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 MCH(2)(+) 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 D(M(+)-CH).