759-764The reversible complexation of cobalt(II) with 2-(irnidaioleazo) benzene (lAB) and 2-(2-aminoethyl)benzimidazole (AEB) has been studied at l=0.3 mol dm-3. At 25°C the values of log KM (KM is the stability constant), ∆H0/KJ mol-1, and ∆S0/JK-1 mol'! 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)62++L Co(OH2)4L2+., (10-4 k/dm-3 mol-1 s-1=11.7±0.4 (lAB), 1.01±0.03 (AEB) at 10°C) are at least 10 times smallerthan the rate constant of water exchange from Co(OH2)62+.Data analysis further indicates that the chelate formation involves Chelation Controlled Substitution mechanism. The dissociation of Co(AEB)2+ is strongly acid catalysed unlike that for Co(IAB)2+
