Accessing metal-carbide chemistry. A computational analysis of thermodynamic considerations.
The electronic structures of terminal metal carbide complexes are calculated using DFT. This study outlines the factors that give rise to stable carbide complexes, which can be used to help in the synthesis of new carbide complexes and to tune their stability as desired. The calculations reveal the presence of a strong Ru≡C triple (σ + 2π) bond. The C atom is nearly unhybridized, such that the C-component of the Ru−C σ-bond has 90% 2p character. This leaves a very stable carbon-based lone pair that is almost entirely 2s in character, which accounts for the lack of Lewis base character exhibited experimentally. Calculations predict a Ru−C bond dissociation energy of 147.4 kcal mol−1 in a typical Ru carbide complex. This large bond strength is not unique to the Ru≡C bond, as revealed by an extension of the study to identify schemes by which to chemically tune the metal−carbide bond strength. Methods examined to achieve this tuning include changing the identity of the central metal and altering the metal ligation scheme. In general, 16-electron square-pyramidal M(C)L4 complexes and 12- or 16-electron tetrahedral M(C)L3 complexes of the 4d elements can possess comparably strong metal−carbide bonds. The calculations also show that the carbide moiety exerts a very strong trans influence, which explains several experimental observations. We conclude that the dearth of terminal carbide complexes is not due to any inherent weakness of M≡C bonds. Many more terminal carbide complexes can be expected in the future as new routes to their formation are found.
Gary, J. Brannon, "Accessing metal-carbide chemistry. A computational analysis of thermodynamic considerations." (2008). Faculty Publications. 70.
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