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(µ-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 δ 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 δ orbitals. The compounds [(ButCO2)3M2]2(µ-SOC–C6H4–COS), where M = Mo and W, have been prepared from the reactions between M2(O2CBut)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.