Principal Resonance Contributors to High-Valent, Transition-Metal Alkylidene Complexes Page: 5,231
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J. Am. Chem. Soc. 1991, 113, 5231-5243
Principal Resonance Contributors to High-Valent,
Transition-Metal Alkylidene Complexes
Thomas R. Cundari and Mark S. Gordon*
Contribution from the Department of Chemistry, North Dakota State University,
Fargo, North Dakota 58105-5516. Received November 8, 1990
Abstract: The results of ab initio calculations are reported for prototypical high-valent, alkylidene complexes. Stationary
points on each potential energy surface are characterized and compared to experimental information where available; as long
as a suitably flexible valence basis set is used, good agreement between theoretically calculated and experimentally determined
geometries is obtained. The complexes of interest include group IVB (Ti, Zr and Hf) and group VB (Nb and Ta) alkylidenes
with hydride ligands as well as models for the four-coordinate, olefin metathesis catalysts (Mo-, W-, and Re-alkylidenes)
which have been recently synthesized and characterized. In light of the fact that much of the discussion concerning the reactivity
of transition-metal carbene complexes has been presented in terms of the resonance contributors derived from rearranging
the electrons in the M-C a and ir orbitals, the minima obtained from the first portion of the study are then subjected to a
further procedure to calculate these contributions. Resonance structures in which the carbon is the negative end of the M-C
bond (i.e., nucleophilic resonance structures) contribute 50% to the ground-state wave function of these complexes. Those
in which the carbon is formally neutral account for much of the remainder (45%). Only 5% is comprised of electrophilic resonance
structures, i.e., those in which the carbon is the positive end of the M-C bond. Furthermore, the metal-carbon double bond
is predominantly comprised of five resonance structures. Four of these resonance structures correspond to models of carbene
bonding which have been discussed previously in the literature. The other resonance structure, which contributes roughly
33% to the ground-state wave function, has hitherto not been considered when examining the chemical reactivity of carbenes.
This large resonance contributor can be described as arising from a dative carbon-to-metal a bond plus a covalent M-C ir
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R R II
R' /~ LMC R'
I l R
Fischer-Tropsch3,29 synthesis (the reduction of CO by H2 to
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0002-7863/91/1513-5231$02.50/0 @ 1991 American Chemical Society
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Cundari, Thomas R., 1964- & Gordon, Mark S. Principal Resonance Contributors to High-Valent, Transition-Metal Alkylidene Complexes, article, July 1991; [Washington, D.C.]. (digital.library.unt.edu/ark:/67531/metadc107773/m1/1/: accessed August 19, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT College of Arts and Sciences.