Methane is spontaneously dehydrogenated in the gas phase by many metal ions of the 5d transition series. In most cases, the MCH2+ produced undergoes further reactions, leading eventually to products such as WC8H16+. The reactivity of the third-row transition-metal ions, both bare and with simple ligands, may be explained in terms of electronic structure considerations. Promotion energy, exchange energy, and the relative and absolute sizes of the valence s and d orbitals all appear to be important.