Metal-to-metal bonding in transition metal monocarbides and mononitrides

The chemical bonding in transition metal carbides and nitrides is compared and contrasted via simplified interaction scheme. Different properties of dd bonding states are exemplified by VC and VN. Different patterns of the metal-to-metal bonding in these two compounds are investigated in terms of overlap populations. A modeling of (i) energy matching and (ii) orbital dimensions is performed to find out electronic reasons for dissimilarities in metal-to-metal bonding. The energy difference between interacting atomic levels is demonstrated as the reason for shift of the pd vs dd competition. In the nitride, the large value of Delta E(pd) (2.4 eV) gives rise to the decreased pd bonding and increased net metal-to-metal bonding. The carbide having a small Delta E(pd) (0.4 eV) displays the strengthened pd bonding, whereas net dd bonding practically vanishes. The larger overlap of atomic orbitals in the carbide results in the pronounced broadening of the p band toward higher binding energies. It does not influence, however, the net metal-to-metal bonding. (C) 1997 Academic Press.