All-organometallic analogues of Zeise's salt for the three group 10 metals

Article abstract of DOI:10.1021/om0502077

Ethene has been found to be able to split the electron-deficient pentafluorophenyl bridging system in [NBu₄]₂[{M(C ₆F₅)₂}(μ-C₆F₅) ₂] to give the corresponding mononucl

Full text of DOI:10.1021/om0502077

Organometallics 2005, 24, 3539-3546
3539
All-Organometallic Analogues of Zeise’s Salt for the
Three Group 10 Metals
Juan Fornie´s,* Antonio Mart´ın, L. Francisco Mart´ın, and Babil Menjo´n
Instituto de Ciencia de Materiales de Arago´n, Facultad de Ciencias,
Universidad de Zaragoza-CSIC, C/ Pedro Cerbuna 12, E-50009 Zaragoza, Spain
Athanassios Tsipis*
Laboratory of Inorganic and General Chemistry, Department of Chemistry,
University of Ioannina, Ioannina 451 10, Greece
Received March 18, 2005
Ethene has been found to be able to split the electron-deficient pentafluorophenyl bridging
system in [NBu4]2[{M(C6F5)2}(µ-C6F5)2] to give the corresponding mononuclear compounds
[NBu4][M(C6F5)3(η2-C2H4)] (M ) Pt (1), Pd (2)) in reasonable yield. Compounds 1 and 2 are
well-behaved species and have been isolated and characterized by analytical and spectroscopic
methods. The crystal structure of 2, as established by X-ray diffraction methods, reveals
that the Pd atom is in an approximately SP-4 environment defined by the ipso-C atoms of
the three σ-bound C6F5 groups (C6F5-κ1) and the midpoint between the doubly bonded C
atoms of the metal π-bound ethene molecule (η2-C2H4). The ethene molecule is coordinated
upright, and the CdC bond length (133.6(6) pm) is the same as in the free ligand (133.7(2)
pm). The nickel homologue [NBu4][Ni(C6F5)3(η2-C2H4)] (3), formed by the low-temperature
reaction of [NBu4]2[Ni(C6F5)4] with B(C6F5)3 in the presence of C2H4, could not be isolated
but only spectroscopically detected in solution. The experimentally established stability of
the [M(C6F5)3(η2-C2H4)]- species has been found to follow the trend calculated by DFT
methods for the M-(η2-C2H4) bond strength: Pt > Pd > Ni. Furthermore, quantitative
estimates of back-bonding in the [M(C6F5)3(η2-C2H4)]- and [MCl3(η2-C2H4)]- anions were
obtained using NBO analyses of electron populations of the relevant donor-acceptor orbitals
and the second-order stabilization energy associated with the charge transfer (CT) interac-
tions describing the back-bonding phenomenon.
Introduction
to the model formulated by Dewar to rationalize the
olefin-to-metal bond7 that was shortly thereafter applied
to the platinum case by Chatt and Duncanson.8 The
molecular structure of Zeise’s anion was definitely
established by three-dimensional single-crystal X-ray9
and neutron diffraction methods.10
In the first half of the 19th century, Zeise prepared
and isolated potassium and ammonium salts of the
[PtCl3(η2-C2H4)]- anion,1 which have been generally
acknowledged to be the first organometallic species ever
isolated in analytically pure form and reported in the
scientific literature.2 Although the initial formulation
of the potassium saltsusually referred to as Zeise’s
saltsas an ethene derivative was challenged by Liebig,3
the discoverer stood by his original opinion,4 a point of
view that was later confirmed by independent work.5
The constitutional nature of the complex anion [PtCl3-
(η2-C2H4)]- as well as that of its bromo analogue6 was,
however, not fully understood until much later, thanks
The analogous palladium derivative [PdCl3(η2-C2H4)]-
is considered to be a key intermediate species in the Pd-
catalyzed oxidation of ethene (Wacker process)11 and has
been spectroscopically detected in solution of nonprotic
solvents.12 The corresponding [NBu4]+ salt was claimed
to have been isolated as a pale yellow solid that lost
(7) Dewar, M. J. S. Bull. Soc. Chim. Fr. 1951, C71. For Dewar’s
comment on this subject, see: Dewar, M. J. S.; Ford, G. P. J. Am. Chem.
Soc. 1979, 101, 783. For a historical perspective on the effect of Dewar’s
bonding model for metal-alkene complexes on the development of
organometallic chemistry, see: Mingos, D. M. P. J. Organomet. Chem.
2001, 635, 1.
(8) Chatt, J.; Duncanson, L. A. J. Chem. Soc. 1953, 2939. See also:
Leigh, G. J. Coord. Chem. Rev. 1991, 108, 1.
(9) Jarvis, J. A. J.; Kilbourn, B. T.; Owston, P. G. Acta Crystallogr.,
Sect. B 1971, 27, 366. Black, M.; Mais, R. H. B.; Owston, P. G. Acta
Crystallogr., Sect. B 1969, 25, 1753. In these papers, reference to
previous studies by two-dimensional X-ray diffraction methods can also
be found.
* To whom correspondence should be addressed. E-mail for J.F.:
juan.fornies@unizar.es. E-mail for A.T. (for comments concerning the
theoretical calculations): attsipis@cc.uoi.gr.
(1) Zeise, W. C. Ann. Phys. Chem. 1831, 21, 497, 542.
(2) Seyferth, D. Organometallics 2001, 20, 2. Thayer, J. S. Adv.
Organomet. Chem. 1975, 13, 1. Thayer, J. S. J. Chem. Educ. 1969, 46,
442.
(3) Liebig, J. Ann. Pharm. 1834, 9, 1. Liebig, J. Ann. Phys. Chem.
1834, 31, 321. Liebig, J. Ann. Pharm. 1837, 23, 12.
(4) Zeise, W. C. Danske Selsk. Skr. 1837, 6, 333. Zeise, W. C. Ann.
Phys. Chem. 1837, 40, 234. Zeise, W. C. Ann. Pharm. 1837, 23, 1.
(5) Griess, P.; Martius, C. A. Ann. Chem. 1861, 120, 324. Birnbaum,
K. Ann. Chem. 1868, 145, 68. Anderson, J. S. J. Chem. Soc. 1934, 971.
Gel’man, A. D.; Litvak, I. B. Izv. Plat. 1939, 16, 29.
(6) Chojnacki, C. Z. Chem. 1870, 419.
(10) Love, R. A.; Koetzle, T. F.; Williams, G. J. B.; Andrews, L. C.;
Bau, R. Inorg. Chem. 1975, 14, 2653.
(11) Jira, R. In Applied Homogeneous Catalysis with Organometallic
Compounds; Cornils, B., Herrmann, W. A., Eds.; Wiley-VCH: Wein-
heim, Germany, 2000; Section 2.4.1, pp 374-393.
10.1021/om0502077 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 06/08/2005

3540 Organometallics, Vol. 24, No. 14, 2005
Fornie´s et al.
Scheme 1
C2H4 fairly rapidly at room temperature, but no analyti-
cal or any other solid-state data were pro-
vided.12d The nickel derivative, [NiCl3(η2-C2H4)]-, has,
to the best of our knowledge, never been detected. The
decrement in information about the [MCl3(η2-C2H4)]-
species for the group 10 metals on going from Pt to Ni
might be related to the M-alkene bond energy, which,
according to recent theoretical calculations, has been
estimated to sharply decrease in the order Pt (33.9 kcal
mol-1) > Pd (19.0 kcal mol-1) > Ni (6.1 kcal mol-1).13
The C6F5 group has been assigned an electron-
withdrawing character only slightly weaker than that
exerted by the chloro ligand.14 In our experience,
however, the C6F5 group has a number of other proper-
ties that, in some instances, make this ligand advanta-
geous over Cl-. Thus, the higher steric requirements of
C6F5 may exert a shielding effect toward other coordi-
nated ligands as well as toward vacant coordination
sites.15 Moreover, metal-coordinated Cl and C6F5 ligands
exhibit very different dissociation abilities, but perhaps
the most distinctive difference may be the much lower
residual nucleophilic character of metal-coordinated
C6F5 groups compared to that of metal-coordinated
chloro ligands. Considering that all these properties
could add to favor the stabilization of metal-coordinated
ethene and encouraged by our success in isolating and
characterizing the complete series of square-planar
carbonyl derivatives cis-[M(C6F5)2(CO)2] (M ) Ni, Pd,
Pt),16 we sought to prepare salts of the [M(C6F5)3(η2-
C2H4)]- anions for all three group 10 metals. The results
obtained are reported and discussed in the present
paper. Moreover, the key factors determining the stabil-
ity of the novel Zeise type salts have been inferred by
electronic structure calculation methods at the DFT
level of theory, while quantitative estimates of back-
bonding were obtained using NBO analyses of electron
populations of the relevant donor-acceptor orbitals and
the second-order stabilization energy associated with
the charge transfer (CT) interactions describing the
back-bonding phenomenon.
Scheme 2
bridging system: [NBu4]2[{M(C6F5)2}2(µ-C6F5)2].17,18 Al-
though several alkyl and aryl groups (e.g. Me, Et, Ph,
mesityl, ...) are known to be able to form [3c,2e] bonds
with main-group elements as well as with transition
metals, the case of the C6F5 group is especially remark-
able because of its high electron-withdrawing char-
acter.14 Indeed, the pentafluorophenyl derivatives
[NBu4]2[{M(C6F5)2}2(µ-C6F5)2] show interesting reactiv-
ity patterns associated with the electron-deficient bridg-
ing system, including different redox processes and
addition of electrophiles to the para position of the C6F5
ring.19 In addition, a good number of donor ligands are
known to cause the splitting of the bridging system that
can occur in a symmetric or an asymmetric way (Scheme
1), depending on the nature of the metal and of the
ligand used.17,20 We will now discuss the results ob-
tained when L ) C2H4.
Synthesis and Characterization of [NBu4][Pt-
(C6F5)3(η2-C2H4)] (1). Bright yellow CH2Cl2 solutions
of [NBu4]2[{Pt(C6F5)2}2(µ-C6F5)2] gradually turn color-
less when put under an ethene atmosphere at room
temperature. This change in color is due to the sym-
metric splitting of the bridging system (Scheme 2), as
clearly seen by 19F NMR spectroscopy: the low-field
resonances characteristic of bridging C6F5 groups disap-
pear upon exposure to C2H4, giving rise to two sets of
signals in a 2:1 ratio which are typical of square-planar
(SP-4) [Pt(C6F5)3(L)]- species. The signal corresponding
to the ortho F atoms of the trans-to-ethene C6F5 group
Results and Discussion
The reaction of the anionic homoleptic species [NBu4]2-
[M(C6F5)4] (M ) Pt, Pd) with the corresponding solvento
derivatives cis-[M(C6F5)2(THF)2] is known to proceed
with extrusion of the very labile THF ligands and
formation of the homodinuclear compounds [NBu4]2[M2-
(C6F5)6] (Scheme 1).17 In these latter homoleptic species,
four C6F5 groups act as terminal σ-bound ligands, while
the remaining two build an electron-deficient, doubly
(12) (a) Bencze, EÄ .; Pa´pai, I.; Mink, J.; Goggin, P. L. J. Organomet.
Chem. 1999, 584, 118. (b) Olsson, A.; Kofod, P. Inorg. Chem. 1992, 31,
183. (c) Olsson, L.-F.; Olsson, A. Acta Chem. Scand. 1989, 43, 938. (d)
Goodfellow, R. J.; Goggin, P. L.; Duddell, D. A. J. Chem. Soc. A 1968,
504.
(13) Stro¨mberg, S.; Svensson, M.; Zetterberg, K. Organometallics
1997, 16, 3165.
(14) Sheppard, W. A. J. Am. Chem. Soc. 1970, 92, 5419. Tolman, C.
A. J. Am. Chem. Soc. 1970, 92, 2953.
(15) Alonso, P. J.; Fornie´s, J.; Garc´ıa-Monforte, M. A.; Mart´ın, A.;
Menjo´n, B. Organometallics 2005, 24, 1269.
(16) Fornie´s, J.; Mart´ın, A.; Mart´ın, L. F.; Menjo´n, B.; Kalamarides,
H. A.; Rhodes, L. F.; Day, C. S.; Day, V. W. Chem. Eur. J. 2002, 8,
4925. Uso´n, R.; Fornie´s, J.; Toma´s, M.; Menjo´n, B. Organometallics
1986, 5, 1581. Uso´n, R.; Fornie´s, J.; Toma´s, M.; Menjo´n, B. Organo-
metallics 1985, 4, 1912.
(17) Uso´n, R.; Fornie´s, J.; Toma´s, M.; Casas, J. M.; Navarro, R. J.
Chem. Soc., Dalton Trans. 1989, 169.
(18) Uso´n, R.; Fornie´s, J.; Toma´s, M.; Casas, J. M.; Cotton, F. A.;
Falvello, L. R.; Llusar, R. Organometallics 1988, 7, 2279.
(19) Uso´n, R.; Fornie´s, J.; Toma´s, M.; Casas, J. M.; Cotton, F. A.;
Falvello, L. R.; Feng, X. J. Am. Chem. Soc. 1993, 115, 4145.
(20) Uso´n, R.; Fornie´s, J.; Toma´s, M.; Mart´ınez, F.; Casas, J. M.;
Fortun˜o, C. Inorg. Chim. Acta 1995, 235, 51.

All-Organometallic Analogues of Zeise’s Salt
Organometallics, Vol. 24, No. 14, 2005 3541
3
shows platinum satellites with J(195Pt,F) ) 480 Hz, a
[Pd(C6F5)3(η2-C2H4)] (2) is formed in a similar way by
reaction of the corresponding dinuclear precursor [NBu4]2-
[{Pd(C6F5)2}2(µ-C6F5)2] in CH2Cl2 solution with C2H4 at
0 °C (Scheme 2). Even at this temperature, the reaction
proceeds considerably faster (5 min) than is observed
for the platinum homologue at room temperature (90
min), a fact that is related to the higher reactivity
usually observed for Pd as compared to that for Pt
derivatives.25 On the other hand, complex 2 is consider-
ably less stable than 1, since it easily reverts to the
parent species under C2H4 release at temperatures
above 0 °C or when the ethene pressure is reduced
(Scheme 2).
The IR vibrational behavior of 2 is roughly similar to
that observed for 1. In this case, however, the three
expected IR-active X-sensitive vibration modes of the
C6F5 groups21 are not well resolved, giving rise to two
defined signals at 784 and 766 cm-1 together with a
shoulder at 797 cm-1. Again, an absorption correspond-
ing to the ν(CdC) vibration could not be located with
certainty in the IR spectrum of 2. However, the pre-
dicted unscaled harmonic vibrational frequencies as-
sociated with the stretching vibration of the coordinated
ethene ligand combined with the symmetric and asym-
metric CH2 scissoring vibrations in complex 2 absorb
at 1337 (symmetric), 1479 (asymmetric), and 1627 cm-1
(symmetric) as very weak absorption bands (relative
intensities less than 1%). The shifting of the ν(CdC)
vibrations to higher frequencies in complex 2 relative
to those of complex 1 is compatible with the CdC bond
length expected in 2 being shorter than in 1 (see below).
As observed in complex 1, the 19F NMR spectrum of
2 also shows two sets of C6F5 signals in a 2:1 ratio. The
C2H4 ligand gives rise to singlets in the 1H and 13C{1H}
NMR spectra of 2 at the following chemical shift
values: δH 5.12 ppm, δC 97.6 ppm. The δH value
observed for 2 is very similar to that reported for the
chloro derivative [PdCl3(η2-C2H4)]- in [2H]tetrahydro-
furan solution (δH 5.09 ppm).12b The fact that the ethene
ligand in the [MR3(η2-C2H4)]- species is considerably
less shielded for Pd than for Pt can be related to the
less efficient π-back-bonding ability of PdII compared
with that of PtII, as will be discussed later on. Since the
original formulation of the π-complex theory of metal-
olefin complexes, put forward by Dewar,7 the metal-
alkene bond is considered to be the result of the
following two components: (1) a dative bond from the
filled π MOs of the alkene to the Lewis acidic MLn metal
fragment (σ donation), and (2) a reverse dative bond
from filled orbitals of the MLn moiety to empty π*
orbitals of the alkene (π back-donation). There is no
general agreement, however, on the relative importance
of these two components,26 probably because of releasing
electron density from π bonding MOs as well as adding
it into π* antibonding MOs should result in a neat CdC
bond weakening. The geometry of the Pd complex
value considerably higher than that observed for the
ortho F atoms belonging to the mutually trans C6F5
groups, for which J(195Pt,F) ) 311 Hz.
3
The addition of ethene-saturated n-hexane to the
reaction medium causes the precipitation of a white
solid, which is identified as [NBu4][Pt(C6F5)3(η2-C2H4)]
(1). When the mother liquor is allowed to stand at -30
°C overnight, a second fraction of 1 is additionally
obtained (60% overall yield). Complex 1 stands without
decomposition for several hours in solution at room
temperature and can be handled in the solid state for
short periods of time in the air; it is therefore reasonably
stable both to heat and against the action of air.
The solid-state IR spectrum of 1 (KBr) shows a
number of absorptions which are typical of the metal-
coordinated C6F5 group. These include three strong and
well-defined absorptions at 798, 785, and 775 cm-1
,
attributable to the so-called X-sensitive vibration modes,21
which, from a symmetry point of view, are known to
behave as the M-R stretching vibrations (C2v, IR active
ΓM-R fundamentals: 2A1 + B1). No absorption in the
IR spectrum of 1 could be unambiguously assigned to
the ν(CdC) vibration.22 B3LYP/LANL2DZ calculations
on complex 1 predicted that the unscaled harmonic
vibrational frequencies associated with the stretching
vibration of the coordinated ethene ligand combined
with the symmetric and asymmetric CH2 scissoring
vibrations occur at 1307 (symmetric), 1501 (asym-
metric), and 1584 (symmetric) cm-1 as very weak
absorption bands (relative intensities less than 3%).
Coordination of ethene to the “Pt(C6F5)3-” fragment in
1 is, however, shown by 1H and 13C NMR spectroscopy.
+
Aside from the signals corresponding to the NBu4
cation, both the 1H and 13C{1H} NMR spectra of 1 show
sharp singlets at the following chemical shift values: δH
4.33 ppm and δC 78.9 ppm. Each of these signals is
flanked by clearly defined platinum satellites with
coupling constants J(195Pt,H) ) 47 Hz and J(195Pt,C)
) 88 Hz, respectively. All of these values are comparable
with those reported for Zeise’s salt: δH ([2H]methanol)
4.36 ppm, 2J(195Pt,H) ) 65.2 Hz;12b δC ∼70 ppm,
1J(195Pt,C) ≈ 195 Hz.23 The marked upfield shifts
observed in the [PtR3(η2-C2H4)]- anions (R ) Cl, C6F5)
with respect to the free ethene values (δH 5.28 ppm, δC
2
1
123.3 ppm)24 denote significant shielding effects exper-
ienced by the ligand upon coordination to the “PtR3
fragments.
-
”
Synthesis and Characterization of [NBu4][Pd-
(C6F5)3(η2-C2H4)] (2). The analogous Pd species [NBu4]-
(21) Maslowsky, E., Jr. Vibrational Spectra of Organometallic
Compounds; Wiley: New York, 1977; pp 437-442. Uso´n, R.; Fornie´s,
J. Adv. Organomet. Chem. 1988, 28, 219.
(22) A weak absorption appearing at ca. 1525 cm-1 in the solid-state
Raman spectrum of Zeise’s salt has been attributed to the ν(CdC)
vibration strongly coupled with CH2 scissoring and CH2 wagging
modes: Mink, J.; Papai, I.; Gal, M.; Goggin, P. L. Pure Appl. Chem.
1989, 61, 973. However, there is no universal agreement about this
kind of assignment, as discussed by: Hartley, F. R. In Comprehensive
Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W.,
Eds.; Pergamon Press: Oxford, U.K., 1982; Vol. 6, Section 39.7.3, pp
632-680.
(25) Hartley, F. R. The Chemistry of Platinum and Palladium;
Applied Science: London, U.K., 1973.
(26) As an example, the ethene ligand in salts of the [PtCl3(η2-
C2H4)]- anion has been assigned a strong σ-donor and a very weak
π-acceptor ability (Chang, T.-H. Zink, J. I. J. Am. Chem. Soc. 1984,
106, 287) and alternatively a good σ-donor and a strong π-acceptor
character (Jaw, H.-R. C.; Chang, T.-H.; Zink, J. I. Inorg. Chem. 1987,
26, 4204). Both contradicting conclusions were drawn from the
interpretation of low-temperature, single-crystal polarized electronic
spectroscopy data.
(23) Mann, B. E.; Taylor, B. F. 13C NMR Data for Organometallic
Compounds; Academic Press: London, U.K., 1981; Table 2.10, pp 184-
199.
(24) Pretsch, E.; Clerc, T.; Seibl, J.; Simon W. Tabellen zur Struk-
turaufkla¨rung organischer Verbindungen mit spektroskopischen Meth-
oden, 3rd ed.; Springer-Verlag: Berlin, Germany, 1986.

3542 Organometallics, Vol. 24, No. 14, 2005
Fornie´s et al.
Table 1. Crystal Data and Structure Refinement
Details for 2
Table 2. Selected Interatomic Distances and
Anglesa and Their Estimated Standard Deviations
for 2
empirical formula
fw
C36H40F15NPd
878.09
Bond Distances (pm)
cryst size (mm)
temp (K)
cryst system
space group
a (pm)
0.48 × 0.31 × 0.13
Pd(1)-C(1)
Pd(1)-C(7)
Pd(1)-C(13)
Pd(1)-C(19)
Pd(1)-C(20)
C(1)-C(2)
206.9(4)
202.8(3)
206.6(4)
225.4(4)
225.7(4)
138.0(5)
137.8(5)
137.0(4)
136.7(4)
138.9(5)
138.1(5)
135.1(4)
136.4(4)
137.5(5)
138.7(5)
136.8(4)
136.8(4)
133.6(6)
95.3(5)
Pd(2)-C(21)
Pd(2)-C(27)
Pd(2)-C(33)
Pd(2)-C(39)
Pd(2)-C(40)
C(21)-C(22)
C(21)-C(26)
C(22)-F(22)
C(26)-F(26)
C(27)-C(28)
C(27)-C(32)
C(28)-F(28)
C(32)-F(32)
C(33)-C(34)
C(33)-C(38)
C(34)-F(34)
C(38)-F(38)
C(39)-C(40)
C(39)-H(39A)
C(39)-H(39B)
C(40)-H(40A)
C(40)-H(40B)
207.4(3)
203.5(4)
206.4(3)
226.3(4)
224.5(4)
139.1(5)
137.9(5)
137.6(4)
136.9(4)
137.1(5)
139.1(5)
135.5(4)
135.8(4)
137.9(5)
138.8(5)
136.9(4)
136.8(4)
131.5(6)
95.1(5)
100(1)
triclinic
P1h
979.65(7)
b (pm)
1894.69(13)
2108.50(14)
68.782(1)
c (pm)
C(1)-C(6)
R (deg)
C(2)-F(2)
â (deg)
86.598(1)
C(6)-F(6)
γ (deg)
89.305(1)
C(7)-C(8)
V (nm3)
Z
3.6417(4)
C(7)-C(12)
C(8)-F(8)
4
dc (g cm-3
)
1.602
C(12)-F(12)
C(13)-C(14)
C(13)-C(18)
C(14)-F(14)
C(18)-F(18)
C(19)-C(20)
C(19)-H(19A)
C(19)-H(19B)
C(20)-H(20A)
C(20)-H(20B)
µ (mm-1
F(000)
θ range (deg)
index range
)
0.614
1776
2.30-25.06
-11 e h e 10
-22 e k e 22
-25 e l e 16
19 325
no. of rflns collected
no. of unique rflns
95.2(5)
95.2(5)
12 624 (Rint ) 0.0263)
98.0
12 624/12/979
R1 ) 0.0423, wR2 ) 0.0982
R1 ) 0.0589, wR2 ) 0.1066
1.060
95.2(5)
95.0(5)
94.9(5)
completeness to θ ) 25.03° (%)
no. of data/restraints/params
final R indices (I > 2σ(I))a
R indices (all data)
95.1(5)
Bond Angles (deg)
88.64(13) C(21)-Pd(2)-C(27) 88.76(13)
C(1)-Pd(1)-C(7)
GOFb on F2
C(1)-Pd(1)-C(13) 176.46(13) C(21)-Pd(2)-C(33) 173.02(14)
C(7)-Pd(1)-C(13)
C(1)-Pd(1)-C(19)
C(1)-Pd(1)-C(20)
87.91(14) C(27)-Pd(2)-C(33) 86.26(13)
92.10(14) C(21)-Pd(2)-C(39) 92.04(16)
92.16(15) C(21)-Pd(2)-C(40) 93.28(17)
2
2
2
a R1 ) ∑(|Fo| - |Fc|)/∑|Fo|; wR2 ) [∑w(Fo - Fc )2/∑w(Fc )2]1/2
;
w ) [σ2(Fo2) + (g1P)2 + g2P]-1; P ) 1/3[max{Fo2,0} + 2Fc ].
2
2
2
b Goodness of fit ) [∑w(Fo - Fc )2/(nobs - nparam)]1/2
.
C(7)-Pd(1)-C(19) 161.13(14) C(27)-Pd(2)-C(39) 163.73(16)
C(7)-Pd(1)-C(20) 164.36(14) C(27)-Pd(2)-C(40) 162.23(16)
C(13)-Pd(1)-C(19) 91.42(14) C(33)-Pd(2)-C(39) 91.38(16)
C(13)-Pd(1)-C(20) 90.94(15) C(33)-Pd(2)-C(40) 92.95(17)
C(19)-Pd(1)-C(20) 34.46(14) C(39)-Pd(2)-C(40) 33.91(17)
[PdCl3(η2-C2H4)]- has been the subject of several theo-
retical studies at different calculation levels.12a,13,30,31
Since, to the best of our knowledge, only spectroscopic
data are currently available for this anionic species,12
we considered it convenient to obtain unequivocal
structural information for the related [Pd(C6F5)3(η2-
C2H4)]- anion.
Pd(1)-C(1)-C(2)
Pd(1)-C(1)-C(6)
C(2)-C(1)-C(6)
Pd(1)-C(7)-C(8)
122.8(3)
122.9(3)
114.3(3)
122.7(3)
Pd(2)-C(21)-C(22) 120.9(3)
Pd(2)-C(21)-C(26) 126.2(3)
C(22)-C(21)-C(26) 112.9(3)
Pd(2)-C(27)-C(28) 121.8(3)
Pd(2)-C(27)-C(32) 123.0(3)
C(28)-C(27)-C(32) 115.2(3)
Pd(2)-C(33)-C(34) 120.7(3)
Pd(2)-C(33)-C(38) 125.7(3)
C(34)-C(33)-C(38) 113.5(3)
Pd(1)-C(7)-C(12) 122.9(3)
C(8)-C(7)-C(12) 114.4(3)
The crystal and molecular structures of 2 have been
established by X-ray diffraction methods. General crys-
tallographic information are collected in Table 1, and a
selection of bond distances and angles is given in Table
2. Two crystallographically independent [Pd(C6F5)3(η2-
C2H4)]- anions were found in the asymmetric unit of
the crystal but, because of their similarity, we will refer
to only one of them (Figure 1). The Pd atom is in an
approximately SP-4 environment defined by the ipso-C
atoms of the three σ-bound C6F5 groups (C6F5-κ1) and
the midpoint between the doubly bonded C atoms of the
metal π-bound ethene molecule (C0). The C2H4 ligand
adopts an upright coordination, the C(19)dC(20) line
forming an angle of 1.8° with the normal to the Pd
coordination plane. Only small angular deviations with
respect to the ideal SP-4 geometry are observed. How-
ever, it is interesting to note that the C-Pd-C angle
between the mutually trans C6F5 groups is slightly less
than 180° (C(1)-Pd(1)-C(13) ) 176.46(13)°), the fluo-
roaryl rings being bent away from the ethene ligand.
The C6F5 rings are almost perpendicular to the Pd
coordination plane, with which they form angles be-
Pd(1)-C(13)-C(14) 123.5(3)
Pd(1)-C(13)-C(18) 122.0(3)
C(14)-C(13)-C(18) 114.4(3)
a A correction for libration resulted in negligible differences in
the geometric parameters given.
tween 86.7° and 89.1°. The ethene ligand is practically
eclipsed with the C6F5 ring in a trans position, the
C(19)dC(20) line forming an angle of 86.2° with the
normal to the referred ring. The two mutually trans
C6F5 rings also adopt an almost eclipsed arrangement
(interplanar angle 4.8°). The CdC bond distance (C(19)-
(27) Bartell, L. S.; Roth, E. A.; Hollowell, C. D.; Kuchitsu, K.; Young,
J. E., Jr. J. Chem. Phys. 1965, 42, 2683.
(28) The R angle is defined as in: Stalick, J. K.; Ibers, J. A. J. Am.
Chem. Soc. 1970, 92, 5333.
(29) Eisenstein, O.; Hoffmann, R. J. Am. Chem. Soc. 1981, 103, 4308.
(30) Hay, P. J. J. Am. Chem. Soc. 1981, 103, 1390.
(31) Filatov, M. J.; Gritsenko, O. V.; Zhidomirov, G. M. J. Mol. Catal.
1989, 54, 462.
Figure 1. Thermal ellipsoid diagram (50% probability) of
one of the two crystallographically independent [Pd(C6F5)3-
(η2-C2H4)]- anions in 2.

All-Organometallic Analogues of Zeise’s Salt
Organometallics, Vol. 24, No. 14, 2005 3543
Scheme 3
C(20) ) 133.6(6) pm) is the same as in the free ligand
(133.7(2) pm).27 However, the C-H bonds are slightly
bent away from the Pd center, the normals of the CH2
units forming the angle R ) 26.4°.28 This R angle is twice
the deviation of the CH2 unit from the original planar
arrangement. The two Pd-C(ethene) distances are
identical within experimental error: Pd(1)-C(19) )
225.4(4) pm and Pd(1)-C(20) ) 225.7(4) pm. These
distances are considerably longer than those observed
in the [PtCl3(η2-C2H4)]- anion, where the mean Pt-C
distance is 213.1(3) pm. Longer M-C bonds and less
distortion of the alkene ligand compared with that of
the free molecule have been considered indicative of
weaker alkene coordination. A weaker M-(η2-C2H4)
interaction should imply a smaller perturbation of the
ethene π bonding system, as had already been predicted
by extended Hu¨ckel calculations on the [MCl3(η2-C2H4)]-
anions (M ) Pd, Pt).29 Consistent with this weak Pd-
(η2-C2H4) bond is the small trans influence exerted by
the C2H4 ligand in 2, showing that the C6F5 group acts
as a better σ-donor ligand toward PdII than does the η2-
coordinated ethene molecule. Thus, the Pd-C6F5 bond
trans to ethene (Pd(1)-C(7) ) 202.8(3) pm) is signifi-
cantly shorter than those corresponding to the mutually
trans C6F5 groups (Pd(1)-C(1) ) 206.9(4), Pd(1)-C(13)
) 206.6(4) pm). The opposite trend was observed in
Zeise’s salt, where the trans-to-ethene Pt-Cl distance
(234.0(2) pm) was found to be longer than those corre-
sponding to the mutually trans Cl ligands (230.2(2)
pm).10 However, according to our B3LYP/LANL2DZ
calculations on Zeise’s salt under vacuum (see below),
the trans-to-ethene Pt-Cl distance (243.1 pm) was
found to be comparable to those corresponding to the
mutually trans Cl ligands (244.5 pm). Therefore, in our
opinion, the different Pt-Cl bond distances observed in
Zeise’s salt are most probably due to electrostatic
interactions between the Cl ligands and the K+ ions in
the crystal lattice.
Scheme 4
clear that the NiII-ethene species 3 is much more
elusive than the related carbonyl derivative.
The powerful Lewis acid B(C6F5)3 has been widely
exploited as an efficient abstractor of organyl ligands
in organotransition-metal chemistry.34 To check whether
it would be able to generate the “Ni(C6F5)3-” fragment
under smooth conditions, we reacted [NBu4]2[Ni(C6F5)4]
with B(C6F5)3 in CH2Cl2 at -78 °C under a CO atmo-
sphere. This reaction was found to quantitatively yield
[NBu4][Ni(C6F5)3(CO)] by IR and NMR spectroscopic
methods (Scheme 4). If a similar reaction is carried out
under a C2H4 atmosphere, the quantitative transforma-
tion of [NBu4]2[Ni(C6F5)4] into a single C6F5-containing
metal species is observed by low-temperature 19F NMR
spectroscopy. The 2:1 integrated ratios of the two
resonance signals corresponding to the ortho F substit-
uents is in keeping with the formation of the desired
species 3 (Scheme 4). Moreover, selective decoupling
experiments demonstrate that the nuclei giving rise to
those signals belong to the same spin system, since there
exists detectable coupling between them (J ) 22 Hz).
Previous examples of through-space coupling between
ortho F substituents of chemically nonequivalent, metal-
coordinated C6F5 groups are already known.35 Such a
behavior was not observed in the heavier metal homo-
Spectroscopic Detection of [NBu4][Ni(C6F5)3(η2-
C2H4)] (3). It is not possible to use for Ni a synthetic
procedure similar to that just discussed for its heavier
metal homologues because of the lack of an appropriate
starting material in this case. All of our attempts to
prepare the dinuclear compound [NBu4]2[{Ni(C6F5)2}2-
(µ-C6F5)2] have failed to date. By reaction of [NBu4]2-
[Ni(C6F5)4]33 with cis-[Ni(C6F5)2(THF)2]16 in noncoordi-
nating solvents, no compound containing µ-C6F5 was
obtainedsat least under the essayed experimental
conditions. A different synthetic approach was therefore
needed in order to obtain the desired product [NBu4]-
[Ni(C6F5)3(η2-C2H4)] (3). Given that [NBu4]2[Ni(C6F5)4]
is known to react with Et2O solutions of HCl(g) under
a CO atmosphere, yielding [NBu4][Ni(C6F5)3(CO)],16
a
similar procedure was followed using C2H4 instead of
CO (Scheme 3); however, no evidence of 3 was found in
the reaction products. Equally unsuccessful was the
reaction of [NBu4]2[{Ni(C6F5)2}2(µ-Cl)2] with AgC6F5 in
a noncoordinating solvent under a C2H4 atmosphere, a
procedure that is also known to render [NBu4][Ni(C6F5)3-
(CO)] in the presence of CO (Scheme 3).16 It becomes
(34) For recent reviews, see: Erker, G. Chem. Commun. 2003, 1469.
Bochmann, M.; Lancaster, S. J.; Hannant, M. D.; Rodr´ıguez, A.;
Schormann, M.; Walker, D. A.; Woodman, T. J. Pure Appl. Chem. 2003,
75, 1183. Chen, E. Y.-X.; Marks, T. J. Chem. Rev. 2000, 100, 1391.
(35) Albe´niz, A. C.; Espinet, P.; Lo´pez-Cimas, O.; Mart´ın-Ruiz, B.
Chem. Eur. J. 2005, 11, 242.
(32) Browning, J.; Goggin, P. L.; Goodfellow, R. J.; Norton, M. G.;
Rattray, A. J. M.; Taylor, B. F.; Mink, J. J. Chem. Soc., Dalton Trans.
1977, 2061.
(33) Uso´n, R.; Fornie´s, J.; Espinet, P.; Navarro, R.; Mart´ınez, F.;
Toma´s, M. J. Chem. Soc., Chem. Commun. 1977, 789. See also ref 16.

3544 Organometallics, Vol. 24, No. 14, 2005
Fornie´s et al.
Table 3. Selected Structural Parameters, Binding
Energies ∆E, and Electronic Properties of the
[MR3(η2-C2H4)]- (M ) Ni, Pd, Pt; R ) C6F5, Cl)
Species Computed at the B3LYP/dgdzvpa Level of
Theory
that the computational level of theory used provides
reliable structural parameters for the compounds under
study. For the [Pd(C6F5)3(η2-C2H4)]- compound the
computed CdC bond distance of 136.3 pm is slightly
longer by 2.7 pm relative to the experimental value. The
same holds also true for the Pd-C(ethene) and Pd-C6F5
bond distances. This is not an unexpected result, since
the computed structural parameters refer to an isolated
molecule in the gas phase. In all of the complexes the
M-R bond trans to the coordinated ethene molecule was
predicted to be significantly shorter than those corre-
sponding to the mutually trans R ligands, in line with
the experimental findings. Our calculations on the
[MCl3(η2-C2H4)]- anions are in excellent agreement with
analogous calculations reported previously.12a,13,30,31
Thus, a CdC bond length of 139.5 pm and a quite short
Pd-C distance of 219.7 pm were obtained using the
BP86 functional in an independent DFT calculation.12a
In contrast, a CdC bond length of 135.4 pm, closer to
our theoretical and experimental value, together with
a much longer Pd-C distance of 235 pm were attained
by ab initio calculations using a relativistic effect core
potential for Pd.30 Very similar results were attained
using the CNDO-S2 methodsa semiempirical SCF MO
calculation procedure.31
Let us now go deeper into the bonding mechanism of
the coordinated ethene ligand in the novel [M(C6F5)3-
(η2-C2H4)]- (M ) Ni, Pd, Pt) molecules and make
comparisons with the [MCl3(η2-C2H4)]- analogues. To
achieve this goal, we carried out NBO analyses of
electron populations of the relevant donor-acceptor
orbitals and computed the second-order stabilization
energy E(2) associated with the charge transfer (CT)
interactions (Table 3) describing the back-bonding
phenomenon.36 It can be seen that the π-acceptor ability
of the coordinated ethene molecule quantified by the
electron population of the acceptor π* MO in both series
of Zeise type salts follows the trend Pt > Ni > Pd. More
specifically, in the [MCl3(η2-C2H4)]- series, the back-
bonding in the Pt compound is 2.7 and 1.8 times
stronger than in the Pd and Ni compounds, respectively.
On the other hand, in the [M(C6F5)3(η2-C2H4)]- series,
the back-bonding effect in the Pt compound is 1.9 and
1.5 times stronger than in the Pd and Ni compounds,
respectively. Noteworthy is the stronger back-bonding
in [MCl3(η2-C2H4)]- than in the [M(C6F5)3(η2-C2H4)]-
compounds, illustrating that the good π-acceptor C6F5
ligand exerts a weaker π back-bonding effect than does
the strong π-donor Cl ligand. The difference in the
extents of π back-bonding in the two series of complexes
is much higher in the Pt compounds, followed by the
Ni compounds, while in Pd compounds the difference is
much smaller (cf. the Pop(π*) values of 0.103 and 0.123
for R ) C6F5, Cl, respectively). The n(dyz) f (π*-MO)
charge transfer (CT) interactions describing the π-back-
bonding effect stabilize the [M(C6F5)3(η2-C2H4)]- and
[MCl3(η2-C2H4)]- complexes by 17.2-36.4 and 25.4-61.8
kcal mol-1, respectively. Noteworthy is the high stabi-
lization (61.8 kcal mol-1) of the n(dyz) f (π*-MO)
interactions in Zeise’s salt, exhibiting the highest π
back-bonding effect. In the series of the [M(C6F5)3(η2-
C2H4)]- compounds the stabilization energies due to the
C6F5/Cl
Ni
Pd
Pt
∆E (kcal mol-1
Re(C-C) (pm)
Re(M-C) (pm)
Re(M-Rt) (pm)
)
10.9/8.2
12.4/16.2
18.6/34.1
136.7/137.9
217.6/210.8
193.2/222.5
196.8/225.8
36.6/38.2
136.3/137.8
237.1/225.5
208.1/237.3
212.8/240.4
33.4/35.6
139.6/142.4
229.3/213.5
205.4/243.1
208.8/244.5
35.4/38.3
Re(M-Rc) (pm)
θe(C-M-C) (deg)
θe(Rc-M-Rc) (deg) 175.0/178.2
θe(Rc-M-Rt) (deg) 87.5/90.9
176.0/176.4
88.0/91.8
2.48/1.68
0.61/0.76
178.2/176.9
89.1/91.6
2.57/2.00
0.64/0.66
η (eV)
2.48/1.65
0.76/0.89
b
qM
qC
-0.41/-0.44 -0.40/-0.42 -0.43/-0.48
bop(C-C)c
0.447/0.375
1.828/1.789
0.129/0.192
1.853/1.830
20.2/25.4
0.487/0.447
1.844/1.762
0.103/0.123
1.882/1.833
17.2/28.3
0.350/0.287
1.750/1.666
0.194/0.337
1.788/1.695
36.4/61.8
Pop(π)d (donor)
Pop(π*) (acceptor)
Pop(dyz) (donor)
E(2)e (kcal mol-1
)
a The LANL2DZ basis set was used for the Pt compounds.
b Natural charges. c Mulliken bond overlap population. d Electron
population of the π and π* MOs of the coordinated ethene molecule
and the nonbonding dyz AO (donor orbital) of the central metal
atom. e The stabilization energy associated with the n(dyz) f (π*-
MO) charge transfer (CT) interactions describing the π-back-
bonding effect, which is given by E(2) ) ∆Eij ) qi(F(i,j)2)/(ꢀj - ꢀi),
where qi is the donor orbital occupancy, ꢀi and ꢀj are diagonal
elements (orbital energies), and F(i,j) is the off-diagonal NBO Fock
matrix element.36
logues 1 and 2. [2H]Dichloromethane solutions of 3 at
-80 °C were found to exhibit, after removal of excess
1
C2H4, a broad signal at δH 4.96 ppm in the H NMR
spectrum that can be assigned to the η2-C2H4 ligand.
Compound 3 was found to decompose in CH2Cl2 solution
above -50 °C, giving complex mixtures of pentafluo-
rophenyl-metal derivatives. It is interesting to note
that, in such mixtures of decomposition products, the
presence of high-frequency resonance signals charac-
teristic of µ-C6F5 groups was observed by 19F NMR
spectroscopy. When CH2Cl2 solutions of 3 were allowed
to reach room temperature under a CO atmosphere,
[NBu4][Ni(C6F5)3(CO)] was obtained as the only species
containing metal-coordinated C6F5 groups (Scheme 4).
The spectroscopic, thermal, and chemical behaviors
of the unstable species 3 are consistent with a formula-
tion as [NBu4][Ni(C6F5)3(η2-C2H4)]. All our attempts to
isolate this compound have failed to date.
Relative Stability and Quantitative Estimates of
Back-Bonding in [MR3(η2-C2H4)]- (M ) Ni, Pd, Pt;
R ) C6F5, Cl). Our experimental results concerning the
compared thermal stability of compounds 1-3 as well
as the structural data of 2 were corroborated by DFT
calculations at the B3LYP level of theory using a basis
set of double-ú valence quality analogous to that used
previously in calculations on the stability of the [MCl3-
(η2-C2H4)]- anions.13 Selected structural parameters,
binding energies, and electronic properties of the [MR3-
(η2-C2H4)]- (M ) Ni, Pd, Pt; R ) C6F5, Cl) compounds
are compiled in Table 3. It can be seen that the salient
features of the structures of all Zeise type salts are
reproduced satisfactorily by the B3LYP calculations.
Interestingly, the B3LYP/dgdzvp calculations reproduce
the experimentally determined CdC bond distance of
133.7(2) pm in the free ethene molecule,27 indicating
(36) Rauk, A. Orbital Interaction Theory of Organic Chemistry;
Wiley: New York, 2001, p 37.

All-Organometallic Analogues of Zeise’s Salt
Organometallics, Vol. 24, No. 14, 2005 3545
n(dyz) f (π*-MO) interactions follow the trend Pt > Ni
> Pd, while in the series of the [MCl3(η2-C2H4)]-
compounds the trend is Pt > Pd > Ni.
were also obtained using NBO analyses of electron
populations of the relevant donor-acceptor orbitals and
the second-order stabilization energy associated with
the charge transfer (CT) interactions describing the
back-bonding. Furthermore, the structural character-
ization of [NBu4][Pd(C6F5)3(η2-C2H4)] (2) by X-ray dif-
fraction methods has enabled us to check the outcome
of our theoretical calculations on the geometry of the
central core of the [Pd(C6F5)3(η2-C2H4)]- anion against
experiment. The C2H4 molecule remains essentially
unaltered upon coordination to the “Pd(C6F5)3-” frag-
ment: the CdC bond length is exactly the same as in
the free ligand, and only a slight degree of pyramidal-
ization is observed at the C atom with a moderate value
of the R angle. It can be concluded that there is
reasonable agreement between the experimentally ob-
tained geometric parameters and those predicted by
theoretical calculations.
The absolute extents of the σ-acceptor ability of the
“M(C6F5)3-” and “MCl3-” fragments may be estimated
by the electron population of the bonding π-MO of the
coordinated ethene molecule, Pop(π) (Table 3). In the
[M(C6F5)3(η2-C2H4)]- compounds the electron density
transferred from the π-MO of ethene to a vacant d
orbital of the metal center follows the trend Pt (0.250
|e|) > Ni (0.172 |e|) > Pd (0.156 |e|), while in the [MCl3-
(η2-C2H4)]- compounds the trend is Pt (0.334 |e|) > Pd
(0.238 |e|) > Ni (0.211 |e|). In summary, the two bonding
interactions in the [M(C6F5)3(η2-C2H4)]- compounds are
the largest for Pt and the smallest for Pd. It should be
stressed that both bonding interactions contribute to the
weakening of the CdC double bond of the coordinated
ethene, by removing electron density from the π-MO and
adding electron density to the π*-MO. The weakening
of the CdC double bond of the coordinated ethene is
reflected in the computed CdC bond lengths, with the
elongation of the bond (Table 3) following the trend Pt
. Ni > Pd in both series of compounds. Finally, the two
bonding interactions are the key factors determining the
stability of the Zeise type salts, both contributing to the
binding energy of the ethene ligand to the metal-
containing fragment. The computed binding energies
(Table 3) were found in the range of 10.9-18.6 kcal
mol-1 for the [M(C6F5)3(η2-C2H4)]- compounds and 8.2-
34.1 kcal mol-1 for the [MCl3(η2-C2H4)]- species. Note
the higher stability of the Pt compounds in both series
of complexes. In general terms, the [M(C6F5)3(η2-C2H4)]-
compounds are less stable than their corresponding
[MCl3(η2-C2H4)]- counterparts, in line with the extents
of the two bonding effects in the complexes. The Pd
compounds have comparable stabilities, and therefore,
both complexes should be isolable under the appropriate
conditions, in line with the experimental findings. Most
important is the slightly higher stability of the [Ni-
(C6F5)3(η2-C2H4)]- compound as compared to that of the
[NiCl3(η2-C2H4)]- analogue. Moreover, the stability of
the [Ni(C6F5)3(η2-C2H4)]- compound is comparable to
that of the isolated [Pd(C6F5)3(η2-C2H4)]- anion and,
therefore, the Ni complex is predicted to exist also as a
stable species, thus confirming its experimental iden-
tification by NMR spectroscopy in solution reported
herein.
Experimental Section
General Procedures and Materials. All reactions and
manipulations were carried out under purified argon using
Schlenk techniques. Solvents were dried by standard methods
and distilled prior to use. The starting materials [NBu4]2[{M-
(C6F5)2}2(µ-C6F5)2] (M ) Pt, Pd),17 [NBu4]2[Ni(C6F5)4],16 and
37
B(C6F5)3 were prepared as described elsewhere. Elemental
analyses were carried out with a Perkin-Elmer 2400-Series II
microanalyzer. IR spectra of KBr disks were recorded on the
following Perkin-Elmer spectrophotometers: 883 (4000-200
cm-1) or Spectrum One (4000-350 cm-1). NMR spectra were
recorded on a Varian Unity-300 or on a Bruker ARX 300
spectrometer. Unless otherwise stated, the spectroscopic mea-
surements were carried out at room temperature.
Synthesis of [NBu4][Pt(C6F5)3(η2-C2H4)] (1). A CH2Cl2
solution (7 cm3) of [NBu4]2[{Pt(C6F5)2}2(µ-C6F5)2] (87.6 mg,
0.047 µmol) was stirred at room temperature under an ethene
atmosphere for 90 min, during which time the initially bright
yellow solution gradually turned colorless. The addition of
C2H4-saturated n-hexane (30 cm3) to the resulting mixture
caused the precipitation of a small amount of white solid. After
this suspension was allowed to stand at -30 °C overnight, the
white solid was filtered and dried at room temperature (1‚
CH2Cl2: 54 mg, 5 µmol, 60% yield). Anal. Found: C, 42.2; H,
3.95; N, 1.4. Calcd for C37H42ClF15NPt: C, 42.25; H, 4.0; N,
1.3. IR (KBr): ν˜max/cm-1 1504 (vs), 1455 (vs), 1057 (vs), 956
(vs; C6F5: C-F),21 881 (m; NBu4+), 798 (s; C6F5: X-sensitive
vibr),21 785 (s; C6F5: X-sensitive vibr), 775 (s; C6F5: X-sensitive
1
2
vibr). H NMR ([2H]chloroform):38 δ 4.33 (s, J(195Pt,H) ) 47
Hz). 13C{1H} NMR ([2H]dichloromethane):38 δ 78.9 (1J(195Pt,C)
) 88 Hz). 19F NMR ([2H]chloroform): δ -117.80 (3J(195Pt,F) )
480 Hz, 2F, o-F), -120.39 (3J(195Pt,F) ) 311 Hz, 4F, o-F),
-164.42 (2F, p-F), -165.00 (1F, p-F), -165.41 (4F, m-F),
-166.18 (2F, m-F).
Concluding Remarks
Suitable synthetic procedures have been devised to
prepare the complete series of [M(C6F5)3(η2-C2H4)]-
species for the group 10 metals (M ) Ni, Pd, Pt). They
are all-organometallic analogues of Zeise’s salt, since the
metal coordination sphere is entirely made of C-donor
ligands. In contrast with the marked stability of the Pt
complex 1, the Pd derivative 2 easily evolves C2H4, as
the result of an intermolecular substitution reaction in
which the dinuclear species [NBu4]2[{Pd(C6F5)2}2(µ-
C6F5)2] is formed. The Ni compound 3 cannot be isolated,
and it decomposes in solution at temperatures above
-50 °C. The key factors determining the stability of the
novel Zeise type salts 1-3 have been inferred using
electronic structure calculation methods at the DFT
level of theory. Quantitative estimates of back-bonding
Synthesis of [NBu4][Pd(C6F5)3(η2-C2H4)] (2). A CH2Cl2
solution (5 cm3) of [NBu4]2[{Pd(C6F5)2}2(µ-C6F5)2] (0.23 g, 0.13
mmol) was stirred at 0 °C under an ethene atmosphere for 5
min. Then, C2H4-saturated n-hexane (30 cm3) was added to
the reaction medium. After the resulting mixture was allowed
to stand at -30 °C for 3 days, a white solid formed that was
filtered and dried at -30 °C (2: 0.13 g, 0.15 mmol, 55% yield).
Anal. Found: C, 49.2; H, 4.6; N, 1.6. Calcd for C36H40F15NPd:
C, 49.2; H, 4.6; N, 1.6. IR (KBr): ν˜max/cm-1 1497 (vs), 1456
(vs), 1056 (s), 1041 (s), 953 (vs; C6F5: C-F),21 881 (m; NBu4+),
797 (sh; C6F5: X-sensitive vibr),21 784 (s; C6F5: X-sensitive
vibr), 766 (s; C6F5: X-sensitive vibr).21 1H NMR ([2H]dichlo-
(37) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 2, 245.
+
(38) Resonances due to the cation NBu4 have been omitted.

3546 Organometallics, Vol. 24, No. 14, 2005
Fornie´s et al.
romethane, -40 °C):38 δ 5.12. 13C{1H} NMR ([2H]dichlo-
romethane, -40 °C):38 δ 97.6. 19F NMR ([2H]dichloromethane,
-40 °C): δ -114.12 (2F, o-F), -116.36 (4F, o-F), -163.26 (2F,
p-F), -164.14 (5F: 4m-F + 1p-F), -165.05 (2F, m-F).
Single crystals suitable for X-ray diffraction purposes were
obtained by slow diffusion of a C2H4-saturated n-hexane layer
(10 cm3) into a solution of 20 mg of 2 in CH2Cl2 (2 cm3) at -30
°C.
Crystal Data for 2. Crystal data and other details of the
structure analysis are presented in Table 1. All diffraction
measurements were made at 100 K on a Bruker Smart CCD
area detector diffractometer using graphite-monochromated
Mo KR radiation. The diffraction frames were integrated using
the SAINT package39 and corrected for absorption with SAD-
ABS.40 Lorentz and polarization corrections were applied. The
structure was solved by Patterson and Fourier methods and
refined using the SHELXL-97 program.41 All non-hydrogen
atoms were assigned anisotropic displacement parameters.
The hydrogen atoms were constrained to idealized geometries
and assigned isotropic displacement parameters equal to 1.2
times the Uiso values of their respective parent atoms (1.5 times
for CH3). The positions of the hydrogen atoms of the ethene
ligands were found in the electron density maps and subse-
quently refined with the C-H distances and H-C-H angles
restrained to sensible values. Full-matrix least-squares refine-
ment on F2 of this model converged to final residual indices
given in Table 1. CCDC reference number: 266540.
δ 4.96. 19F NMR ([2H]dichloromethane, -80 °C):42 δ -116.64
(pseudo quartet, 2F, o-F), -119.05 (pseudo triplet, 4F, o-F).
Computational Methods. DFT calculations at the B3LYP
level of theory were carried out on the [MR3(η2-C2H4)]- anionic
species (M ) Ni, Pd, Pt; R ) Cl, C6F5) using the GAUSSIAN03
program suite.43 The geometries of all species were fully
optimized by Becke’s three-parameter hybrid functional44,45
combined with the Lee-Yang-Parr46 correlation functional,
using a basis set of double-ú quality (DZVP basis set)47 for the
Ni and Pd compounds, while the LANL2DZ basis set, being
also of double-ú quality, was used for the Pt compounds. Full
geometry optimization was performed for each structure using
Schlegel’s analytical gradient method,48 and the attainment
of the energy minimum was verified by calculating the
vibrational frequencies that result in the absence of imaginary
eigenvalues. The computed electronic energies were corrected
for zero-point energy (ZPE) differences. The stabilization
energy E(2) associated with the charge transfer (CT) interac-
tions between the relevant donor-acceptor orbitals was com-
puted from the second-order perturbative estimates of the Fock
matrix in the natural bond orbital (NBO) analysis.48,50
Acknowledgment. This work has been supported
by the Spanish MCYT (DGI)/FEDER (Project BQU2002-
03997-CO2-02) and Promerus LLC.
Supporting Information Available: Crystallographic
data (CIF) for [NBu4][Pd(C6F5)3(η2-C2H4)] (2) and Cartesian
coordinates and energies of the Zeise type salts (Tables S1 and
S2, respectively). This material is available free of charge via
the Internet at http://pubs.acs.org.
Synthesis of [NBu4][Ni(C6F5)3(η2-C2H4)] (3). A CH2Cl2
solution (10 cm3) of [NBu4]2[Ni(C6F5)4] (0.38 g, 0.32 mmol) was
cooled at -78 °C and put under an ethene atmosphere. Then
a precooled (-78 °C) solution of B(C6F5)3 (0.18 g, 0.35 mmol)
in CH2Cl2 (10 cm3) was added. After 1 h of reaction, 3 was the
only species detected in solution that contains metal-coordi-
nated C6F5 groups. 1H NMR ([2H]dichloromethane, -80 °C):38
OM0502077
(43) Frisch, M. J. T.; et al. Gaussian 03, Revision B.02; Gaussian,
Inc.: Pittsburgh, PA, 2003. See the Supporting Information for the
remaining 80 authors.
(44) Becke, A. D. J. Chem. Phys. 1992, 96, 215.
(45) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(46) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. Lett. 1998, B37, 785.
(47) (a) Godbout, N.; Salahub, D. R.; Andzelm, J.; Wimmer, E. Can.
J. Chem. 1992, 70, 560. (b) Sosa, C.; Andzelm, J.; N.; Elkin, B. C.;
Wimmer, E.; Dobbs, K. D.; Dixon, D. A. J. Phys. Chem. 1992, 96, 6630.
(48) Schlegel, H. B. J. Comput. Chem. 1982, 3, 214.
(49) Weinhold, F. In The Encyclopedia of Computational Chemistry;
Schleyer, P. v. R., Ed.; Wiley: Chichester, U.K., 1998; pp 1792-1811.
(50) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88,
899.
(39) SAINT, version 6.02; Bruker Analytical X-ray Systems, Madi-
son, WI, 1999.
(40) Sheldrick, G. M. SADABS empirical absorption program, ver-
sion 2.03; University of Go¨ttingen, Go¨ttingen, Germany, 1996.
(41) Sheldrick, G. M. SHELXL-97, Program for the refinement of
crystal structures from diffraction data; University of Go¨ttingen,
Go¨ttingen, Germany, 1997.
(42) No clear-cut distinction could be made between the signals
corresponding to C6F5 groups attached to boron or nickel in the high-
field region, due to extensive superposition. This circumstance pre-
cluded the reliable assignment of the meta F and para F signals for
compound 3.