The reduction of pentaammine(phenanthroline-2-carboxamido-N)cobalt(III) by titanium(III) and vanadium(II)

Abstract Pentaammine(phenanthroline-2-carboxamido- N )cobalt(III) ( I ) reacts with Ti 3+ (aq.) in two separable steps. The first step involves rapid formation of a binuclear intermediate with Ti 3+ forming a chelate with the carboxamido oxygen and the adjacent ring nitrogen. The rate law is k obs = k 1 [Ti 3+ ]+ k r [H + ] where k 1 =240±10 and k r =6.7±0.4 M −1 s −1 at 25 °C and I =1.0 M (LiCl). The second reaction involves electron transfer and follows the rate law k obs = a [H + ] −1 [Ti 3+ ]+ b [H + ] −1 where a =0.0046±0.0005 s −1 and b =(1.3±0.1)x10 −4 M −1 s −1 . The first term in the rate law of the second step is assigned to outer-sphere reduction by Ti(OH) 2+ of the binuclear precursor formed in the first reaction. The second term involves intramolecular electron transfer within the conjugate base of the binuclear precursor with k et , estimated as 0.13 s −1 . The factors affecting intramolecular electron transfer within Co(III)- and Ru(III)-Ti(III) intermediates are discussed. The reaction of I with V 2+ (aq.) also occurs in two distinct reactions. The first reaction follows the same rate law as the analogous Ti 3+ reaction with k 1 =2010±100 and k r =600±50 M −1 s −1 at 25 °C, I =1.0 M (LiClO 4 ). The second reaction is first-order in [V 2+ ] and independent of [H + ] with a second-order rate constant of 0.36 ±0.02 M t1 s −1 . The latter redox reaction is thought to involve outer-sphere reduction of the binuclear precursor formed in the first step.