SEMILOCALIZED BONDING SCHEMES IN EXTENDED SYSTEMS - ORBITAL LOCALIZATION IN Y2CL3 AND INTERSTITIAL DERIVATIVES

A semilocalized description of bonding in extended metal-metal-bonded systems is described for the prototype title compounds. Beginning with calculated band orbitals, we explicitly construct localized orbitals (Wannier functions) for the metal-metal-bonding orbitals of a M6X12 cluster and the Y2Cl3 chain compound. Face-localized three-center bond orbitals for a Zr6Cl18 cluster are remarkably similar to the HOMO for a (Zr6Cl12H7)Cl6(3-) cluster in which hydrogens cap seven of eight Zr6 cluster faces. The (Zr6Cl12H7)Cl6(3-) cluster was chosen to model (Th6Br(i)12H7)Br(a-a)6/2, reported recently by Simon and co-workers. A semilocalized bonding picture for semiconductors with the Gd2Cl3 structure type is constructed, providing clarification of both structure-property relationships and trends in interstitial chemistry of these compounds. For the metal-metal bonding, a scheme with two 4c-2e bonds and one 2c-2e bond per cell is extracted from the band structure. The insertion of hydrogens into the tetrahedral interstices of these chains (to form structures of the beta-Y2Cl3N type) is seen to be plausible and chains formulated as Y4Cl6H2(2+) are electronically viable.