Kinetic studies on the reactions of Cr 2+ and V 2+ with the μ-amido-μ-phosphatobis[tetra-amminecobalt(III)] complex

Abstract

Details of the kinetics of the reactions of the μ-amido-μ-phosphatobis[tetra-amminecobalt(III)] complex with Cr 2+ and V 2+ in perchlorate solutions, μ = 2·0M, (LiClO 4), are reported. The reduction with chromium(II), 2Cr 2+ + (Co III ) 2 → 2Co 2+ + (Cr III) 2, proceeds in two stages each involving the attack of chromium(II) and reduction of cobalt(III). The kinetics of the formation of a cobalt(III)-chromium(III) intermediate in the first stage have been studied by the stopped-flow method. The rate law is of the form (i), − d ln [( Co III) 2] dt = k 1K 1[ Cr 2+] K 1 + [ H +] where at 25°, K 1 (0·033 mole l −1) is the first acid dissociation constant of the diprotonated μ(NH 2, H 2PO 4) complex and k 1 (31·6 l mole −1 sec −1) is the rate constant for the reaction of the μ(NH 2, HPO 4) complex with Cr 2+. The second stage of the reduction is slower and can be studied by conventional techniques. The data give a good fit to (ii) − d ln[ Co III · Cr III] dt = (k 2 + k 3[ H +] −2)[ Cr 2+] and at 30° K 2 = 0·098 l mole −1sec −1, k 3 = 0·043 sec −1 and k 4 = 0·035 l −1mole sec −1. Spectra of the intermediate and final product have been obtained. The reduction with V 2+ also proceeds in two stages, the first of which is directly measurable. The rate law is of the form (iii) − d ln [( Co III) 2] dt = (k 5K 1 + k 6[ H +])[ V 2+] K 1 + [ H +] where at 25° k 5 (2·91 l mole −1 sec −1) and k 6 (0·64 l mole −1 sec −1) are for the reactions of the μ(NH 2, HPO 4) and μ(NH 2, H 2PO 4) complexes respectively. Activation parameters have been obtained for k 1, k 5 and k 6.