Electronic structure of the dipositive transition metal hydrides ScH²⁺, TiH²⁺, VH²⁺, CrH²⁺ and MnH²⁺

Abstract

The electronic structure of the dipositive transition metal hydrides ScH²⁺, TiH²⁺, VH²⁺, CrH²⁺ and MnH²⁺ has been studied, using MCSCF and internally contracted MRCI techniques. All electronic states that correlate with the lowest term of the dipositive transition metal and H(²S) have been studied as a function of internuclear separation, and we find that ScH²⁺, TiH²⁺, VH²⁺, as well as several states of CrH²⁺, are thermodynamically stable. The potential energy curves of MnH²⁺ cross the relevant repulsive M⁺ + H⁺ curves at sufficiently large separations to ensure that the character of the molecule at equilibrium is well represented by Mn²⁺—H. The ordering of the electronic states, their bond lengths and dissociation energies are discussed. Most interesting is the nature of the bonding and its variation as one goes from ScH²⁺ to MnH²⁺. All of these molecules have a large electrostatic component to the bonding that arises from the cation polarizing the H atom. Additionally, they have a conventional sigma bond between the 3d_σ orbital and the H1s orbital, which decreases in importance as one goes from ScH²⁺ to MnH²⁺ and is essentially gone at CrH²⁺. This reflects the contraction of the 3d_σ orbitals, the increasing ionization energy as well as the increasingly larger exchange energy in the high-spin 3d shell, as one goes from ScH²⁺ to MnH²⁺. Rather than disrupt the high-spin coupling to bond to a 3d_σ orbital, the H atom essentially couples its spin antiferomagnetically to the intact spin state of the transition metal dication.