ELECTROCHEMICAL REDUCTION OF THE BIS(CYCLO-OCTADIENE)RHODIUM MONOCATION - GENERATION AND FATE OF THE 17-ELECTRON RADICAL

The electrochemical reduction of the monocation of bis-(cyclo-octadiene)Rh[(CODRh+] has been studied in chlorinated hydrocarbons and d(6)-acetone by cyclic voltammetry, chronoamperometry and exhaustive coulometry. Successive one-electron reductions are observed for the couples (CODRh+/(COD)(2)Rh and (COD)(2)Rh/(CODRh- at -1.34 V vs. Fc and -1.93 V vs. Fc respectively. The 17-electron Rh(0) radical (COD)(2)Rh abstracts a Cl atom from CH2Cl2 to give the dinuclear complex [(COD)Rh(mu-Cl)](2) in high yield at 298 K. At subambient temperatures this reaction is suppressed and the dominant decomposition product is apparently (COD)Rh(C8H13), formed by H atom abstraction by (COD)(2)Rh from solvent and/or adventitious water. Electrolysis products were characterized by electron spin resonance (ESR), nuclear magnetic resonance (NMR) and mass spectrometry. The reactivity of the radical is rationalized by a bonding model in which the lowest unoccupied molecular orbital (LUMO) is d(x2-y2) with some diolefin mixing. ESR measurements are consistent with this model and suggest that the COD ligands form a ligand field around Rh which is closer to square planar than to tetrahedral.