Structural and bonding trends in osmium carbonyl cluster chemistry: metal–metal bond lengths and calculated strengths in the anions [Osx(CO)y]2- , hydrides [Osx(CO)yHz] and hydride anions [Osx(CO)yHz ]c-

Abstract

The metal–metal bond distances [d(M–M)] in the known structurally characterised 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 × 10 ¹³ [d(Os–Os)] ^-4.6 , itself derived from published structural and enthalpy data. Summation of the metal–metal bond enthalpy terms, to give the total metal–metal bond enthalpy, ΣE(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 metal atom, ΣE(Os–Os)/x, and the number of ligand electrons per metal atom, the data falling on a curve which includes bulk osmium metal and [Os(CO) ₅ ] at the extremes. Correlations are also noted between ΣE(Os–Os) and the number of skeletal electron pairs (polyhedral skeletal electron pair theory) or number of formal two-centre two-electron (2c2e) bonds (18-electron rule). These correlations show that the electrons are used more efficiently for metal–metal bonding in larger clusters with fewer ligands. Thus, the metal–metal bond enthalpy per electron pair available (using the 18-electron rule) increases as the cluster becomes larger, indicating the error in models based on assigning fixed energies to notional 2c2e Os–Os bonds. Trends in ΣE(Os–Os) were 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 to a prediction of limiting values of Os–H bond enthalpy terms. Trends in ΣE(Os–Os) were examined for series of closely related clusters, including 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 ΣE(Os–Os) is shown to be a single parameter which quantifies the overall effect of small changes in metal–metal distances in osmium carbonyl clusters.