Molecular orbital calculations for surface-bound metal species: rhodium(I) dicarbonyls on alumina or silica

Abstract

The oxide-supported Rh I(CO) 2 species, frequently generated on heterogeneous rhodium catalysts, has been modeled for extended Hückel molecular orbital calculations using, as oxide supports, simple oxyanion structures composed of Si or Al atoms in tetrahedral or octahedral oxygen environments. Calculations for different orientations of Rh I(CO) 2 with respect to the oxyanion structures showed the lowest energy configuration to be that in which the metal car bonyl fragment binds to two oxygen atoms, forming a square planar geometry about the Rh I. Rearrangement to a five-coordinate species with Rh I(CO) 2 bound to three oxygens requires between 0.51 and 1.94 eV of energy, depending upon the oxyanion structure and whether the central atom is Si or Al. The lowest energy for conversion from four- to five-coordination about Rh I occurs upon octahedral anions. The square planar→ five-coordinate conversion is discussed as a surface migration pathway, with relative activation barriers to the migration depending upon the energy of this conversion.