X-ray crystallographic and theoretical comparison of Ge[2,4,6-(CF3)3C6H2]2 and Ge[N(SiMe3)2]2 as ligands in (Ph3P)2NiGeX2 complexes

Article abstract of DOI:10.1021/om9900521

A new transition metal germylene complex, (Ph₃P)₂NiGe[2,4,6-(CF₃)₃C ₆H₂]₂ (2), has been synthesized and structurally characterized. The metrical parameters of complex 2 are compared and contrasted with the previously synthesized complexes (Ph₃P)₂NiGe[N(SiMe₃)₂]₂ (3) and (Et₃P)₂PtGe[N(SiMe₃)₂]₂ (1). Nonlocal BP86 density functional calculations support the presence of Ni-Ge d-p π bonding in complexes 2 and 3.

Full text of DOI:10.1021/om9900521

Organometallics 1999, 18, 1547-1552
1547
X-r a y Cr ysta llogr a p h ic a n d Th eor etica l Com p a r ison of
Ge[2,4,6-(CF 3)3C6H2]2 a n d Ge[N(SiMe3)2]2 a s Liga n d s in
(P h 3P )2NiGeX2 Com p lexes
J . E. Bender IV,†,1 A. J . Shusterman,§ M. M. Banaszak Holl,*,‡ and J . W. Kampf‡
Department of Chemistry, Brown University, Providence, Rhode Island 02912,
Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055, and
Department of Chemistry, Reed College, Portland, Oregon 97202-8199
Received J anuary 26, 1999
A new transition metal germylene complex, (Ph3P)2NiGe[2,4,6-(CF3)3C6H2]2 (2), has been
synthesized and structurally characterized. The metrical parameters of complex 2 are
compared and contrasted with the previously synthesized complexes (Ph3P)2NiGe[N(SiMe3)2]2
(3) and (Et3P)2PtGe[N(SiMe3)2]2 (1). Nonlocal BP86 density functional calculations support
the presence of Ni-Ge d-p π bonding in complexes 2 and 3.
In tr od u ction
clearly demonstrates that M-Si bond length is not a
reliable indicator of the degree of M-Si π-character.
The extent of multiple bonding between transition
metal elements and group 14 elements, and the conse-
quence of this bonding behavior with respect to reactiv-
ity and structure, has been of intense interest to
chemists for decades. This line of study was greatly
encouraged by the isolation of the first metal carbenes2
and subsequently driven by the large number of impor-
tant stoichiometric and catalytic transformations in
which carbenes have been observed to take part.3
Considerable interest has also focused on the isolation,
structural characterization, and reactivity of MdEX2 (E
) Si, Ge, Sn, Pb) fragments.4 Key issues explored for
these heavier congeners include the effect of stabilizing
ligands bound to the group 14 element upon the
magnitude of the MdE bonding interaction and MdE
reactivity toward substrates. The work for the heavier
congeners mirrors the seminal work exploring the
stability and reactivity of carbenes that resulted in the
assignment of two general classes: the alkylidene or
Schrock carbene and the Fischer carbene.2 Considerable
experimental work on heteroatom effects in silylene
ligands has been published.5-10 This series of studies
In a recent study of platinum germylene structure and
reactivity, crystallographic analysis of (Et3P)2PtGe-
[N(SiMe3)2]2 (1) indicated a short Pt-Ge distance of
2.304(1) Å, 0.11-0.17 Å less than related germyl species
in square-planar Pt complexes. Reaction of 1 with CO2
and PhNO yields four-membered metallacycles remi-
niscent of the 2+2 type chemistry observed for car-
benes.11,12 Recent results also indicate that 1, 2, and 3
are photosensitive.13 The sum of these structural and
reactivity studies suggest that M-Ge π bonding may
contribute significantly to the chemistry of group 10
germylenes.14 Note that unlike the test cases utilized
by J acobsen and Ziegler in which the germylene must
compete for metal back-bonding with CO ligands,10 and
the previously characterized pair of complexes (CO)5-
CrGe[CH(SiMe3)2]2 and (CO)5CrGe[S(2,4,6-(CH3)3C6-
H2]2,15,16 the germylene has considerably less competi-
tion for π back-bonding in the complexes of general
formula (R3P)2MGeX2 discussed in this paper. The two
bis(trimethylsilyl)amide groups in the Ge[N(SiMe3)2]2
complex may compete with the (PPh3)2M fragment for
Ge π bonding. Thus, the role of M-Ge π bonding in
causing the short Pt-Ge bond distance could be mini-
mal, especially given the results of calculations for
heteroatom-stabilized platinum silylenes.9
† Brown University.
‡ University of Michigan.
§ Reed College.
(1) Current address: Chemistry Department, Grand Valley State
University, Allendale, MI 49401-9403.
(2) (a) Fischer, E. O.; Maasbol, A. Angew. Chem., Int. Ed. Engl. 1964,
3, 580. (b) Schrock, R. R. J . Am. Chem. Soc. 1974, 96, 1974.
(3) Doetz, K. H. Transition Metal Carbene Complexes; Verlag
Chemie: Weinheim, 1983.
(4) (a) Lappert, M. F. Main Group Met. Chem. 1994, 17, 183. (b)
Tokitoh, N.; Matsuhashi, Y.; Shibata, K.; Matsumoto, T.; Suzuki, H.;
Saito, M.; Manmaru, K.; Okazaki, R. Main Group Met. Chem. 1994,
17, 55. (c) Lappert, M. F.; Rowe, R. S. Coord. Chem. Rev. 1990, 100,
267. (d) Petz, W. Chem. Rev. 1986, 86, 1019. (d) Herrmann, W. A.
Angew. Chem., Int. Ed. Engl. 1986, 25, 56.
(5) (a) Grumbine, S. D.; Tilley, T. D.; Arnold, F. P.; Rheingold, A. L.
J . Am. Chem. Soc. 1994, 116, 5495. (b) Grumbine, S. D.; Mitchell, G.
P.; Straus, D. A.; Tilley, T. D.; Rheingold, A. L. Organometallics 1998,
17, 5607.
(8) Grumbine, S. D.; Straus, D. A.; Tilley, T. D.; Rheingold, A. L.
Polyhedron 1995, 14, 127.
(9) Grumbine, S. D.; Tilley, T. D.; Arnold, F. P.; Rheingold, A. L. J .
Am. Chem. Soc. 1993, 115, 7884.
(10) J acobsen, H.; Ziegler, T. Inorg. Chem. 1996, 35, 775.
(11) Litz, K. E.; Henderson, K. H.; Gourley, R. W.; Banaszak Holl,
M. M. Organometallics 1995, 14, 5008.
(12) Litz, K. E.; Kampf, J . W.; Banaszak Holl, M. M. J . Am. Chem.
Soc. 1998, 120, 7484.
(13) Litz, K. E.; Bender, J . B.; Banaszak Holl, M. M.; Kampf, J . W.
Unpublished results.
(14) Similar effects have also recently been noted in (PR3)2PtSiR2
complexes: Feldman, J . D.; Mitchell, G. P.; Nolte, J .-O.; Tilley, T. D.
J . Am. Chem. Soc. 1998, 120, 11184.
(6) Grumbine, S. D.; Tilley, T. D.; Rheingold, A. L. J . Am. Chem.
Soc. 1993, 115, 358.
(7) Grumbine, S. D.; Chadha, R. K.; Tilley, T. D.; Rheingold, A. L.
J . Am. Chem. Soc. 1992, 114, 1518.
(15) Lappert, M. F.; Miles, S. J .; Power, P. P.; Carty, A. J .; Taylor,
N. J . J . Chem. Soc., Chem. Commun. 1977, 458.
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Schubert, U. Chem. Ber. 1978, 111, 606.
10.1021/om9900521 CCC: $18.00 © 1999 American Chemical Society
Publication on Web 03/26/1999

1548 Organometallics, Vol. 18, No. 8, 1999
Bender et al.
for 60 min for thorough removal of toluene. Sparkling, deep-
burgundy microcrystals of 2 were obtained by filtration and
recrystallization from hexane in two crops (385 mg, 80% yield).
1H NMR (C6D6): δ 7.87 (4H, s), 7.47 (20H, m), 6.88 (10H, m).
19F NMR (C6D6): δ -55.58 (s, 12F), -63.34 (s, 6F). 31P{1H}
NMR (C6D6): δ 39.61 (s). Anal. Calcd for C54H34F18P2GeNi: C,
53.2; H, 2.8. Found: C, 51.92; H, 2.97.
X-r a y Da ta Collection , Str u ctu r e Deter m in a tion , a n d
Refin em en t of 2. A small, deep red, irregular platelike crystal
(0.04 × 0.10 × 0.28 mm) was mounted on a standard Siemens
SMART CCD-based X-ray diffractometer equipped with a
normal focus Mo-target X-ray tube (λ ) 0.710 73 Å) operated
at 2000 W power (50 kV, 40 mA) and a LT-2 low-temperature
device. The X-ray intensities were measured at 153 K; the
detector was placed at a distance of 4.577 cm from the crystal.
A total of 2132 frames were collected with a scan width of 0.3°
in ω and an exposure time of 30 s/frame. The frames were
integrated with the Siemens SAINT software package with a
narrow frame algorithm. The integration of the data using a
primitive monoclinic unit cell yielded a total of 55 786 reflec-
tions to a maximum 2θ value of 61°, of which 13 919 were
independent and 8442 were greater then 2σ(I). The final cell
constants (a ) 12.3206(1) Å, b ) 23.017(1) Å, c ) 18.0758(1)
Å, â ) 104.29(1)°, V ) 4967.33(5) Å3, Z ) 4) were based on
the xyz centroids of the 8192 reflections above 10σ(I). Analysis
of the data showed negligible decay during data collection; the
data were collected for absorption using an empirical method
(SADABS) with transmission coefficients ranging from 0.825
to 0.978. The structure was solved and refined (Siemens
SHELXTL (5.03)) using space group P2(1)/c with Z ) 4 for
the formula C54H34F18P2GeNi. All non-hydrogen atoms were
refined anisotropically with the hydrogens placed in idealized
positions. Final full-matrix refinement based on F2 converged
at R1 ) 0.0852 and wR2 ) 0.1152 (based on observed data);
R1 ) 0.1545 and wR2 ) 0.1368 (based on all data); the largest
peak in the final difference map was 1.12 e/Å3 and was
associated with Ni.
F igu r e 1. Germylene and metal-germylene complexes
discussed herein.
To further explore the issue of M-Ge π bonding, we
have recently prepared a new species of stable, electron-
withdrawing germylene, bis[2,4,6-tris(trifluoromethyl)-
phenyl]germanium(II),17 and synthesized two related
nickel germylene complexes, (Ph3P)2NiGe[2,4,6-(CF3)3-
C6H2]2 (2) and (PPh3)2NiGe[N(SiMe3)2]2 (3),18 to allow
a direct comparison of the effects of germylene ligand
substitution upon the M-Ge bond length. The set of
complexes is unique because the germylenes used for
complexes 2 and 3 are both stable as free molecules.
Thus, direct structural comparisons can be made to Ge-
[2,4,6-(CF3)3C6H2]2 (4) and Ge[N(SiMe3)2]2 (5) as well.
The key complexes discussed in this paper are sum-
marized in Figure 1. In trying to assess the significance
of bonding formalisms such as Mr:GeR2 and MdGeR2,
crystal structures often do not provide clear guidance.
Therefore, a density functional and natural bond order
analysis of the bonding was also undertaken for both
complexes.
Com p u ta tion a l Meth od s
Density functional calculations were performed using stan-
dard routines included in Spartan 5.0 (BP86/DN**) and J aguar
3.5 (pseudospectral BP86/LACVP**).21,22 BP86 is a nonlocal
functional that employs Becke’s 1988 gradient correction23 to
Slater’s exchange functional,24 and Perdew’s 1986 gradient
correction25 to the Perdew-Zunger local correlation func-
tional.26 DN** is a numerical all-electron basis set of split-
valence+polarization quality (all atoms). LACVP** is a split
valence+polarization basis set that employs an effective core
potential for Ni (Ne core)27 and Ge (Ar core)28 and uses the
6-31G** basis for all remaining atoms.29 A singlet spin state
was assumed for all molecules, and symmetry was not
employed (C1 point group). Molecular geometries were deter-
mined using crystallographic coordinates for all heavy atoms.
The coordinates of hydrogen atoms in the PH3 ligands were
assigned by using Spartan to delete the atoms of the phenyl
groups; this effectively replaces each phenyl group with a
hydrogen lying 1.42 Å from phosphorus on the original PCipso
vector. The coordinates of all other hydrogens were determined
Exp er im en ta l Section
All procedures utilized air-free technique and dry, deoxy-
genated solvents.19 Ge[2,4,6-(CF3)3C6H2]2 (4)17 and Ge[N-
(SiMe3)2]2 (5)20 were prepared according to literature proce-
dures. Ni(COD)2 was purchased from Strem Chemicals and
used as received. 1H (300 MHz), 19F (282 MHz), 13C (90.6 MHz),
and 31P (146 MHz) NMR spectra were obtained in C6D6 or
THF-d8 and referenced to the residual solvent protons, CFCl3
in C6D6 at 0.0 ppm, the natural abundance 13C in solvent, and
external 85% H3PO4 in D2O. IR spectra were taken using a
Nicolet 5DXB spectrometer.
(P h 3P )2NiGe[2,4,6-(CF 3)3C6H2]2 (2). Ph3P (207 mg, 0.788
mmol) and [2,4,6-(CF3)3C6H2]2 (250 mg, 0.394 mmol) were
added sequentially to a toluene solution (10 mL) of Ni(COD)2
(108 mg, 0.393 mmol) at 20 °C, allowing 30 min of stirring
between the additions. The solution became opaque dark red
on addition of phosphine, followed by a slight darkening to
burgundy when the germylene was added. The solution was
stirred for an additional 60 min at 20 °C. The solvent was
removed in vacuo, and the solids were allowed to vacuum-dry
(21) Spartan v5.0; Wavefunction, Inc.: 18401 Von Karman Avenue,
Suite 370, Irvine, CA, 92612, 1997.
(22) J aguar v3.5; Schro¨dinger, Inc.: Portland, OR, 1997.
(23) Becke, A. D. Phys. Rev. A 1988, 38, 3098.
(17) Bender, J . E.; Banaszak Holl, M. M.; Kampf, J . W. Organome-
tallics 1997, 16, 2743.
(18) Litz, K. E.; Bender, J . B.; Kampf, J . W.; Banaszak Holl, M. M.
Angew Chem., Int. Ed. Engl. 1997, 36, 496.
(19) Shriver, D. F.; Drezdzon, M. A. The Manipulation of Air-
Sensitive Compounds, 3rd ed.; Wiley: New York, 1986.
(20) Gynane, M. J . S.; Harris, D. H.; Lappert, M. F.; Power, P. P.;
Rivie`re, P.; Rivie`re-Baudet, M. J . Chem. Soc., Dalton Trans. 1977,
2004.
(24) Slater, J . C. Quantum Theory of Molecules and Solids, Vol. 4:
The Self-Consistent Field for Molecules and Solids; McGraw-Hill: New
York, 1974.
(25) (a) Perdew, J . P. Phys. Rev. B 1986, 33, 8822. (b) Perdew, J . P.
Phys. Rev. B 1986, 34, 7406.
(26) Perdew, J . P.; Zunger, A. Phys. Rev. B 1981, 23, 5048.
(27) Hay, P. J .; Wadt, W. R. J . Chem. Phys. 1985, 82, 299.
(28) Hay, P. J .; Wadt, W. R. J . Chem. Phys. 1985, 82, 270, 284.
(29) Hariharan, P. C.; Pople, J . A. Theor. Chim. Acta 1973, 28, 213.

Ge[2,4,6-(CF3)3C6H2]2 and Ge[N(SiMe3)2]2 as Ligands
Organometallics, Vol. 18, No. 8, 1999 1549
by a partial PM3(tm) geometry optimization (frozen coordi-
nates for heavy atoms and PH3). Calculations on “bare”
germylene ligands used the same atomic coordinates derived
for the nickel-germylene complexes. Natural population
analyses were performed using NBO v. 4.0.30
Resu lts a n d Discu ssion
As part of a previous study involving the catalytic
synthesis of an amide germane, we reported the syn-
thesis and structure of (Ph3P)2NiGe[N(SiMe3)2]2 (3).18
This complex exhibited a rather short Ni-Ge bond
length of 2.206(1) Å, 0.064 Å less than that expected
for a Ni-Ge single bond on the basis of covalent radii31
and 0.11 Å less than a related nickel germyl bond
length.32 However, comparison to complexes such as
[Cp′(CO)2MndGedMn(CO)2Cp′] (Cp′ ) C5H4Me) that
have bond lengths > 0.25 Å shorter than corresponding
germyls,33,34 and consideration of counterexamples with
short bonds such as the sulfur-stabilized silylenes,9
suggested that no claims of multiple bonding should be
made for a complex such as 3 without additional
supporting evidence. To experimentally probe the degree
of heteroatom stabilization afforded by the bis(trimeth-
ylsilylamide) groups, and the degree to which they
compete with the d10 nickel atom for π bonding with the
empty Ge p orbital, a new nickel germylene was
synthesized using Ge[2,4,6-(CF3)3C6H2]2 (4).
The synthesis of a nickel germylene containing 4
proceeded in direct analogy to complex 3. The general
route of Fahey and Mahan to d10 nickel complexes,
based upon addition of the free Lewis base ligands to
Ni(COD)2, was employed,35 resulting in deep burgundy
microcrystals of (Ph3P)2NiGe[2,4,6-(CF3)3C6H2]2 (2). A
single-crystal X-ray analysis of 2 confirmed that a
monomeric three-coordinate nickel complex was ob-
tained (Figure 2). The three structural parameters that
provide the most information regarding the character
of the Ni-Ge bonding are the Ni-Ge distance, the
PNiGeN dihedral angle, and the C-Ge-C angle. The
Ni-Ge bond distance of 2.1814(7) Å found for 2 is 0.025
Å shorter than that obtained for amide-stabilized 3. By
way of comparison, the Ru-Si bond in the ruthenium
dimethylsilylene complex is 0.031 and 0.048 Å shorter
than related sulfur-stabilized ruthenium silylenes.5 A
significant difference was also observed for the PNiGeX
dihedral angle: 76.1° for 3 and 59.9° for 2. However,
given the difference in steric bulk of the ligands, and
the interactions of the ortho-CF3 groups with the
germanium, the best interpretation of the 16.2° differ-
ence in angle is not readily apparent. The C-Ge-C
bond angle for 2 is 104.3(2)° as compared to 99.95(10)°
in the free germylene. On the basis of literature
precedent,36 this angle appears to be diagnostic of the
F igu r e 2. ORTEP diagram for 2 (50% probability).
Selected bond distances (Å) and angles (deg): Ni-P1,
2.1811(12); Ni-P2, 2.1752(12); Ni-Ge, 2.1814(7); Ge-C37,
2.051(4); Ge-C46, 2.057(4); P1-Ni-P2, 118.10(5); C25-
Ge-C46, 104.3(2); Ge-F3, 2.858(3); Ge-F9, 2.851(3).
degree of s and p character in bonds to germanium and
could indirectly reveal the nature of the metal-ger-
mylene bond. The literature contains an example of a
structurally characterized germanium double bond to
carbon, and the C-Ge-C angle observed is 113.4°.37
Complexes containing Ge-Ge double bonds exhibit a
wide range of C-Ge-C angles (110-128°).38,39 Struc-
turally characterized, σ-bonded alkylmetal carbenes
have C-C-C angles in the 109-112° range.40 The
C-Ge-C bond angle observed for 2 is considerably
smaller than that observed for authentic complexes
containing double bonds to germanium; however, the
angle is appreciably larger than that observed for the
free germylene despite the increased steric congestion
due to the (PPh3)2Ni fragment. It is also interesting to
note that the Ni-P distances observed for 2 of 2.1811-
(12) and 2.1752(12) Å are about 0.03 Å longer than the
average distance observed for Ni-P bonds in Ni(PPh3)3
(2.147(7) Å).41 The Ni-P distances in 3 are also some-
what greater (2.157(1) and 2.164(1) Å). The P-Ni-P
(37) Lazraq, M.; Escudie, J .; Couret, C.; Satge, J .; Drager, M.;
Dammel, R. Angew. Chem., Int. Ed. Engl. 1988, 27, 828.
(38) (a) Hitchcock, P. B.; Lappert, M. F.; Miles, S. J .; Thorne, A. J .
J . Chem. Soc., Chem. Commun. 1984, 480. (b) Snow, J . T.; Murakami,
S.; Masamune, S.; Williams, D. J . Tetrahedron Lett. 1984, 25, 4191.
(c) Drager, M.; Escudie, J .; Couret, C.; Ranaivonjatovo, H.; Satge´, J .
Organometallics 1988, 7, 1010. (d) Batcheller, S. A.; Tsumuraya, T.;
Tempkin, O.; Davis, W. M.; Masamune, S. J . Am. Chem. Soc. 1990,
112, 9394. (e) Tokitoh, N.; Matsumoto, T.; Manmaru, K.; Okazaki, R.
J . Am. Chem. Soc. 1993, 115, 8855. (f) Kira, M.; Iwamoto, T.;
Mariyama, T.; Kabuto, C.; Sakurai, H. Organometallics 1996, 15, 3767.
(g) Weidenbruch, M.; Stu¨rmann, M.; Kilian, H.; Pohl, S.; Saak, W.
Chem. Ber. 1997, 130, 735.
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275.
(30) Glendening, E. D.; Badenhoop, J . K.; Reed, A. E.; Carpenter,
J . E.; Weinhold, F. NBO 4.0; Theoretical Chemistry Institute: Uni-
versity of Wisconsin, Madison, WI, 1996.
(31) Dean, J . A., Ed. Lange’s Handbook of Chemistry; McGraw-
Hill: New York, 1985; pp 3-121 to 3-126.
(32) Bender, J . E.; Banaszak Holl, M. M. Unpublished results.
(33) Herrmann, W. A.; Kneuper, H.-J .; Herdtweck, E. Chem. Ber.
1989, 122, 433.
(34) Ga¨de, W.; Weiss, E. J . Organomet. Chem. 1981, 213, 451.
(35) Fahey, D. R.; Mahan, J . E. J . Am. Chem. Soc. 1977, 99, 2501.
(36) (a) Fjeldberg, T.; Haaland, A.; Schilling, B. E. R.; Lappert, M.
F.; Thorne, A. J . J . Chem. Soc., Dalton Trans. 1986, 1551. (b) Grev. R.
S.; Schafer, H. F. Organometallics 1992, 11, 3489. (c) Heinemann, C.;
Herrmann. W. A.; Thiel, W. J . Organomet. Chem. 1994, 475, 73.
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Organomet. Chem. 1977, 139, C68. (b) Herrmann, W. A.; Hubbard, J .
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1984, 23, 2978. (c) Angermund, K.; Grevels, F. W.; Kruger, C.; Skibbe,
V. Angew. Chem., Int. Ed. Engl. 1984, 23, 904. (d) Curtis, M. D.;
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1550 Organometallics, Vol. 18, No. 8, 1999
Bender et al.
angles for 2 and 3 are 118.10(5)° and 113.81°, respec-
tively. Another point of comparison between Ni(PPh3)3
and 2 is the interactions of the ortho-CH bonds of the
phenyl rings. Stephan et al. noted that three close Ni-H
interactions existed at 2.87, 2.74, and 2.91 Å in Ni-
(PPh3)3 and suggested that these may be related to the
facile decomposition observed via orthometalation. 2 has
two such contacts: the Ni-H12 contact of 2.748 Å and
the H2-Ge contact of 3.036 Å. Indeed, the C-H bond
of C2 is oriented directly over the Ni-Ge bond vector.
Despite the presence of the same magnitude of Ni-H
contact, 2 is observed to be quite thermally stable and
well-behaved as opposed to Ni(PPh3)3, which was quite
difficult to obtain in pure form, in part because of facile
side-reactions.41,42 Complex 3 also has close Ni-H and
Ge-H contacts. The closest Ni-H contacts are Ni-
H44B, Ni-H39C, and Ni-H30 at 2.954, 3.028, and
3.022 Å, respectively. The contacts from H44B and
H39C arise from -SiMe3 groups that bracket the Ni on
either side of the NiP2 plane. H30 is an ortho-C-H bond
from one of the triphenylphosphine ligands. The closest
Ge-H contacts are Ge-H2, Ge-H37C, and Ge-H43C
at 3.045, 3.140, and 3.059, respectively. Unlike Ni-
(PPh3)3, none of the (R3P)2NiGeY2 complexes isolated
to date have exhibited a tendency to orthometalate.
Apparently, germylenes 4 and 5, as well as Ge[CH-
(SiMe3)2]2, dramatically stabilize the (Ph3P)2Ni fragment
upon formation of the three-coordinate complex in a way
that adding a third PPh3 group is unable to achieve.
The most obvious difference between PPh3 and the
germylenes is the presence of the empty p orbital on
the germylene available for π back-bonding.
Complex 2 has a number of close Ge-F contacts. The
two shortest are at 2.858 Å (Ge-F3) and 2.851 Å (Ge-
F9), and there are two longer range interactions at 3.002
Å (Ge-F17) and 3.058 Å (Ge-F12). These distances are
all longer than the four closest contacts observed for the
free germylene species of 2.566(2), 2.554(1), 2.790(2),
and 2.781(2) Å.17 Indeed, in the free germylene the two
closest contacts are located directly above and below the
empty p orbital, suggesting a donor-acceptor interac-
tion between the fluorine lone pairs and the ger-
mylene.43 In addition, the C-F bond lengths of the close
contacts in the free germylene show a small but statisti-
cally significant increase of ∼0.04 Å consistent with a
bonding interaction, whereas none of the C-F bonds of
2 show any elongation. Calculations indicate that bind-
ing of the (PPh3)2Ni center has served to reduce the
electrophilicity of the germanium center via π back-
bonding, resulting in lengthened Ge-F contacts (vide
infra). The CF3 contacts in 2 are unique for the
germanium compounds studied to date containing the
[2,4,6-(CF3)3C6H2]- substituent.17,44 This is the only
case in the [2,4,6-(CF3)3C6H2]-Ge system where only
one of the ligands engages in short Ge-F contacts (each
∼2.8 Å) with both of its CF3 groups. The other [2,4,6-
(CF3)3C6H2]- substituent has leaned back and does not
make any close Ge-F contacts. The Ge-C bonds in 2
F igu r e 3. Ni-Ge π bond calculated for (A) (H3P)2NiGe-
[2,4,6-(CF3)3C6H]2 (2H) and (B) (H3P)2NiGe[N(SiMe3)2]2
(3H). Coordinates from the X-ray structures of 2 and 3 were
used; however the phenyl groups on phosphorus were
replaced by hydrogen atoms. The calculation was at the
BPDN** level as implemented by Spartan 5.0.
are 2.057(4) and 2.051(4) Å; however the Ni-C distances
are very different: 3.936(4) (Ni-C46) and 3.650(4) (Ni-
C37) Å, with the shorter distance corresponding to the
[2,4,6-(CF3)3C6H2]- substituent with F-contacts to ger-
manium. The Ni-F3 distance in this case is 3.286(3)
Å, longer than one would expect for a contact. (Ph3P)2-
NiGe[N(SiMe3)2]2 (3) shows no such asymmetry in the
amide ligands.
In an effort to understand the nature of the M-Ge
bonding in greater detail, we have undertaken a com-
putational investigation of 2 and 3 using nonlocal
density functional methods. BP86/DN** calculations on
(H3P)2NiGe[2,4,6-(CF3)3C6H2]2 (2H) reveal molecular
(Kohn-Sham) orbitals with NiGe π and π* character.
The π orbital is occupied (HOMO-4) and arises from Ni-
Ge d-p overlap (Figure 3a). The orbital is polarized
toward Ni and contains small NiP antibonding contri-
butions. The π* orbital is unoccupied (LUMO) and is
mainly localized on Ge. These observations suggest that
there is a NiGe π bond in 2H that is highly polarized
toward Ni. Interpretation of the molecular orbitals of
(H3P)2NiGe[N(SiMe3)2]2 (3H) is more difficult. Several
occupied orbitals reveal out-of-plane NiGe bonding
interactions, but only HOMO-4 shows simultaneous π
bonding overlap above and below the NiGe axis (Figure
3b). HOMO-4 is highly polarized toward Ni and also
contains small GeN antibonding contributions from p
orbitals on the two N. Apparently, NiGe π bonding is
(42) Tolman, C. A.; Seidel, W. C.; Gerlach, D. H. J . Am. Chem. Soc.
1972, 94, 2669.
(43) Shusterman, A. J .; Banaszak Holl, M. M.; Bender, J . E.
Unpublished results.
(44) For other crystallographically characterized germanium com-
pounds containing this ligand see: Bender, J . E.; Banaszak Holl, M.
M.; Mitchell, A. M.; Wells, N. J .; Kampf, J . W. Organometallics 1998,
17, 5166.

Ge[2,4,6-(CF3)3C6H2]2 and Ge[N(SiMe3)2]2 as Ligands
Organometallics, Vol. 18, No. 8, 1999 1551
Ta ble 1. Su m m a r y of Key Metr ica l P a r a m eter s for Str u ctu r a lly Ch a r a cter ized Th r ee-Coor d in a te Gr ou p 10
Ger m ylen e Com p lexes
M-Ge
(Å)
av M-P
(Å)
av Ge-X
P-M-P
(deg)
X-Ge-X
(deg)
PMGeX dihedral
(deg)
complex
(Å)
(Et3P)2PtGe[N(SiMe3)2]2 (1)
(Ph3P)2NiGe[2,4,6-(CF3)3C6H2]2 (2)
(Ph3P)2NiGe[N(SiMe3)2]2 (3)
2.3039(10)
2.1814(7)
2.2064(6)
2.2615(7)
2.178(4)
2.160(5)
1.870(5)
2.054(4)
1.88(1)
114.98(10)
118.10(5)
113.81(4)
106.3(3)
104.3(2)
105.05(11)
77.8
59.9
76.1
Ta ble 2. NBO Occu p a n cies for 2H a n d 3H
Assu m in g No Ni-Liga n d Bon d in g Or bita ls a n d
F ive Ni Lon e P a ir Or bita ls
less important in 3H than it is in 2H. Nevertheless, both
complexes exhibit NiGe π bonding, and this may affect
NiGe bond distances and mediate the photochemical
and 2+2 type of reactivity previously noted in this
system.11-13
A more definitive picture of NiGe bonding is obtained
through natural bond orbital (NBO) analysis of the
delocalized Kohn-Sham orbitals.30 The orbitals are first
transformed into a set of natural atomic orbitals (NAO)
that provide atomic orbital populations and atomic
charges and then transformed into localized natural
bond orbitals (NBO) that can identify bonding interac-
tions. Each transformation is carried out in a way that
maximizes the occupancy of certain orbitals (up to two
electrons) while minimizing the occupancy of others.
Deviations from the ideal occupancies provide an effec-
tive measure of metal-ligand bonding.
orbital occupancy (electrons) and hybridization
atom
Ni
2H
3H
1.98 (d)
1.94 (d)
1.93 (d)
1.88 (d)
1.66 (d)
1.98 (d)
1.94 (d)
1.92 (d)
1.87 (d)
1.72 (d)
0.51 (“empty” s)
0.52 (“empty” s)
Ge
1.41 (sp2.4
)
1.51 (sp1.3
)
0.52 (“empty” p)
0.55 (“empty” p)
P
P
1.74 (sp2.6
1.73 (sp2.7
)
)
1.75 (sp2.3
1.75 (sp2.3
1.76 (p)
)
)
N
N
1.77 (p)
mainly due to different degrees of delocalization within
each germylene ligand and not differences around the
metal center. NBO analysis of the “bare” germylene
ligands assigns 1.9% of the electron density (5.9 elec-
trons) in the diarylgermylene (derived from 2H) to outer
orbitals, but only 0.9% of the electron density (1.8
electrons) in the diaminogermylene (derived from 3H)
to outer orbitals. This is not surprising since the
benzene rings and the trifluoromethyl groups in the
former are all highly delocalized.
The NAO populations for 2H and 3H are quite
similar. The Ni atom and phosphine ligands in 2H each
carry a small positive charge, qNi ) 0.08 (electron
configuration ) d9.36s0.54p0.02) and qPH3 ) 0.08 and 0.09,
indicating net electron transfer from (PH3)2Ni to the
germylene, qGeAr2 ) -0.25. The same trend is seen in
3H, but to a lesser degree: qNi ) 0.05 (electron config-
uration ) d9.40s0.55p0.02), qPH3 ) 0.04 and 0.05, qGeN2
)
-0.14. Unfortunately, these charges only define net
electron transfer and cannot be used to describe the
degree of NiGe π bonding. Separating σ and π electron
transfer requires construction of natural bond orbitals.
NBO analysis of 2H and 3H reveals that each
molecule is a strongly delocalized complex, indicating
that several alternative sets of bond orbitals are pos-
sible. Given that nickel is nearly neutral, we first
selected a set of bond orbitals in which each ligand
carries standard bonds and lone pairs. The ligating
atoms, Ge and P, each carry a single lone pair, and Ni
carries five lone pairs and no metal-ligand bonds. This
set of NBO allows easy identification of metal-to-ligand
and ligand-to-metal electron transfer. Metal-to-ligand
electron transfer is indicated by metal lone pair orbitals
with “low” occupancies (,2 electrons) and outer ligand
orbitals with “high” occupancies (>0 electrons). Con-
versely, ligand-to-metal electron transfer is indicated by
ligand lone pair orbitals with low occupancies and outer
metal orbitals with “high” occupancy.
Although the bonding pattern described above was
arbitrarily selected, it provides a fair description of the
electron density in 2H and 3H. 97.9% of the electron
density (362.4 electrons) in 2H is assigned to core and
valence orbitals, and only 2.1% (7.6 electrons) is as-
signed to outer orbitals. 3H is described even better by
this set of orbitals; 98.8% of the electron density (270.7
electrons) is assigned to core and valence orbitals, and
only 1.2% (3.3 electrons) is assigned to outer orbitals.
The higher occupancy of outer orbitals in 2H indicates
that the diarylgermylene complex is more delocalized
than the diaminogermylene complex; however, this is
The NBO occupancies in 2H and 3H reveal that both
complexes are stabilized by nickel-to-germanium elec-
tron transfer and π back-bonding, but to a different
degree (Table 2). The occupancies of the lone pair (σ
donor) orbitals on the ligating atoms are relatively low
(∼1.5 Ge, 1.7 P), and the occupancy of the outer (σ
acceptor) Ni s orbital is high (∼0.5). This is consistent
with partial ligand-to-metal electron transfer via σ
bonds. NBO analysis also reveals a low occupancy (∼1.7)
Ni d orbital and a high occupancy (∼0.5) “empty” Ge p
orbital in each complex. These results are consistent
with partial nickel-to-germanium electron transfer via
π back-bonding.
We can show that NiGe π back-bonding is more
effective in 2H than in 3H by comparing the NBO
occupancies of the metal complexes with those of the
“bare” ligands. The “empty” Ge p orbital in the “bare”
diarylgermylene (derived from 2H) is occupied by less
than 0.01 electron. This suggests that the 0.52 electron
occupying the “empty” Ge orbital in 2H is derived
almost entirely from π back-bonding. Note that this
theoretical result is consistent with the experimentally
observed change in the lengths of the Ge-F contacts in
4 versus 2. Ni-Ge π back-bonding reduces Ge electro-
philicity, resulting in longer Ge-F contacts. The “empty”
Ge p orbital in the “bare” diaminogermylene (derived
from 3H) has an occupancy of 0.28 electrons. This means
that the 0.55 electrons occupying the “empty” Ge orbital
in 3H are derived from a combination of π back-bonding
and NfGe electron transfer. Comparison of N lone pair
occupancies in the “bare” germylene and in 3H shows

1552 Organometallics, Vol. 18, No. 8, 1999
Bender et al.
Ta ble 3. NBO Occu p a n cies for 2H a n d 3H Assu m in g Tw o NiGe Bon d in g Or bita ls a n d F ou r Ni Lon e P a ir
Or bita ls
orbital occupancy (electrons), polarization, hybridization
orbital
2H
3H
NiGe σ
1.82
1.85
32% Ni (sd0.8)/68% Ge (sp1.5
)
)
32% Ni (sd1.0)/68% Ge (sp1.0
)
)
NiGe σ*
NiGe π
NiGe π*
0.85
0.89
68% Ni (sd0.8)/32% Ge (sp1.5
1.94
68% Ni (sd1.0)/32% Ge (sp1.0
1.95
83% Ni (d)/17% Ge (p)
0.26
17% Ni (d)/83% Ge (p)
86% Ni (d)/14% Ge (p)
0.33
14% Ni (d)/86% Ge (p)
that NiGe back-bonding reduces the amount of NfGe
electron transfer in 3H. Therefore, the difference in Ge
p orbital occupancies (0.28 f 0.55 electron) represents
a lower limit on the amount of NiGe electron transfer
in 3H. However, even when we allow for changes in
NfGe electron transfer, the orbital occupancies still
indicate that π back-bonding is more effective in 2H
than in 3H.
We repeated the NBO analysis with a bonding pattern
in which the Ge lone pair and one Ni lone pair were
replaced by two NiGe bonding orbitals (all remaining
orbitals were identical to those described above). The
characteristics of the NiGe bonding and antibonding
orbitals are listed in Table 3. The NiGe σ bonding
orbitals are polarized toward Ge. The occupancies of
these orbitals are relatively low, and the occupancies
of the corresponding σ* orbitals are very high, indicating
weak and highly delocalized σ bonding. The hybridiza-
tion of Ge in the σ bonding orbital changes substantially
between 2H (41% s) and 3H (49% s), which is also
consistent with changes in Ge lone pair hybridization
obtained using the previous orbital definition (Table 2).
Increasing s character in 3H should lead to a shorter
NiGe σ bond and offset the weaker π bond found in this
complex (vide infra).
NBO analysis also finds NiGe π bonding orbitals
involving NiGe d-p orbital overlap. The π orbitals are
highly polarized toward Ni, consistent with the usual
π back-bonding model. Interestingly, the orbital oc-
cupancies, π > σ and π* , σ*, indicate that the π
bonding electrons are localized between Ni and Ge to a
much higher degree than are the σ bonding electrons.
Comparison of the π orbitals of 2H and 3H indicates
that NiGe π bonding is slightly more polarized in 3H.
The π* occupancy in 3H is also significantly higher,
indicating that π bonding is less effective in this
complex.
The use of two different sets of bonding orbitals (no
Ni-ligand bonds vs NiGe double bond) for NBO analysis
may seem to introduce an arbitrary element into the
description of NiGe bonding. However, it should be clear
that both analyses agree on key points: (1) Ge acts as
a σ donor/π acceptor ligand, (2) Ni acts as a σ acceptor/π
donor metal, (3) the hybridization of the Ge σ donor
orbital changes from “low” s-character in 2H to “high”
s-character in 3H, and (4) NiGe π bonding is more
effective in 2H. The similarity of NiGe bond distances
in 2H and 3H may simply reflect the different roles of
σ and π bonding in the two complexes; 2H seems to
contain a weaker σ bond and a stronger π bond than
3H. A complete description of bonding in 2H and 3H
should also explain trends in other structural param-
eters, such as P-Ni-P and X-Ge-X bond angles, NiP
distances, and the P-Ni-Ge-X dihedral angle. These
require a more detailed analysis, however, and are
beyond the scope of this paper. A detailed study of the
role of π bonding in group 10 metal germylenes is in
progress.43
Con clu sion s
Comparison of the bond lengths of the metal ger-
mylene complexes with related metal germyls, and with
tabulated covalent radii, indicates M-Ge bond lengths
∼ 0.1 Å shorter than expected for the nickel and
platinum complexes. The experimental evidence sug-
gests that some type of multiple bonding might be
occurring; however no firm conclusions could be drawn
based upon the experimental data alone. Nonlocal
density functional calculations indicate the presence of
Ni-Ge π bonds and significant amount of metal-to-
germylene electron transfer for the two nickel ger-
mylenes studied. The interaction is consistent with
typical metal-to-ligand π back-bonding. Additional theo-
retical studies are under way to assess the significance
of the Ni-Ge π bonding in the observed bond length
and angle changes and the relationship to the reactivity
patterns observed in these three-coordinate metal-
germylene systems.43,45
Ack n ow led gm en t. M.M.B.H. thanks the University
of Michgan for support of this work. J .E.B. thanks the
NSF for a graduate fellowship. T. D. Tilley and D. H.
Berry are acknowledged for fruitful discussions regard-
ing this chemistry.
Su p p or tin g In for m a tion Ava ila ble: Crystal data for 2.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM9900521
(45) Aviyente, V. Work in progress.