STRUCTURAL AND BONDING TRENDS IN OSMIUM CARBONYL CLUSTER CHEMISTRY - METAL-METAL BOND LENGTHS AND CALCULATED STRENGTHS IN THE ANIONS [OS-X(CO)(Y)](2-), HYDRIDES [OS-X(CO)(Y)H-Z] AND HYDRIDE ANIONS [OS-X(CO)(Y)H-Z](C-)

The metal-metal bond distances [d(M-M)] in the known structurally char acterised osmium carbonyl anions, [Os-x(CO)(y)](2-), neutral carbonyl hydrides, [Os-x(CO)(y)H-z] and carbonyl hydride anions, [Os-x(CO)(y)H- z](c-), have been used to calculate bond enthalpy terms E(Os-Os) using the relationship E(Os-Os) = 1.928 x 10(13) [d(Os-Os)](-4.6), itself d erived from published structural and enthalpy data. Summation of the m etal-metal bond enthalpy terms, to give the total metal-metal bond ent halpy, Sigma E(Os-Os), has revealed the varying efficiency with which these compounds use their electrons for metal-metal bonding. There is a strong correlation between the total metal-metal bond enthalpy per m etal atom, Sigma E(Os-Os)/x, and the number of ligand electrons per me tal atom, the data falling on a curve which includes bulk osmium metal and [Os(CO)(5)] at the extremes. Correlations are also noted between CE(Os-Os) and the number of skeletal electron pairs (polyhedral skelet al electron pair theory) or number of formal two-centre two-electron ( 2c2e) bonds (18-electron rule). These correlations show that the elect rons are used more efficiently for metal-metal bonding in larger clust ers with fewer ligands Thus, the metal-metal bond enthalpy per electro n pair available (using the Ig-electron rule) increases as the cluster becomes larger, indicating the error in models based on assigning fix ed energies to notional 2c2e Os-Os bonds. Trends in Sigma E(Os-Os) wer e explored as Os(CO)(4), Os(CO)(3) or Os(CO)(2) fragments are added to clusters in cluster build-up processes as CO ligands are replaced by H-, and on oxidative addition of H-2 to clusters, the latter leading t o a prediction of limiting values of Os-II bond enthalpy terms. Trends in CE(Os-Os) were examined for series of closely related clusters, in cluding those derivable from [Os-4(CO)(14)] by replacing CO by H- or H -2, and a series of clusters derived from [Os-6(CO)(18)]. The sum Sigm a E(Os-Os) is shown to be a single parameter which quantifies the over all effect of small changes in metal-metal distances in osmium carbony l clusters.