Reaction of pentaammine(urea-O)cobalt(III) cation with chlorine and hypochlorous acid in aqueous solution

Abstract

The Co(NH3)5[OC(NH2)2]3+ cation in aqueous acid reacts with chlorine and hypochlorous acid, with two sequential steps observed in each case. Rate constants for both steps show a first-order dependence on [oxidant], with k1/k2 always <20, but varying with the choice of reactant and acid. Rate constants with Cl2 as reactant are faster than with HOCl, possibly related to preferential attack by Cl+ compared with OCl− on the bound urea. Competition by ions (HSO 4 − , Cl− or NO 3 − ) measured by product analysis of reactions conducted in 1 M acid produced competition ratios R (R=[CoX]/[CoOH2][X]) which are similar to values determined with a range of leaving groups previously, indicating a mechanistic constancy. No formation of Co(NH3)5Cl2+ was observed in reactions conducted in H2SO4 or HNO3, implying that free Cl− is not generated at the reaction site and captured by the metal ion. Electronic and vibrational spectra of the intermediate formed in the two stage reaction is indicative of a change from an O-bound to an N-bound ligand in forming that intermediate, although it cannot be a simple isomerization due to the dependence on [oxidant]. A possible mechanism is discussed.