Complex formation of cobalt(II) with 2-(imidazoleazo)benzene and 2-(2-aminoethyl)benzimidazole: A kinetic and equilibrium study

The reversible complexation of cobalt(II) with 2-(imidazoleazo)benzene (IAB ) and 2-(2-aminoethyl)benzimidazole (AEB) has been studied at I = 0.3 mol d m(-3). At 25 degrees C the values of log K-M (K-M is the stability constant ), Delta H-0/kJ mol(-1), and Delta S-0/JK(-1) mol(-1) are 2.09 +/- 0.03, 14 .4 +/- 2.4, 88 +/- 8 for Co(IAB)(2+) and 2.65 +/- 0.01, 20.0 +/- 7.2, 120 /- 21 for Co(AEB)(2+) respectively. The rate constants of formation of CoL2 + chelate via the reaction: Co(OH2)(6)(2+)+LCo(OH2)(4)L2+, (10(-4) k(f)(L)/ dm(3) mol(-1) s(-1) = 11.7 +/- 0.4 (IAB), 1.01 +/- 0.03 (AEB) at 10 degrees C) are at least 10 times smaller than the rate constant of water exchange from Co(OH2)(6)(2+). Data analysis further indicates that the chelate forma tion involves Chelation Controlled Substitution mechanism. The dissociation of Co(AEB)(2+) is strongly acid catalysed unlike that for Co(IAB)(2+).