz character. Finally, the LUMO 61
of [κ3-S,S,S-B(mimH)3], which has largely boron pz character,
becomes LUMO 61 of [κ4-B(mimH)3]Ir(PH3)Cl with mainly
t
B(mimBu )3]Ir(CO)(PPh3)H is in the range observed for
2
trivalent Ir(PR3)2(CO)(X)(Bcat)H derivatives (1984-2118
iridium dz character.
cm-1).16
The essential feature of a general M-B bonding interac-
tion is illustrated by the simplified molecular orbital diagram
shown in Figure 12, which reduces the discussion to the
situation for a 2-center interaction. Thus, the interaction
between a filled metal-based orbital and the empty orbital
on boron results in a filled M-B bonding orbital and an
unoccupied M-B antibonding orbital. Since the M-B
bonding orbital is occupied by a pair of electrons that were
originally on the metal, a metal center that originally
possessed a dn configuration becomes dn-2 upon coordination
to boron.
The transition from a dn to dn-2 configuration upon
coordination of a BR3 fragment bears a close analogy to the
change resulting from the interaction of a metal center with
another Lewis acid, namely H+. Thus, it is widely recognized
that protonation of a dn metal center results in the formation
of a metal hydride in which the metal center has a dn-2
configuration.35,54 As with the formation of a MfBR3 bond,
protonation of a dn metal center results in a dn-2 configuration
because the metal-hydride bond requires a pair of electrons
and the only source of these electrons is the metal center.
The use of oxidation numbers to determine the value of dn,
It is instructive to consider in more detail how coordination
of the boron to the d8 iridium center causes the iridium to
adopt a d6 configuration. For this purpose, it is useful to
consider the molecular orbital diagram of a hypothetical [κ3-
S,S,S-B(mimH)3]Ir(PH3)Cl species in which the boron does
not coordinate to the iridium center (Figure 8b). Examination
of Figure 10 illustrates that the HOMO of such a species
2
possesses dz character and is the σ-antibonding component
2
2
of the Ir-Cl bond (the dx -y orbital is responsible for binding
the PH3 and three sulfur ligands and is at higher energy);
coupled with the six electrons that occupy the dxz, dyz, and
dxy orbitals, the iridium is assigned a d8 configuration.
Upon approaching the iridium center, the empty boron pz
2
orbital interacts with the iridium dz -derived HOMO. How-
2
ever, since the dz orbital also interacts with the Cl pz orbital,
a classic 3-center-4-electron interaction ensues, thereby
resulting in bonding, nonbonding, and antibonding B-Ir-
Cl orbitals (Figure 11). The outcome of this interaction is,
therefore, the transfer of a pair of electrons from a metal-
52
2
based dz orbital to a ligand-based orbital. As a result, the
4
H
2
“dz ” B-Ir-Cl σ-antibonding orbital of [κ -B(mim )3]Ir-
(PH3)Cl is unoccupied and the iridium center possesses a d6
configuration.
(53) In addition to MO 52 , MO 53 also has Ir-Cl σ-bonding character.
MO 52 and MO 53 differ due to interaction of the Ir-Cl σ-bonding
orbital with an in-phase and out-of-phase combination of sulfur p
orbitals in the [IrS3P] equatorial plane.
2
(52) While this ligand-based orbital is nonbonding with respect to dz , it is
stabilized by interaction with the metal pz orbital and thus may be
regarded as a metal-ligand bonding orbital.
(54) For examples of the interaction of H+ with d8 metal centers, see:
Canty, A. J.; van Koten, G. Acc. Chem. Res. 1995, 28, 406-413.
2594 Inorganic Chemistry, Vol. 45, No. 6, 2006

Analysis of the Bonding in Metal Borane Compounds
Figure 11. Molecular orbitals corresponding to the 3-center-4-electron interactions in [κ3-B(mimH)3]Ir(PH3)Cl (left) and [κ4-B(mimH)3]Ir(PH3)Cl (right).
therefore, fails when there exists a metal-to-ligand MfZ
dative bond. As such, the failure to predict the correct dn
configuration provides a further illustration of how oxidation
numbers may give a misleading description of the bonding
situation in molecules.
in valence and a corresponding decrease in the dn count of
the metal. In essence, therefore, the reaction is “oxidative”
in nature on the basis that the metal must use two of its
electrons to achieve the cleavage.
Conclusions
In view of the above analysis of the MfBR3 interaction,
it is evident that the proposed dn configuration in other
{[B(mimR)3]M} derivatives should be re-evaluated. For
example, [κ4-B(mimMe)3]Pt(PPh3), which has been described
as the first five-coordinate zerovalent d10 platinum complex,14
is more appropriately described as possessing a divalent d8
Pt center.
In summary, a series of iridium and rhodium complexes
that feature MfB dative bonds, namely [κ3-B,S,S-B(mimR)3]-
t
Ir(CO)(PPh3)H (R ) But, Ph) and [κ4-B(mimBu )3]M(PPh3)-
Cl (M ) Rh, Ir), has been synthesized and structurally
characterized by X-ray diffraction. Details of the MfB
interaction in these complexes have been elucidated by
molecular orbital analyses which indicate that the iridium
centers possess d6 configurations. The d6 configuration is in
accord with the value predicted by using a method that
employs the valence to determine dn but is not in accord
with the d8 configuration that is predicted by using the
oxidation number. Thus, even though B(mimR)3 may be
regarded as a neutral closed-shell ligand, coordination to a
dn transition metal via the boron results in the formation of
Finally, it has been noted that the term “oxidative addition”
is not appropriate for the reaction of a metal center, M, with
a B-H bond of a [R3B-H]- derivative because the oxidation
number of M does not change in forming the product
[M(BR3)H]-.13 However, while it is certainly true that there
is no change in oxidation number of the metal if BR3 is
viewed as a neutral ligand, the reaction of a metal center
with a B-H bond of [R3B-H]- does result in an increase
Inorganic Chemistry, Vol. 45, No. 6, 2006 2595

Landry et al.
the volume of the sample was further reduced to deposit more
material. The mixture was filtered, and the solid was washed with
t
pentane (5 mL) and dried in vacuo giving [κ3-B,S,S-B(mimBu )3]-
Ir(CO)(PPh3)H as a white powder (32 mg, 25%). Anal. Calcd for
C40H49BIrN6OPS3: C, 50.0%; H, 5.1%; N, 8.8%. Found: C, 50.4%;
H, 5.3%; N, 9.5%. IR Data (KBr pellet, cm-1): 2963 (w), 2923
(m), 2853 (w), 2100 (w), 1989 (s), 1479 (w), 1435 (m) 1388 (s),
1342 (m), 1254 (w), 1188 (s), 1158 (s), 1094 (s), 1027 (m), 821
(w), 800 (w), 746 (w), 695 (s), 516 (m). 1H NMR data (C6D6, ppm):
-10.90 [s, Ir-H], 1.14 [s, 9H of But], 1.39[s, 9H of But], 1.83 [s,
9H of But], 6.47 [m, 1H of imidazole ring], 6.64 [d, JH-H ) 2 Hz,
1H of imidazole ring], 6.66 [m, 1H of imidazole ring], 6.67 [d,
JH-H ) 2 Hz, 1H of imidazole ring], 6.70 [d, JH-H ) 2 Hz, 1H of
imidazole ring], 6.73 [d, JH-H ) 2 Hz, 1H of imidazole ring], 6.95
[m, 3H of PPh3], 7.05 [m, 6H of PPh3], 7.81 [m, 6H of PPh3]. 31
P
NMR (C6D6): 5.9 ppm [s]. Mass spectrum: m/z ) 959.7 {M -
1}+.
Synthesis of [κ3-B,S,S-B(mimPh)3]Ir(CO)(PPh3)H. A mixture
of Ir(PPh3)2(CO)Cl (19 mg, 0.024 mmol) and [TmPh]Li (17 mg,
0.031 mmol) was treated with benzene (2 mL) and occasionally
shaken at room temperature over a period of 12 h. Over this period,
the yellow solution became colorless and a pale brown deposit
formed. The mixture was filtered, and pentane (0.5 mL) was added
to induce precipitation. The volume of the sample was further
reduced to deposit more material. The mixture was filtered, and
the solid was dried in vacuo giving [κ3-B,S,S-B(mimPh)3]Ir(CO)-
Figure 12. Generic molecular orbital diagram for coordination of a BR3
moiety to a dn transition metal center. The formation of the M-B bond
requires a pair of electrons that must be supplied by the metal center, and
so the metal center in the adduct adopts a dn-2 configuration.
1
(PPh3)H as a white powder (5 mg, 20%). H NMR data (C6D6,
a complex in which the metal center possesses a dn-2
configuration because the metal must supply both electrons
for the M-B bond. As such, the present study emphasizes
that the use of oxidation numbers to determine the dn
configuration fails when there is a metal-to-ligand MfZ
dative bond.
ppm): -10.8 [broad s, 1H of Ir-H], 6.46 [m, 1H of imidazole ring
A], 6.56 [m, 1H of imidazole ring B], 6.61 [d, JH-H ) 2 Hz, 1H of
imidazole ring C], 6.73-6.75 [m, 3 aryl H and 1H of imidazole
ring B], 6.77 [d, JH-H ) 2 Hz, 1H of imidazole ring A], 6.80 [d,
JH-H ) 2 Hz, 1H of imidazole ring C], 6.89 [m, 6 aryl H], 6.90-
7.15 [m, 9 aryl H], 7.02-7.06 [m, 2 aryl H], 7.25-7.28 [m, 2 aryl
H], 7.72-7.77 [m, 6 aryl H], 7.81-7.84 [m, 2 aryl H]. 31P NMR
data (C6D6, ppm): 4.56 [s, PPh3].
Experimental Section
t
Synthesis of [κ4-B(mimBu )3]Rh(PPh3)Cl. A mixture of (COD)-
t
Rh(PPh3)Cl (15 mg, 0.03 mmol) and [TmBu ]K (15 mg, 0.03 mmol)
General Considerations. All manipulations were performed
using a combination of glovebox, high-vacuum, and Schlenk
techniques under a nitrogen or argon atmosphere. Solvents were
was treated with benzene (3 mL) to give a solution which deposited
yellow-orange crystals over a period of 3 days. The crystals were
isolated by decanting the solution and extracted into MeCN (2 mL).
The extract was allowed to stand at room temperature, thereby
1
purified and degassed by standard procedures. H and 13C NMR
spectra were measured on Bruker 300 DRX, Bruker 400 DRX, and
Bruker Avance 500 DMX spectrometers. Chemical shifts are
reported in ppm relative to SiMe4 (δ ) 0) and were referenced
internally with respect to the protio solvent impurity (δ ) 7.16 for
C6D5H and 1.94 for CD2HCN). 31P NMR spectra were referenced
relative to 85% H3PO4 (δ ) 0) using P(OMe)3 in C6D6 as an
external reference (δ ) 141.0). Coupling constants are given in
hertz. Infrared spectra were recorded on Nicolet Avatar 370 DTGS
spectrometers and are reported in wavenumbers. Mass spectra were
obtained on a Micromass Quadrupole-Time-of-Flight mass spec-
trometer using fast atom bombardment (FAB) and a m-nitrobenzyl
alcohol matrix. (COD)RhCl(PPh3)55 and (COD)Ir(PPh3)Cl56 were
obtained by the literature methods.
t
depositing [κ4-B(mimBu )3]Rh(PPh3)Cl as yellow-orange needles57
which were isolated by decanting and dried in vacuo (15 mg, 58%).
Anal. Calcd for C39H48BClRhN6PS3: C, 53.4%; H, 5.5%; N, 9.6%.
1
Found: C, 52.4%; H, 5.2%; N, 9.2%. H NMR data (CD3CN,
ppm): 1.39 [s, 18H of 2 But], 1.66 [s, 9H of But], 6.31 [d, JH-H
)
2 Hz, 1H of imidazole ring], 6.47 [br, 2H of imidazole ring], 6.86
[d, JH-H ) 2 Hz, 2H of imidazole ring], 6.94 [d, JH-H ) 2 Hz, 1H
of imidazole ring], 7.17-7.80 [m, 15 H of PPh3]. 31P NMR data
1
(CD3CN, ppm): 27.5 [d, JRh-P ) 121 Hz, Rh-PPh3]. Mass
spectrum: m/z ) 876.3 {M - 1}+.
t
Generation of [κ4-B(mimBu )3]Rh(PPh3)H. The hydride complex
t
1
[κ4-B(mimBu )3]Rh(PPh3)H may be observed via H NMR spec-
t
Synthesis of [κ3-B,S,S-B(mimBu )3]Ir(CO)(PPh3)H. A mixture
troscopy as an intermediate in the reaction of (COD)Rh(PPh3)Cl
t
of Ir(PPh3)2(CO)Cl (100 mg, 0.128 mmol) and [TmBu ]K (132 mg,
t
t
with [TmBu ]K. In addition, [κ4-B(mimBu )3]Rh(PPh3)H may be
t
generated by treating [κ4-B(mimBu )3]Rh(PPh3)Cl with LiBH4. A
0.256 mmol) was treated with benzene (5 mL) and stirred overnight
at room temperature. Over this period, the yellow solution became
colorless and a white deposit formed. The mixture was filtered,
and the filtrate was concentrated to approximately half the initial
volume. Pentane (5 mL) was added to induce precipitation, and
t
solution of [κ4-B(mimBu )3]Rh(PPh3)Cl (5 mg) in CD3CN (0.7 mL)
was treated with LiBH4 (3 mg). The reaction was monitored by 1H
t
NMR spectroscopy and demonstrated the formation of [κ4-B(mimBu )3]-
(57) Note that X-ray diffraction indicates that the needles obtained prior
t
(55) Chatt, J.; Venanzi, L. M. J. Chem. Soc. 1957, 4735-4741.
(56) Crabtree, R. H.; Morris, G. E. J. Organomet. Chem. 1977, 135, 395-
403.
to desolvation are of composition [κ4-B(mimBu )3]Rh(PPh3)Cl‚2MeCN‚
2C6H6; as such, it is important not to dry the crystals and remove all
traces of benzene before crystallizing from acetonitrile.
2596 Inorganic Chemistry, Vol. 45, No. 6, 2006

Analysis of the Bonding in Metal Borane Compounds
Table 1. Crystal, Intensity Collection, and Refinement Data
t
t
t
[κ3-B,S,S-B(mimBu )3]-
[κ3-B,S,S-B(mimPh)3]-
[κ4-B(mimBu )3]-
[κ4-B(mimBu )3]-
Ir(CO)(PPh3)H
Ir(CO)(PPh3)H
Ir(PPh3)Cl
Rh(PPh3)Cl
lattice
formula
fw
space group
a/Å
b/Å
c/Å
triclinic
C44H55BN8OPS3Ir
1042.12
P1h
11.247(1)
14.301(1)
16.423(1)
99.379(1)
97.000(1)
103.366(1)
2500.4(2)
2
triclinic
C48H40BN7OPS3Ir
1061.03
P1h
12.243(1)
14.218(8)
16.425(1)
95.879(1)
110.785(1)
113.132(1)
2358.9(2)
2
triclinic
monoclinic
C55H66BClN8PS3Rh
1115.5
P21/m
9.765(1)
19.775(1)
14.192(1)
90
92.316(2)
90
2738.4(3)
2
C53H76BClN6O3.5PS3Ir
1218.81
P1h
14.527(4)
17.832(5)
24.242(7)
89.748(6)
78.524(4)
85.921(5)
6138(3)
4
R/°
â/°
γ/°
V/Å3
Z
temp (K)
radiation (λ, Å)
F (calcd), g cm-3
µ (Mo KR), mm-1
θ max, deg
no. of data
no. of params
R1
243
0.71073
1.384
2.867
28.3
10 918
520
0.0295
0.0815
1.079
243
0.71073
1.494
3.040
28.3
15 998
556
0.0227
0.0633
243
0.71073
1.319
2.39
23.3
17 189
937
0.0694
0.144
243
0.71073
1.353
0.549
28.3
19 261
344
0.0343
0.0613
1.033
wR2
GOF
1.035
1.067
Rh(PPh3)H after 24 h at room temperature. 1H NMR data (CD3CN,
ppm): -16.2 [dd, JRh-H ) 17 Hz, JP-H ) 6 Hz, Rh-H), 1.56 [s,
18H of 2 But], 1.68 [s, 9H of But], 6.70 [d, JH-H ) 2 Hz, 1H of
imidazole ring], 7.02 [br, 1H of imidazole ring], 7.06 [br, 2H of
imidazole ring], 7.21 [d, JH-H ) 2 Hz, 1H of imidazole ring], 7.32-
7.38 and 7.55-7.61 [m, 15 H, PPh3].
Computational Details. All calculations were carried out using
DFT as implemented in the Jaguar 5.0 and 6.0 suite of ab initio
quantum chemistry programs.59 Geometry optimizations were
performed with the B3LYP functional and the 6-31G** (C, H, N,
B, O, S, P) and LACVP (Rh, Ir) basis sets. Molecular orbital
analyses were performed with the aid of JIMP,60 which
employs Fenske-Hall calculations61 and visualization using
MOPLOT.62
t
Synthesis of [κ4-B(mimBu )3]Ir(PPh3)Cl. A mixture of (COD)-
t
IrCl(PPh3) (34 mg, 0.06 mmol) and [TmBu ]K (30 mg, 0.06 mmol)
was treated with benzene (1 mL) and allowed to stand at room
temperature for 1 h. Acetonitrile (70 µL) was added to the solution
and stirred, resulting in the deposition of small yellow-orange
crystals after standing at room temperature for a period of 2 days.
The crystals were isolated by filtration and extracted into THF (2
mL). The extract was allowed to stand at room temperature, thereby
Acknowledgment. We thank the National Science Foun-
dation (CHE-03-50498) for support of this research.
Supporting Information Available: Cartesian coordinates for
geometry optimized structures (5 pages); crystallographic data in
CIF format. This material is available free of charge via the Internet
at http://pubs.acs.org.
t
depositing [κ4-B(mimBu )3]Ir(PPh3)Cl as large yellow-orange crys-
tals, which were isolated by decanting and dried in vacuo (ca. 10
1
mg, 18%). H NMR data (CD3CN, ppm): 1.39 [s, 18H of 2 But],
1.66 [s, 9H of But], 6.30 [d, JH-H ) 2 Hz, 1H of imidazole ring],
6.46 [d, JH-H ) 2 Hz, 2H of imidazole ring], 6.95 [d, JH-H ) 2
Hz, 2H of imidazole ring], 6.99 [d, JH-H ) 2 Hz, 1H of imidazole
ring], 7.12-7.39 [m, Rh-PPh3]. Mass spectrum: m/z ) 966.5
{M - 1}+.
X-ray Structure Determinations. X-ray diffraction data were
collected on a Bruker P4 diffractometer equipped with a SMART
CCD detector and crystal data, data collection, and refinement
parameters are summarized in Table 1. The structures were solved
using direct methods and standard difference map techniques and
were refined by full-matrix least-squares procedures on F2 with
SHELXTL (Version 5.10).58
IC051975T
(58) Sheldrick, G. M. SHELXTL, An Integrated System for SolVing, Refining
and Displaying Crystal Structures from Diffraction Data; University
of Go¨ttingen: Go¨ttingen, Germany, 1981.
(59) Jaguar 6.0; Schro¨dinger, Inc.: Portland, 2005.
(60) Manson, J.; Webster, C. E.; Hall, M. B. JIMP DeVelopment Version
1.1.5 (built for Windows PC and Redhat Linux 7.3); Department of
Chemistry, Texas A&M University: College Station, TX, (http://
www.chem.tamu.edu/jimp/), 2005.
(61) Hall, M. B.; Fenske, R. F. Inorg. Chem. 1972, 11, 768-775.
(62) Lichtenberger, D. L. MOPLOT, Version 2.0; Department of Chemistry,
University of Arizona: Tuscon, 1993.
Inorganic Chemistry, Vol. 45, No. 6, 2006 2597