Electronic coupling in covalently-linked M-M quadruple bonds:: M2δto ligand π conjugation

Dimetal tetracarboxylates, M-2(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' [M-2(O2CR)(3)](2)(bridge), or extended chains. A ring of formula [M-2(O2CR)(2)(bridge)](4) is an alternative to a chain [M-2(O2CR)(3)](2)(bridge)(2)](infinity). and 'molecular squares' of formula [(RNCIINR)(2)M-2(bridge)](4) have recently been reported by Cotton and Murillo, where M = Mo and Rh and R = p-McOC(6)H(4) The electronic coupling between the M-2 units manifests itself in electrochemical data and in the electronic spectra of the compounds. The electronic coupling occurs by M(2)delta to ligand pi-conjugation and specific examples are illustrated for the bridging ligands oxalate, perfluoroteraphthalate, 1,8-anthracenedicarboxylate and 2,7-dioxynaphthyridine. Correlations of a variety of spectro-scopic data and computations employing density functional theory are presented. (C) 2002 Elsevier Science B.V. All rights reserved.