On the Role of Aromaticity in Bonding Hydrocarbons to Metal Surfaces

Abstract

The stable bonding configurations adopted by cyclic C₅ and C₆ hydrocarbons on Os(0001), Ir(111) and Pt(111) surfaces is discussed in terms of the energetics of competing metal-carbon σ-bonding and 6-electron aromatic π-bonding. Previous work which Pt(111) has shown that these are the adsorbed cyclopentadienyl and benzene respectively which are both π-bondcd in a manner which retains a significant level of 6-clectron aromatic stabilisation. New work on Ir(111) is presented to show that while benzene adsorbs in a stable π-configuration, the stable species arising from the thermal evolution of both c-C₅H₆ and c-C₅H₈ adsorption is an adsorbed c-C₅H₃; the bonding of which is dominated by carbon-iridium σ-coordination. Less stable adsorbed cyclopenladienyl and c-C₅H₆ species have been identified as intermediates in the thermal evolution of c-C₅H₆ and c-C₅H₈ respectively, on Ir(111). The stable adsorbed c-C₅H₃ species appears to be the analogue of a recently reported stable c-C₆H₄ species derived benzene adsorption on Os(000l). The sequence of stability of these adsorbed species is qualitatively correlated with the DRE of the aromatics and the strength of carbon-metal σ-bonding.