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

Abstract

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 [(Bu^tCO₂)₃M₂]₂(µ-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.