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

The electronic structure of the dipositive transition metal hydrides ScH2+, TiH2+, VH2+ CrH2+ and MnH2+ has been studied, using MCSCF and internally c ontracted MRCI techniques. All electronic states that correlate with the lo west term of the dipositive transition metal and H(S-2) have been studied a s a function of internuclear separation, and we find that ScH2+, TiH2+, VH2 +, well as several states of CrH2+, are thermodynamically stable. The poten tial 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 electr onic states, their bond lengths and dissociation energies are discussed. Mo st interesting is the nature of the bonding and its variation as one goes f rom ScH2+ to MnH2+. All of these molecules have a large electrostatic compo nent to the bonding that arises from the cation polarizing the H atom. Addi tionally, they have a conventional sigma bond between the 3d(sigma) orbital and the His orbital, which decreases in importance as one goes from ScH2to MnH2+ and is essentially gone at CrH2+. This reflects the contraction of the 3d(sigma) orbitals, the increasing ionization energy as well as the in creasingly larger exchange energy in the high-spin 3d shell, as one goes fr om ScH2+ to MnH2+. Rather than disrupt the high-spin coupling to bond to a 3d(sigma) orbital, the H atom essentially couples its spin antiferomagnetic ally to the intact spin state of the transition metal dication.