Surface-supported metal cluster carbonyls. Chemisorption, decomposition and reactivity of Rh₆(C0)₁₆ supported on silica

A partially decarbonylated metal cluster is quickly formed on the surface of silica by oxidation at room temperature; it is possible to regenerate the initial cluster compound under a carbon monoxide atmosphere at 200°C. Decarbonylation of Rh₆(CO)₁₆ at higher temperature produces a new metallic material on the surface, characterized by two v(CO) vibration bands at 2048 ± 7 cm^-1 and 1893 ± 10 cm^-1. These two bands have been respectively assigned to a terminal carbonyl group and a bridged carbonyl group bonded to two rhodium atoms. Oxidation of this compound occurs very easily under oxygen at room temperature and gives an oxidized material presumably of the same nuclearity; adsorption of carbon monoxide produces two intense sharp bands at 2093 and 2038 cm^-1 which have been assigned to the symmetric and asymmetric stretching modes of two CO molecules bonded to a single oxidized Rh site as Rh^I(CO)₂. The conversion from the oxidized surface species to the metallic one can be performed under mild conditions, but attempts to regenerate the initial cluster compound were unsuccessful.