COMPLEX-FORMATION BETWEEN NICKEL(II) AND SOME PENTAMINE-(SUBSTITUTED SALICYLATO)COBALT(III) IONS

The kinetics of reversible complexation of Ni(II) with pentamine(subst ituted salicylato)cobalt(III) ions, [Co(N5){O2CC6H3(X)OH}]2+ [N5 = 5NH 3, (en)2(NH3) (cis isomer, en = ethane-1,2-diamine) or tetren (tetraet hylenepentamine), X = 3-NO2; N5 = 5NH3, X = 5-NO2], was investigated b y the stopped-flow technique at 15-35-degrees-C, pH 5.70-6.90 and I = 0.30 mol dm-3 (ClO4-). The formation of [(Co(N5){O2CC6H3(X)O}Ni]3+ occ urs via the reaction of [Ni(OH2)6]2+ with the phenoxide form of the co balt(III) substrates. The rate and activation parameters have been det ermined for the formation and dissociation of the binuclear species in which nickel(II) is chelated by the salicylate moiety. The data are c onsistent with an I(d) mechanism. The rate constant for spontaneous di ssociation of the binuclear species to the reacting partners is sensit ive to the nature of the pentamine moiety and decreases in the sequenc e tetren > (en)2(NH3) almost-equal-to 5NH3. The acid-catalysed dissoci ation of cis-[(en)2(NH3)Co{O2CC6H3(NO2-3)0}Ni]3+ conforms to a two-ste p process.