µ-Vinylidene Binuclear Ni Complexes
Organometallics, Vol. 15, No. 6, 1996 1695
Nonetheless, the spectroscopic data for [Ni2(µ-CO)-
(dppm)2Cl2] should be regarded with caution when
compared to that for the other bona fide A-frame
compounds considered in this study. With this caveat
concerning the experimental data, there is still a
reasonable basis to conclude that stronger π-acceptor
ligands in the bridgehead position result in lower
HOMO/LUMO separations.
tions is that for nickel A-frame complexes bridged by
dmpm or dppm, significant differences in ground-state
electronic structure are not to be expected. Certainly,
in the case of the new dmpm-bridged nickel A-frames
(1a -c) reported here and compared to the dppm-bridged
systems (2a -c),4 the similarities in molecular and
electronic structure far outweigh the differences.
We consider next the effects of changing the terminal
halide or pseudohalide ligands. Both the dmpm-bridged
(1a -c) and dppm-bridged (2a -c) A-frames show elec-
tronic transitions that in energy follow the order Cl >
Br > I, Table 1. Although this is the normal ordering
for the empirical “spectrochemical series” of increased
d-orbital splittings by halide ligands, it is useful to
consider the effect within the context of our EHMO
calculations. Again, a primarily metal-based HOMO
and vinylidene π*-based LUMO is important in this
trend. The dσ and dπ interactions between the nickel
atoms and the halogen ligands lead to metal-based
[Ni2(PH2CH2PH2)2(X)2]2+ fragment orbitals that in en-
ergy follow the order Cl < Br < I. This translates
directly to the decreasing order of the primarily metal-
based HOMO’s (Cl < Br < I) and to increasing HOMO/
LUMO separations (Cl > Br > I) for the series.
Exp er im en ta l Section
Gen er a l P r oced u r es. All manipulations were carried out
under nitrogen using standard Schlenk line and drybox
techniques. Solvents were degassed and purified by distilla-
tion under nitrogen from the appropriate drying agents. Ni-
(cod)2 (cod ) 1,5-cyclooctadiene), bis(dimethylphosphino)-
methane (dmpm), and bis(diphenylphosphino)methane (dppm)
were purchased from Strem Chemicals or Aldrich (dppm) and
used without further purification. 1H NMR spectra were
recorded on General Electric QE 300 or Varian XL-200
spectrometers with chemical shifts in ppm referenced to
internal SiMe4. 31P{1H} NMR spectra were recorded on
General Electric QE 300 or a Varian XL-200 spectrometers
operating at 121.4 and 80.96 MHz, respectively. 31P{1H} NMR
chemical shifts were reported in ppm with respect to an
external 85% H3PO4 reference. UV-vis electronic absorption
spectra were recorded on an IBM 9420 spectrophotometer.
Elemental analyses were determined by Galbraith Laborato-
ries, Inc., Knoxville, TN.
We consider finally the effects of different bridging
diphosphine ligands. The dmpm-bridged complexes
(1a -c) show absorption maxima in their UV-vis elec-
tronic absorption spectra which are shifted to higher
energy by approximately 40 nm compared to the related
dppm-bridged complexes (2a -c), Table 1. A direct
comparison of the electronic absorption spectrum of
[Ni2(µ-CdCH2)(dmpm)2Cl2] (1a ) and [Ni2(µ-CdCH2)-
(dppm)2Cl2] (2a ) is also made in Figure 1. For the
purposes of these calculations, PH2CH2PH2 (dHpm) was
used to model the bridging diphosphine ligands. Struc-
tural parameters were obtained from the crystal struc-
ture of 1a where hydrogens were substituted for methyl
groups. Classically, the π-accepting ability of phos-
phines was believed to be solely an interaction between
the phosphorus 3dπ orbitals and metal d-orbitals.25 In
recent years it has been shown that the π-accepting
ability of phosphine ligands is affected by the mixing of
the phosphorus 3dπ orbitals and the P-R σ* orbitals.26-29
We find that the π-accepting ability of the diphosphines
in our systems has only a minor influence on the overall
electronic structure of these compounds. The dmpm
ligand is a stronger σ-donor than dppm. Consequently,
dmpm increases the amount of electron density on the
metal centers, thus raising the energy of the d-orbital
manifold. This strengthens the interaction with the b2
orbital of the vinylidene fragment and leads to a larger
HOMO-LUMO gap. The result is seen as a blue shift
in the UV-vis spectra of the dmpm compounds relative
to their dppm counterparts. The EHMO calculations
thus do provide a model for considering the generally
higher energy electronic transitions of the dmpm-
bridged A-frames compared to the dppm-bridged sys-
tems. Perhaps, the real lesson of the EHMO calcula-
Syn th esis of [Ni2(µ-CdCH2)(d m p m )2Cl2] (1a ). A 100 mL
Schlenk flask was charged with Ni(cod)2 (1.0 g, 3.64 mmol),
20 mL of THF, and a stir bar. In a separate flask, dmpm (0.57
mL, 3.64 mmol) was dissolved in 10 mL of THF. The dmpm
solution was transferred by cannula into the Schlenk flask
with continuous stirring. To the resulting red solution was
added 1,1-dichlorovinylidene (0.15 mL, 1.82 mmol) by syringe.
The solution was stirred vigorously for 5 min after which time
30 mL of hexane was added. The reaction flask was then
placed in a freezer at -20 °C overnight and then filtered to
produce a brick red microcrystalline solid and a dark red
solution. The solid was placed on a fine frit, and a minimum
amount of THF was used to extract the soluble portion through
the frit, leaving a small quantity of an insoluble black solid
presumed to be nickel metal. The extract was placed in a
small round bottom flask. Clusters of needlelike crystals were
obtained by layering an equal volume of hexane over the
filtrate solution and storing in a freezer (yield: 0.43 g, 48%
based on Ni). Anal. Calcd for Ni2C12H30P4Cl2: C, 29.62; H,
6.21; P, 25.46. Found: C, 29.50; H, 6.66; P, 25.18. 31P{1H}
NMR (CD2Cl2): δ -0.1 (s). 1H NMR (CD2Cl2): δ 4.86 (p,
CdCH2, J (PH) ) 3.30 Hz), 1.79, 1.55 (m, PCHAHBP), 1.48, 1.44
(s, PMeAMeB). UV-vis data appear in Table 1.
Syn th esis of [Ni2(µ-CdCH2)(d m p m )2Br 2] (1b). 1a (0.20
g, 0.41 mmol) was added to a 50 mL acetone solution of KBr
(0.49 g, 4.1 mmol). The mixture was stirred for 5 min. The
acetone was removed under vacuum, and the remaining solid
was extracted with CH2Cl2 and recrystallized from CH2Cl2/
hexane. This resulted in the formation of a red microcrystal-
line solid (yield: 0.17 g, 71% based on 1a ). Anal. Calcd for
Ni2C12H30P4Br2: C, 25.05; H, 5.25; Br, 27.77. Found: C, 25.19;
H, 5.45; Br, 27.44. 31P{1H} NMR (CD2Cl2): δ -0.2 (s). 1H
NMR (CD2Cl2): δ 4.89 (p, CdCH2, J (PH) ) 1.2 Hz), 1.85, 1.14
(m, PCHAHBP), 1.58, 1.47 (s, PMeAMeB). UV-vis data appear
in Table 1.
Syn th esis of [Ni2(µ-CdCH2)(d m p m )2I2] (1c). 1a (0.20 g,
0.41 mmol) was added to a 50 mL acetone solution of KI (0.68
g, 4.1 mmol). The mixture was stirred for 5 min. The acetone
was removed under vacuum, and the remaining solid was
extracted with CH2Cl2 and recrystallized from CH2Cl2/hexane.
This resulted in the formation of a dark purple microcrystalline
solid (yield: 0.21 g, 78% based on 1a ). Anal. Calcd for
Ni2C12H30P4I2: C, 21.53; H, 4.52. Found: C, 21.71; H, 4.77.
(25) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 4th
ed.; Wiley: New York, 1980 p 87.
(26) Xiao, S.-X.; Trogler, W. C.; Ellis, D. E.; Berkovitch-Yellin, Z. J .
Am. Chem. Soc. 1983, 105, 7033-7037.
(27) Marynick, D. S. J . Am. Chem. Soc. 1984, 106, 4064-4065.
(28) Braga, M. Inorg. Chem. 1985, 24, 2702-2706.
(29) Orpen, A. G.; Connelly, N. G. J . Chem. Soc., Chem. Commun.
1985, 1310-1311.
Heise, Jerald D.
