Modeling metal-metal multiple bonds with multireference quantum chemical methods

Several examples of metal-metal multiple bonds will be presented. Their properties will be analyzed based on multireference quantum chemical calculations, from which it is possible to quantify the degree of multiple bonding by using the concept of effective bond order. The Re-Re bond in Re2Cl82- is discussed first. This system was discovered in 1965 by F. A. Cotton and C. B. Harris and marked the beginning of the era of multiple bonds. Group VI diatomic molecules, namely, Cr2, Mo2 and W2 are then reported. The differences in their multiple bonds will be analyzed. Metal-metal bonds in complexes containing these bimetallic units, recently synthesized by various groups, are also reviewed, together with some systems presenting a Fe-Fe multiple bonding. The importance of ligand effects in the formation of metal-metal multiple bonds are examined. Synthetically, compounds with the metals in low formal oxidation states can be obtained only because of the ligand choice. The role of quantum chemistry in understanding metal-metal multiple bonds and its latest achievements are discussed.