MECHANISM OF HALIDE-INDUCED DISPROPORTIONATION OF M(CO)3(PCY3)2+ 17-ELECTRON RADICALS (M = FE, RU, OS) - PERIODIC TRENDS ON REACTIVITY AND THE ROLE OF ION-PAIRS

Halide-induced disproportionation of the 17-electron M(CO)3(PCy3)2+ radicals was probed by double potential step chronocoulometry, rotating-ring-disk electrochemistry, bulk coulometry, and cyclic voltammetry. The products of disproportionation, M(CO)3(PCy3)2 and MX2(CO)2(PCy3)2, were observed directly for M = Ru and Os, and X = Cl- or Br-. At a constant concentration, [Bu4NPF6] + [Bu4NX] = 0.2 M, the kinetics of disproportionation followed the rate law k2[Bu4NX][M(CO)3(PCy3)2PF6]. Conductivity measurements in the reaction solvent CH2Cl2 were fit to the Fuoss equation, and K(d), the dissociation constant for the ion pairs, ranged between 2.57 x 10(-5) for [Bu4N]Cl and 4.26 x 10(-5) for [Bu4N]PF6 at 25-degrees-C. The [Fe(CO)3(PCy3)2]PF6 species has K(d) almost-equal-to 1.7 x 10(-4). Unlike 18-electron systems, the periodic effect on the reactivity of 17-electron M(CO)3(PCy3)2+ showed only a slight variation. For [Bu4N]Cl at 25-degrees-C the rates k2 were 66 M-1 s-1 (Fe), 280 M-1 s-1 (Ru), and 27 M-1 s-1 (Os). For [Bu4N]Br hardly any variation occurred along the series: 17 M-1 s-1 (Fe), 26 M-1 s-1 (Ru), and 20 M-1 s-1 (Os). The pseudo-first-order rate constants k(obs) showed a nonlinear dependence on the concentration of the inert electrolyte [Bu4N]PF6, and are zero order in [Bu4N]X in the absence of [Bu4N]PF6. This data, however, fit well to a kinetic scheme that involved equilibration of contact ion pairs, followed by unimolecular substitution within the contact ion pair {[M(CO)3(PCy3)2]X}ip. The internal substitution rates and the equilibrium constants for ion pair exchange, measured at 25-degrees-C, are as follows (k (s-1), K): for [Bu4N]Cl, 15 +/- 3, 0.9 +/- 0.2 (Fe), 56 +/- 5, 0.79 +/- 0.07 (Ru), 4.8 +/- 0.9, 0.7 +/- 0.1 (Os); for [Bu4N]Br, 3.3 +/- 0.1, 0.98 +/- 0.03 (Fe), 5.3 +/- 0.5, 1.02 +/- 0.05 (Ru), 4.4 +/- 0.7, 0.9 +/- 0.1 (Os). Activation parameters obtained for M(CO)3(PCy3)2+/Bu4NCl systems are as follows (DELTA-H double dagger (kcal mol-1), DELTA-S double dagger (cal mol-1 K-1)): 19.7 +/- 0.9, 10 +/-3 (Fe); 18.7 +/- 1.6, 12 +/- 5 (Ru); 12.1 +/- 0.7, -14 +/- 3 (Os).