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., NiII<PdII<PtII; AgI<PdII<RhI) 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 RhI and PdII 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 eg and t2g 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.