Metal-metal and metal-ligand bond strengths in metal carbonyl clusters

The limited experimental thermochemical information about metal carbonyl clusters, and the more extensive literature on structural studies of such compounds, provide a means of exploring trends in their stabilities. This review surveys that literature for selected metals, showing how the enthalpy of disruption of gaseous M-x(CO)(y), dusters into gaseous metal atoms and carbon monoxide can be partitioned into two components representing the strengths of metal-metal and metal-ligand bonds. In doing so, it is assumed that the bond enthalpies, E(M-M), of metal-metal bends vary smoothly with their length, d(M-M), according to a relationship E(M-M) = A[d(M-M)](-4.6), for which a justification is provided. The structure of a cluster thus provides a means of determining the total metal-metal bond enthalpy of that cluster. Application of this method to thermodynamically characterised clusters demonstrates that the average metal-ligand bond enthalpy, E(M-CO), in carbonyl clusters M-x(CO)(y). varies slightly with the ligand to metal ratio, y/x; a carbonyl ligand binds mon strongly to a metal when it is competing with few other ligands. We demonstrate that for binary osmium carbonyl dusters, Os-x(CO)(y), the distances d(Os-C) and d(C-O) are also functions of the ligand to metal ratio, y/x, providing evidence for the familiar synergistic bonding of the carbonyl ligand, and that these distances are a function of the metal-ligand bond enthalpy, E(Os-CO). Trends in cluster stability, as determined by the total metal-metal bond enthalpy, Sigma E(M-M), for anionic and carbonyl hydride clusters of osmium, rhenium and rhodium, [M-x(CO)(y)H-z](c-), are presented. Similar trends for clusters of rhenium and rhodium containing core or interstitial carbon, nitrogen or other atoms are also explored, and partition of the atomisation enthalpy of binary metal carbides, MC and M2C into metal-metal and metal-carbon components is investigated to provide insight into the strength of binding of core carbon atoms surrounded by octahedral arrays of metal atoms. (C) 2000 Elsevier Science S.A. All rights reserved.