Abstract
In this study, digital simulations were performed for cyclic voltammograms (CV) obtained at different scan rates (0.2 to 1.0 V/s) for the oxidatively-promoted CO-substitution reaction by PPh3 in the bimetallic FvCo(2)(CO)(4),1, in CH2Cl2/0.05 M [NBu4][B(C6F5)(4)] in order to establish the possible mechanism by which this reaction occurs. For the simulation input parameters, transfer coefficients (a) were kept at 0.5 and diffusion coefficients values of D-o = 1.53 x 10(-5) and 1 x 10(-5) cm(2) s(-1) were used for 1 and the substitution product FvCo(2)(CO)(3)PPh3, 2, respectively. An associative mechanism that involves four heterogeneous electron-transfer reactions (ks) for 1(0/+), 1(+/2+), 2(+/0), 2(+/2+), and two homogeneous chemical reactions (k(f)) of 1(+) and 1(2+) with PPh3 gave best fitting to the experimental CVs. Although the inclusion of the reaction between PPh3 and 1(2+) only slightly improves the fitting, it suggests that the rates and equilibrium constants for CO-substitution processes in 1(+) and 1(2+) to be of the same order of magnitude. Importantly, the values of both K-eq(1) (k(f)(1)/k(b)(1)) and k(f)(1) were shown to be quite large (>= 105 M-1 and >= 5 x 10(3) M-1 s(-1), respectively). This finding clearly demonstrates that the substitution of CO by PPh3 in the radical cation 1(+) is thermodynamically highly favored and kinetically rapid on the voltammetric time scale.