Complex formation between nickel(II) and some pentamine (substituted salicylato)cobalt(III) ions

Abstract

The kinetics of reversible complexation of NiII with pentamine(substituted salicylato)cobalt(III) ions, [Co(N₅){O₂CC₆H₃(X)OH}]²⁺[N₅= 5NH₃, (en)₂(NH₃)(cis isomer, en = ethane-1,2-diamine) or tetren (tetraethylenepentamine), X = 3-NO₂; N₅= 5NH₃, X = 5-NO₂], was investigated by the stopped-flow technique at 15–35 °C, pH 5.70–6.90 and I= 0.30 mol dm⁻³(ClO₄⁻). The formation of [(Co(N₅){O₂CC₆H₃(X)O}Ni]³⁺ occurs via the reaction of [Ni(OH₂)₆]²⁺ with the phenoxide form of the cobalt(III) substrates. The rate and activation parameters have been determined for the formation and dissociation of the binuclear species in which nickel(II) is chelated by the salicylate moiety. The data are consistent with and I_d mechanism. The rate constant for spontaneous dissociation of the binuclear species to the reacting partners is sensitive to the nature of the pentamine moiety and decreases in the sequence tetren > (en)₂(NH₃)⁻ 5NH₃. The acid-catalysed dissociation of cis-[(en)₂(NH₃)Co{O₂CC₆H₃(NO₂⁻³)O}Ni]³⁺ conforms to a two-step process.