Principal Resonance Contributors to High-Valent, Transition-Metal Alkylidene Complexes Page: 5,236

                                
5236  J. Am. Chem. Soc., Vo. I13, No, 14, 1991

The basis B and B+ equilibrium geometries for the group V
alkylidene complexes are identical. The addition of diffuse
functions to carbon yields stationary points whose bond lengths
agree to the nearest 0.01 A and whose bond angles agree within
a degree with those results obtained with basis B. In the Ta
alkylidene complex, as for the Hf alkylidene complex, there is a
significant contraction of the metal-carbon bond upon switching
from basis A to those employing the SBK ECPs and valence basis
set; other geometrical parameters were little changed. The eclipsed
rotamers are transition states on the potential energy surfaces;
the imaginary mode in each case corresponds to rotation about
the M-C bond and leads to the staggered conformation. The
staggered conformations are minima.
Several test calculations were performed on Hf and Ta al-
kylidenes employing the SBK pseudopotentials and a 3-21G basis
for C and H (i.e. basis B without a polarization function on the
C). This is referred to as basis B-. The purpose of these cal-
culations is to see if the difference in equilibrium geometries
between basis A and basis B results from improving the basis set
on the ligand. Those geometry optimizations which employed basis
B- yield virtually identical equilibrium geometries to those ob-
tained with basis B and B+. This suggests that the changes in
geometry for the third-row alkylidenes are due to improvement
of the metal valence basis set and/or ECPs. The calculated Ta==
bond length in the staggered conformation for the model 10-
electron complex H3TaCH2 (basis B and B+) is 1.90 A. This
calculated bond length is X=0.13 A shorter than that measured
for 18-electron complexes such as TaCp2(CH3)CH." However,
the calculated Ta=C bond length is in excellent agreement when
compared to electron-deficient compounds such as the 1.90 A
measured by neutron diffraction for the 14-electron" complex
[TaCI3(PMe3)(=CHCMe )j2.4     The calculated Ta-H bond
lengths (average of 1.81 A in basis B and B+) are in good
agreement with the average Ta-H bond length of 1.78 A measured
with use of neutron diffraction for the complex CpTaH3." The
average Nb-H bond length of 1.69 A in Cp2NbH3, as measured
by X-ray diffraction," is rather short compared with the calculated
value of 1.81 A from basis B. Note, however, that the uncertainty
in the X-ray value (*0.06 A) is much larger than that in neutron
diffraction (-0.01 A) due to the instability of the Nb complex."
Indeed, the Nb-H bond lengths in Cp2NbH3 are expected to be
very similar to those for the Ta analogue since Nb and Ta possess
virtually identical covalent and ionic radii as well as comparable
electronegativities."t'
As was the case for the group IV alkylidenes there is no vi-
brational data for M-C bonds of group VB alkylidenes. The
intrinsic w, for the Nb-H and Ta-H bonds (basis B+) are 1777
and 1793 cm-a, respectively. IR bands at 1710 (Cp2Nb(H)3) and
1735 cm-' (CpTa(H)3) have been assigned to M-H stretches.'0
Related hydrido complexes of these metals also have IR bands
in this region which have been assigned to vMH modes.7,
The bonding about the methylidene ligand is also different from
that of its main group analogues, the phosphorus ylides, in that
the carbene C possesses a planar coordination geometry in the
former compounds. This deviation from ideal trigonal-planar
geometry in phosphorus ylides has been ascribed to interaction
of the P-H o bonds with the C lone pair,6-  A calculation on
H3PCH, (using the HW ECPs for phosphorus, with a d exponent
of 0.45, and the 3-21G(d) basis for C, H) shows an admixture
of 23% P and H character in the HOMO which is concentrated
largely on the carbon. In contrast, the HOMO in H3Ta=CH2
(basis B+, staggered geometry) is 46% Ta dr and 54% C pr with
virtually no electron density on the hydrogens bound to tantalum
(<0.5%). A nearly zero rotational barrier about the M-C bond
is predicted for these compounds, as is the case for the phosphorus

(68) The various electronic counting schemes are discussed in ref 6a.
(69) Wilson, R. P.; Koetzle, T. F.; Hart, D. W.; Kvick, A.; Tipton, D. L.;
Bau, R. S. Am. Chem. Soc. 1977, 99, 1775.
(70) Maslowsky, E. Vibrational Spectra of Organometallic Compounds;
Wiley: New York, 1979; p 353.
(71) Albright, T. A.; Burdett, J. K.; Whangbo, M. H. Orbital Interactions
In Chemistry; Wiley: New York, 1985; p 178.

Chart IV. C, Geometry of Models for Olefin Metathesis Catalysts

'-"w    x

Mi - ReX =C
M -Mo. W; X- = N

4
Chart V. Schematic Representation of Interaction between w
Orbitals on C and N with Metal dr AOs
C

5a

Sb

Sc

analogues."5 Rotational barriers of 17-21 kcal mol-' have been
measured for Cp2Ta(--CHR)(X) complexes with use of data
obtained from variable-temperature NMR.4,63 The differences
between the Cp2M and H2M barriers apparently arise from the
strong r interaction between the rings and the metal.72'3
c.   High-Valent, Lewis Acid Free Metathesis Catalysts
Carbenes have been implicated in the olefin metathesis reaction
since the seminal work of Herrison and Chauvin in the early
1970s." Many of the early metathesis catalysts were poorly
characterized mixtures,22-26"45"    Schrock et al.?23"336 and
others34.36j'67n have recently synthesized a family of well-char-
acterized, high-valent alkylidene complexes for use as olefin
metathesis catalysts. The minimum-energy geometries for the
metathesis catalyst models, 4, were calculated with both basis sets
A and B (Table ill) under the constraint of C, symmetry. All
of the alkylidene complexes maintain a distorted-tetrahedral co-
ordination environment about the central metal atom. Evidence
for the bonding as shown in 4 (M=C double bond, M=N triple
bond) is provided by an analysis of the wave functions obtained
for the minima and the geometries about the C (trigonal planar)
and N (linear). There are three r pseudosymmetry metal d
orbitals (corresponding to the t2 set of Td symmetry). Since these
complexes are formally d0, the most favorable conformation for
the r-bonding ligands will be that which allows for maximum
in-phase bonding between the metal dr and the ligand pr orbitals
without having the ligands compete for the dr orbitals. If the
M, C, and N atoms lie in the xz plane with the metal at the origin
and carbon along the positive z axis, 4, then the carbon pr will
interact with the d, Sa. The N pl (tN     Px) and p, (=N p,)
(72) See refs 12c, 12d, and 60 for a discussion of this point.
(73) The rotational barrier for the TaCp(CH2)Me complex is calculated
to be 28 kcal mrol' at the relativistic extended HOckel level of theory. This
is in reasonable agreement with the experimentally determined value* of a21
kcal mol-'. In contrast to this H2TaCH, and H3NbCH2 are calculated to have
rotational barriers of less than kcal mol-' by use of both relativistic Extended
HOckel and ab initio wave functions. These rotational barriers were calculated
with the program P[TERtEx (kindly provided by P. Pyykk, Helsinki) and by
keeping all geometric parameters frozen except for the methylene torsional
angle. Lauher and Hoffman6 have used EHT to calculate a rotational barrier
of 28 kcal mol-' in the isoelectronic TiCp(CH2)Me- complex.
(74) Mocella, M. T,; Busch, M. A.; Muetterties, E, L. J. Am. Chem. Soc.
1976, 98, 1283.
(75) Basset, J. M.; Taarit, Y. B.; Coudurier, G.; Praliaud, H, J. Orgno-
met. Chem 1974. 74, 167.
(76) (a) Ginsburg, E. J.; Gorman, C, B.; Marder, S R.; Grubbs, R. H. J.
Am. Chem. Soc. 1989, 111, 7621. (b) Swager, T. M.; Grubbs, R. H. J. Am.
Chem. Soc. 1989. 111, 4413.
(77) (a) Kress, J.; Osborn, J. A. J. Am. Chem. Soc. 1983, 105, 6346. (b)
Kress, J.; Osborn, J. A.; Greene, R. M. E.; Ivin, K. J.; Rooney, R. R. J. Am.
Chem. Soc. 1987, 109, 899.

Cundari and Gordon