Adsorption and co-ordination of unsaturated hydrocarbons with metal surfaces and metal atoms

Abstract

A survey is first made of the co-ordination of olefins and chelating diolefins to metal atoms in organometallic complexes: with d⁸ and d¹⁰ complexes, the observed stability sequences for ethylene co-ordination (e.g., Ni^II<Pd^II<Pt^II; Ag^I<Pd^II<Rh^I) are interpreted in terms of the spatial direction and extension of the d-orbitals. Measurements of the power of diolefins to inhibit the isomerization of 1-pentene catalyzed by methanolic RhCl₃ confirm their stronger co-ordination which is attributed to an entropy effect. The principal phenomena of homogeneous catalysis by organometallic complexes are consistent with the above stability sequences, optimum activity occurring with Rh^I and Pd^II complexes: comparison of these phenomena with those in heterogeneous hydrogenation catalysis leads to the working hypothesis that the reactive state of adsorbed unsaturated hydrocarbons is also a π-complex.

A molecular orbital model appropriate to a face-centred cubic metal is outlined, and the direction of emergence of the e_g and t_2g orbitals at the (100), (110) and (111) planes obtained: the degree of occupation of the orbitals is discussed, and possible models for π-adsorbed states of olefins, diolefins and alkynes are sketched. The (111) plane is the least suited for their adsorption. A brief attempt is made to apply molecular orbital concepts to mechanisms in hydrogenation catalysis.