| This thesis examines the electronic structures and chemical bonding of several AB(,2)X(,2) phases, as well as of the C(,1) fragments on metal surfaces in the Fischer-Tropsch reaction.; In Chapter I the prototypic BaAl(,4) structure and its derivatives--the CaAl(,2)Zn(,2), CeMg(,2)Si(,2) phases are discussed. The electronic structure of BaAl(,4) has a delocalized five-center six-electron bonding in each hollow of the square lattice in BaAl(,4), closely related to the electron deficient bonding in B(,5)H(,9). The bonding in CeAl(,2)Ga(,2) and CeMg(,2)Si(,2) can be derived similarly, by perturbing the original BaAl(,4) case.; In Chapter II we examine the ThCr(,2)Si(,2) phase. Among the many examples of the ThCr(,2)Si(,2) structural type is a set of AB(,2)X(,2) structures, B = transition metal, X = group 14 or 15 element, in which one X...X contact varies over the range of bonding between no bond and a fully formed X-X single bond. The tuning is provided by variation in the transition metal, X-X bonding promoted by metals on the right-hand side of the Periodic Table.; In Chapter III we analyze the CaBe(,2)Ge(,2) phase. The Be(,2)Ge(,2) part of the CaBe(,2)Ge(,2) structural type is built up of two distinguishable, isomeric layers, the Be and Ge occupying distinct sublattices within one layer, switching their positions in the next layer. A consequence of this differential site occupation results in difference in the dispersions of the Be-Ge bonding and antibonding bands. The high dispersion in the occupied band (mostly of Ge states) is a destabilizing factor within the layer, but a better supporter of interaction with an inverted layer in a donor-acceptor mode.; In Chapter IV the CaAl(,2)Si(,2) phase is studied. The Al(,2)Si(,2)('2-) two-dimensional networks in the CaAl(,2)S(,2) structure may be derived by a conceptual splitting of the wurtzite lattice and a subsequent reconstruction. The reconstruction suggests some analogies with organic propellanes: The "handle" bond in the SiAl(,4) tetrahedron is always longer than the "rib" bond.; In Chapter V the binding and coupling of C(,1) fragments on metal surfaces are analyzed. We also explored the electronic controlling factors for the binding and coupling of C(,1) fragments on the surface. |
