Electronic coupling in 1,4-(COS)2C6H4 linked MM quadruple bonds (M = Mo, W): the influence of S for O substitution.

Electronic structure calculations employing density functional theory on the compounds [(HCO2)3M2]2(mu-X-C6H4-X) where M = Mo and W and -X = -CO2, -COS and -CS2 reveal that the successive substitution of oxygen by sulfur leads to enhanced electronic coupling as evidenced by the increased energy separation of the metal delta orbital combinations which comprise the HOMO and HOMO-1. This enhanced coupling arises principally from a lowering of the LUMO of the X-C6H4-X bridge which, in turn, increases mixing with the in-phase combination of the M2 delta orbitals. The compounds [(Bu(t)CO2)3M2]2(mu-SOC-C6H4-COS), where M = Mo and W, have been prepared from the reactions between M2(O2CBu(t))4 and the thiocarboxylic acid 1,4-(COSH)2C6H4 in toluene and the observed spectroscopic and electrochemical data indicate stronger electronic coupling of the M2 centers in comparison to the closely related terephthalate compounds.