THE DELTA IN 18- IMPORTANCE OF THE METAL(DELTA ELECTRON COMPLEXES )LIGAND INTERFACE FOR THE SUBSTITUTIONAL REACTIVITY OF RE(0) COMPLEXES (ALPHA-DIIMINE(-))RE-I(CO)(3)(X)/

The factors determining the electron transfer-induced halide labilizat ion in complexes (alpha-diimine)Re(CO)(3)(Hal), Hal = Cl and Br, were systematically studied via EPR and cyclic voltammetry in the presence of substituting ligands such as triphenylphosphine, cyanide, or aceton itrile. The alpha-diimines employed were the four isomeric bidiazines (bdz) 3,3'-bipyridazine, 2,2'-bipyrazine, and 2,2'- and 4,4'-bipyrimid ine and the nonaromatic alpha-diimines 1,4-di-tert-butyl-1,4-diaza-1,3 -butadiene (dab) and 1,3-di-tert-butylsulfurdiimine (sdi). For compari son, the complexes (L)Re(CO3)Cl, L = 2,2'-bipyridine, 1,4,7,10-tetraaz aphenanthrene, and eta(2)-2,2',2 ''-terpyridine, and the new cationic species [(bdz)Re(CO)(3)(CH3CN)](+) were also investigated. In a furthe r experiment, in situ EPR spectroelectrochemistry was employed to stud y the primary paramagnetic intermediates during the reduction of the p rototype compound, (bpy)Re(CO)(3)Cl, under a CO2 atmosphere. The susce ptibility to substitution was found to be dependent not on the redox p otential but on the pi molecular orbital coefficients at the metal-coo rdinating nitrogen centers which are reflected by N-14, Re-185,Re-187, and P-31 EPR coupling constants. The most labile systems were thus fo und among the complexes of the small dab and sdi ligands, despite thei r facile reduction. In contrast, the complexes of these nonaromatic co mpounds showed an electrochemically reversible one-electron oxidation which, in comparison to the absorption maximum, allowed us to estimate contributions to the reorganization energy of the MLCT excited state in two cases. For the reductive labilization, it is primarily the smal l but variable and EPR-detectable ligand-to-metal electron (spin) tran sfer at the metal/ligand interface which determines the extent of acti vation in 18 + delta valence electron intermediates.