Through-ring bonding in edge sharing dimers of octahedral complexes

A study of the preferred structures for the M2X2 rings in the binuclear com plexes of types [M-2(mu-XR2)(2)L-8] and [M-2(mu-XR3)(2)L-8] is presented, b ased on qualitative orbital arguments supported by extended Huckel calculat ions on Cr compounds. The main conclusions are confirmed by DFT calculation s on key compounds of Cr and Mn and agree well with the results of a struct ural database analysis. With the simplified electron counting scheme deduce d, complexes with six or four electrons available for bonding of the M2X2 f ramework are predicted to have two possible minimum energy structures, with either a short M-M or X-X distance, whereas compounds with eight framework electrons are expected to present no short through-ring distance. Such a b ehavior is consistent with the framework electron rules reported earlier fo r compounds with different coordination spheres and provides a general desc ription of the structure and bonding in a variety of compounds with M2X2 di amonds. Metal-metal bonding across the ring can be equally predicted taking into account only the bonding characteristics of the t(2g)-like orbitals f or the XR2- but not for the XR3-bridged complexes. In addition, the framewo rk electron counting scheme has the advantage of being independent of the f ormal oxidation state assigned to the metal atom.