Theoretical Study of Transition-Metal Hydrides. 5. HfH⁺ through HgH⁺, BaH⁺, and LaH⁺

We present ab initio calculations (generalized valence bond plus configuration interaction, using relativistic effective core potentials) on the monopositive diatomic metal hydride ions of the third transition-metal series (HfH⁺ through HgH⁺ plus BaH⁺ and LaH⁺). We analyze the trends in bond energies, equilibrium geometries, bond character, and excitation energies in terms of (1) the atomic configuration of the metal, (2) the orbital sizes of the metal, and (3) the exchange and promotion energies on bonding hydrogen to the high-spin metal. The bond dissociation energies are found to be significantly larger than in either first- or second-row transition-metal hydride cations.