VIBRATIONAL STUDIES OF INTERSTITIAL CARBIDE ATOMS IN NICKEL AND RHENIUM CARBONYL CARBIDE CLUSTERS

A vibrational spectroscopy study of the interstitial carbon atom in a series of Re and Ni carbide clusters enabled the M-C vibrational modes to be assigned and the effect of the metal atoms capping the Re6(μ6-C) and Ni8(μ8-C) cores to be investigated. The assignment of the M-C vibrational modes has been confirmed by 13C isotopic labeling of the interstitial carbide atom. A single force constant value accounts for the observed frequencies in the nickel carbide clusters, whether capped or uncapped, and in the uncapped rhenium carbide clusters. In contrast, the vibrational analysis for the capped rhenium clusters indicates that the force field around the carbon atom should be described by slightly different axial and equatorial force constants. A rationalization of the capping effect in terms of structural and electronic effects is proposed.