Kinetics and mechanism of complex formation between O-bonded pentaammine(valinato/prolinato)cobalt(III) ions and nickel(II) in aqueous medium

The kinetics of reversible complexation of Ni(OH2)(6)(2+) with oxygen-bonded valinato/prolinatocobalt(III) substrates, (NH3)(5)Co(val/pro)(3+) have been investigated by stopped flow technique at 25degreesC, 6.1 less than or equal to pH less than or equal to 6.70 and I = 0.3 mol dm(-3). The formation of binuclear species, [(NH3)(5)Co(val/pro)Ni](4+), occured via the reaction of Ni(OH2)(6)(2+) with deprotonated (amine and imine functions of valine and proline, respectively) form of the cobalt(III) substrates, (NH3)(5)Co(val/pro)(2+). The results indicate the formation of mono-bonded binuclear species through entry of the imine/amine functions into the coordination sphere of Ni(II) with Ni-OH2 bond dissociation is limiting (I-d mechanism). The binuclear species exist in dynamic equilibrium involving the mono-dentate and chelated forms with chelated forms predominating. The small values of dissociation rate constants, despite the intrinsic electrostatic repulsion between the like charge centers, also support the chelate nature of the binuclear species.