Through-ring bonding in edge sharing dimers of octahedral complexes

A study of the preferred structures for the M2X2 rings in the binuclear complexes of types [M-2(mu-XR2)(2)L-8] and [M-2(mu-XR3)(2)L-8] is presented, based on qualitative orbital arguments supported by extended Huckel calculations on Cr compounds. The main conclusions are confirmed by DFT calculations on key compounds of Cr and Mn and agree well with the results of a structural database analysis. With the simplified electron counting scheme deduced, complexes with six or four electrons available for bonding of the M2X2 framework 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 behavior is consistent with the framework electron rules reported earlier for compounds with different coordination spheres and provides a general description of the structure and bonding in a variety of compounds with M2X2 diamonds. Metal-metal bonding across the ring can be equally predicted taking into account only the bonding characteristics of the t(2g)-like orbitals for the XR2- but not for the XR3-bridged complexes. In addition, the framework electron counting scheme has the advantage of being independent of the formal oxidation state assigned to the metal atom.