Electronic coupling in covalently-linked MM quadruple bonds: M2δ to ligand π conjugation

Dimetal tetracarboxylates, M2(O2CR)4, where R=alkyl and M=Mo or W, can be linked in a perpendicular or parallel manner through the agency of a variety of ligands to give ‘dimers of dimers’ [M2(O2CR)3]2(bridge), or extended chains. A ring of formula [M2(O2CR)2(bridge)]4 is an alternative to a chain [M2(O2CR)3]2(bridge)2]∞ and ‘molecular squares’ of formula [(RNCHNR)2M2(bridge)]4 have recently been reported by Cotton and Murillo, where M=Mo and Rh and R=p-MeOC6H4. The electronic coupling between the M2 units manifests itself in electrochemical data and in the electronic spectra of the compounds. The electronic coupling occurs by M2δ to ligand π-conjugation and specific examples are illustrated for the bridging ligands oxalate, perfluoroteraphthalate, 1,8-anthracenedicarboxylate and 2,7-dioxynaphthyridine. Correlations of a variety of spectroscopic data and computations employing density functional theory are presented.