A valence bond study of the bonding in first row transition metal hydride cations: What energetic role does covalency play?

The transition metal hydride cations, TMH+ (TM = first transition metal row , Sc, Ti, V, Cr, Mn, Co, Ni, Cu, and Zn), have been studied using valence b ond (VB) theory to elucidate the bonding in these systems through VB concep ts. Although the bonds appear extremely covalent by virtue of charge distri bution, this appearance conceals key contributions to bonding, such as cova lent-ionic resonance energy (RECS) and relaxation energy of the inactive el ectrons (Delta E-relax(inactive)). The RECS term is seen to increase from S cH+ toward ZnH+, becoming significant in the late TMH+ molecules. The Delta E-relax(inactive) term, which accountsfor the nonbonding 3d(n) electrons a nd the 3s(2)3p(6) core electrons, is always significant. Furthermore, fdr a ll of the bonds from CrH+ to CuH+, the relaxation term makes a major contri bution to the bond energy. It appears therefore, that in these TM-Ht bonds, the spin pairing of the bonding electrons can act as a trigger for the non bonding and adjacent core electrons to relax their Pauli repulsion and ther eby strengthen the binding of TM+ and H. As a result of the general weaknes s of TM bonds, the relaxation is expected to frequently be an important bon ding contribution. The major function of the inactive and core electrons sh ows that the traditional role of "covalency" must be reassessed in a system atic manner.