Energetics of Third-Row Transition Metal Methylidene Ions MCH₂⁺ (M = La, Hf, Ta, W, Re, Os, Ir, Pt, Au)

High-level ab initio calculations, involving multireference configuration interaction and moderately large basis sets, have been performed to determine the metal-carbon bond energies in the metal methylidene ions MCH₂⁺ of the 5d transition series. On the basis of our calculations and available experimental data, the recommended bond energies D(M⁺-CH₂) are 98 ± 1.5 (La), 104 ± 5 (Hf), 115 ± 5 (Ta), 111 ± 3 (W), 97 ± 4 (Re), 113 ± 3 (Os), 123 ± 5 (Ir), 123 ± 5 (Pt), and 94 ± 2 (Au) kcal/mol. These bond energies are consistent with the experimentally observed reactivity of the metal ions M⁺ with methane. The double-humped pattern is explained in the context of promotion and exchange energies. The arguments are extended in order to estimate metal-methylidyne bond strengths Z)(M⁺-CH).