RELATION OF STRUCTURAL-CHANGES TO ELECTRON-TRANSFER PARAMETERS IN FULVALENEDIYL DIRHODIUM COMPLEXES DEMONSTRATING QUASI-REVERSIBLE 2-ELECTRON VOLTAMMETRY

The oxidations of two fulvalenediyl (Fv) dirhodium complexes [(Fv)Rh-2 (CO)(4), 1, and (Fv)Rh-2(CO)(2)(mu-dppm), 2] have been studied by volt ammetry, coulometry, IR spectroelectrochemistry, UV-VIS spectroscopy, and X-ray crystallography. Both complexes undergo net two-electron oxi dations to dications having a Rh-Rh bond. The M-M bond distances are 2 .837(1) Angstrom in 2(2+) and 4.511(1) Angstrom in 2. Quasi-reversible cyclic voltammetric responses were exploited over a range of sweep ra tes to give CV curves successfully fit to an EE mechanism by digital s imulations: for 1, E(o)'(1) = -0.10 V (vs Fc), k(s1) = 0.013 cm s(-1), (1 - alpha(1)) = 0.66, E(o)'(2) = -0.24(5) V, k(s2) = 0.044 cm s(-1), alpha(2) = 0.5; for 2, E(o)'(1) = -0.76 V, k(s1) = 0.035 V, (1 - alph a(1)) = 0.75, E(o)'(2) = -0.776 V, k(s2) > 0.2 cm s(-1), alpha(2) = 0. 5. Complementary results were obtained for the reduction of the dicati ons 1(2+) or 2(2+). The effect of the bridging fulvalenediyl ligand is to retard the oxidation process by an amount consistent with the calc ulated barrier to rotation around the C-C bond of the eta(5),eta(5)'-C 10H8 ligand. In both complexes the 0/1+ redox process is slower than t he 1+/2+ process. The electron transfer rates are consistent with a pr ogressive increase in the Rh-Rh bond order in the 0/1+/2+ charged comp lexes rather than full M-M bond formation and cleavage in a single ste p.