X-X through-cage bonding in Cu, Ni, and Cr complexes with M3X2 cores (X=S, As)

Density functional calculations on trinuclear complexes bridged by two sulfur atoms, [(tmeda)(3)Cu(3)(mu-S)(2)](3+), [(tmeda)(3)Ni(3)(mu-S)(2)](2+), and [(tmeda)(3)Ni(3)(mu-S(2))](4+), as well as on the formation of [(tmeda)(3)Cu(3)(mu-S)(2)](3+) from a dinuclear [(tmeda)(2)Cu(2)(mu-S(2))](2+) complex and a mononuclear [(tmeda)Cu(eta(2)-S(2))](+) fragment, are reported. A qualitative orbital analysis of the M(3)X(2) framework bonding is presented for the case in which each metal atom M has a square planar coordination sphere completed by one bidentate or two monodentate ligands (that is, [(L(2)M)(3)X(2)] compounds). It is concluded that a framework electron count (FEC) of 12 corresponds to systems with six M-X bonds but no X-X bond through the cage, while an FEC of 10 favors the formation of an X-X bond. Framework electron counting rules are also presented for related M(3)X(2) cores in [(L(5)M)(3)X(2)] complexes, based on a qualitative molecular orbital (MO) analysis supported by DFT calculations on [(OC)(15)Cr(3)(mu-As(2))].