Bonding in an Unusual Nickel Carbide

Abstract

Abstract The bonding in an unusual nickel carbide polymer network synthesized by Musanke and Jeitschko is analyzed using qualitative, band-structure calculations in the framework of extended Hiickel tight-binding theory. The carbide features infinite, one-dimensional, vertex-sharing chains of Ni squares. Each square is centered by a carbon and flanked by C2 units. - In this electron-rich polymer, the Ni atoms act as electron acceptors through their s and p orbitals. The orbital interaction schemes reveal that there is little Ni-Ni bonding and essentially no Ni to C2 back-donation. Molecular orbital interaction diagrams of some molecular models are used to describe the bonding in 1 ∞[Ni3Cs]8-. We also discuss the stability of the planar building block of the polymer, (CNi4)4- (which resembles planar CH4) with respect to a “tetrahedral” alternative and we do so for the extended 1 ∞[ Ni3C]4-and 1 ∞[Ni3C5]8-chains. In each model case the tetrahedral alternative is favored, and there is no sign of stabilization of a crucial CL4 orbital. Such stabilization is partially achieved through bonding to two Ca axial to the C in the center of the Ni4 ring, but it takes the entire inter-chain calcium network to effect the observed stabilization.