Inequivalence of equivalent bonds: Symmetry breaking in Co(CH ₃)₂⁺

In a theoretical study of the gas phase insertion of transition-metal cations into the C-H and C-C bonds of simple alkanes, an unusual aspect of the metal-carbon bond was discovered. Using ab initio methods (generalized valence bond and configuration interaction), it was found that the two methyl groups in Co(CH₃)₂⁺ do not bond to equivalent sd hybrid orbitals as one might expect. Instead, using a single valence bond (VB) spin coupling, we found two distinctly different bonds: one to a Co 4s-like orbital and the other to a 3d-like orbital, leading to a distortion of the molecule from its symmetrical geometry. With the resonance of two valence bond configurations, the bond distances become equivalent and symmetry is restored, however, the bonding orbitals in each configuration remain quite inequivalent. Similar behavior was observed on the potential-energy surface of CoH ₂⁺ and this description was found to carry over to Co(H)(CH₃)⁺, where one VB configuration dominates: the hydrogen bonds to the Co 4s orbital while the methyl group bonds to a 3d orbital. The behavior of the cobalt complexes contrasts dramatically with that of the isoelectronic complexes Rh(R₁) (R₂)⁺ and Ir(R₁) (R₂)⁺ (R₁,R ₂=H,CH₃), in which the ligands form bonds to sd hybrid orbitals. These orbitals are essentially the same in the case of R₁=H and R₂=CH₃ and strictly equivalent in the case of R ₁=R₂.