ELECTRON-TRANSFER IN DINUCLEAR COBALT COMPLEXES WITH NON-INNOCENT BRIDGING LIGANDS

Reaction of tripod cobalt(II) templates [{CH3C(CH2PAr2)(3)}Co-II] with potentially bridging ligands L generates the dinuclear compounds [(tr ipod)-Co-L-Co(tripod)](2+). With L = oxalate (C2O42-) a biscobalt(II) complex (1) is Formed, while with L = C6H2O42-, the dianion derived fr om 2,5-dihydroxy-1,4-benzoquinone (anilic acid), two-electron transfer within the dimetallic unit occurs and a biscobalt(III) charge distrib ution results (2a), as shown by X-ray structural analyses of 1 and 2 a , NMR spectroscopy, and theoretical investigations by the INDO method, Complex 2a exhibits an unusually intense, low-energy absorption in it s electronic spectrum; this is explained with a simple MO model. One-e lectron reduction of 2 a generates the corresponding mixed-valence com plex, which is highly stabilised through extensive electron delocalisa tion. Substituents at the 3,6 posi tions of the bridging ligand (Cl, B r, I, NO2, Me, iPr. Ph: 2b-h) as well as alkyl substitution at the aro matic rings of the tripod ligands (3,4) influence the optical and elec trochemical properties consistent with the proposed model of charge di stribution, Formal replacement of one [(tripod)Co-III](3+) moiety by [ CH2](2+) leads to the mononuclear complex 6, which is shown to be a ty pical [(tripod)Co-III-catecholato)](+) complex, Therefore the substant ially different optical and electrochemical properties of the dinuclea r complexes with respect to those of 6 result from strong metal-metal interactions mediated by the bridging ligand.