Pyramidality effect on metal-metal single bonds

A structural database study has been made of the transition-metal compounds M2X8Ln with metal-metal single bonds. The existence of a pyramidality effect in d(7)-d(7) bonds is supported by qualitative extended-Huckel molecular orbital calculations and experimental data for binuclear rhodium compounds. The results for several families of binuclear complexes of Co-II, Ru-I, Os-I and Pt-III show that the metal-metal distance decreases with increasing pyramidality and with increasing torsion angles. The importance of the central metal atom and of the presence of axial ligands is also discussed.