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 complexes reveal details of both the equilibrium concentrations of monomers and dimers (at high CV sweep rates) and the rates of interconversion between the monomers and dimers (at lower sweep rates). Voltammetry was performed in CH2Cl2/0.1 M [Bu(4)N]-[PF6] electrolyte. At room temperature and slow to moderate scan rates, a single Nernstian wave is seen for both complexes, arising from reduction of the monomer by one electron, E(o) = -0.82 V vs Fc for 2/2(-) and -0.93 V for 3/3(-). High sweep rates (up to several thousand volts per second) are required to freeze out the monomer/dimer rates at T < 250 K. Digital simulations of CV curves corrected for background currents and ohmic effects establish 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 permethylated analogue 3 at 228 K, 1.7 x 10(7) M(-1) s(-1). These values are compared to those previously reported in laser flash photolysis studies at room temperature.