DIFFUSIVE SEPARATION AND ASSOCIATIVE ELECTRON-TRANSFER IN THE OXIDATION OF THIOCYANATE BY BIS(1,4,7-TRIAZACYCLONONANE)NICKEL(III)

Oxidation of excess thiocyanate by [Ni(tacn)2]3+ shows kinetic inhibition by the product, [Ni(tacn)2]2+, and with a large excess of Ni(II) the reaction exhibits pseudo-second-order behavior. In the presence of the spin trap DMPO, the rate law is pseudo first order with k(obs) = (2k1[SCN-] + 2k2[SCN-]2) at pH 3, mu = 0.1 M, and 25-degrees-C. The rate constants are k1 = 0.046 +/- 0.003 M-1 s-1 and k2 = 2.04 +/- 0.012 M-2 s-1. Under these conditions inhibition by Ni(II) is significant only at relatively high Ni(II) concentrations. On the basis of a LFER, the magnitude of k1 confirms that it represents electron transfer to form Ni(II) and the SCN radical, with diffusive product separation as the rate-limiting step. The k2 step is interpreted as corresponding to formation of Ni(II) and the radical (SCN)2- with electron transfer concerted with S-S bond formation.