MEASUREMENT OF FAST DIMERIZATION RATES BY CYCLIC VOLTAMMETRY - COMPARISON OF THE 17E RADICALS (ETA(5)-C(5)R(5))CR(CO)(3) (R=H, CH3) AT LOW-TEMPERATURES

The 17e complexes (eta(5)-C5H5)Cr(CO)3 (2) and (eta(5)-C(5)Me(5))Cr(CO )(3) (3) are in equilibrium with their metal-metal-bonded dimers 2(2) and 3(2), respectively. Cyclic voltammetry (CV) studies of the two com plexes reveal details of both the equilibrium concentrations of monome rs and dimers (at high CV sweep rates) and the rates of interconversio n between the monomers and dimers (at lower sweep rates). Voltammetry was performed in CH2Cl2/0.1 M [Bu(4)N]-[PF6] electrolyte. At room temp erature and slow to moderate scan rates, a single Nernstian wave is se en for both complexes, arising from reduction of the monomer by one el ectron, E(o) = -0.82 V vs Fc for 2/2(-) and -0.93 V for 3/3(-). High s weep rates (up to several thousand volts per second) are required to f reeze out the monomer/dimer rates at T < 250 K. Digital simulations of CV curves corrected for background currents and ohmic effects establi sh a dimerization rate constant for 2 in this medium at 243 K of 8.4 x 10(5) M(-1) s(-1), whereas a larger value is found for the permethyla ted analogue 3 at 228 K, 1.7 x 10(7) M(-1) s(-1). These values are com pared to those previously reported in laser flash photolysis studies a t room temperature.