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 M3X2 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, [(L2M)(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 all FEC of 10 favors the formation of an X-X bond. Framework electron counting rules are also presented for related M3X2 cores in [(L5M)(3)X-2] complexes. based on a qualitative molecular orbital (MO) analysis supported by DFT calculations on [(OC)(15)Cr-3(mu-As-2)].