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(-X-C ₆H₄-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 [(Bu^tCO₂)₃M₂] ₂(-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 M ₂ centers in comparison to the closely related terephthalate compounds.