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

Abstract

The reversible complexation of cobalt(II) with 2-(irnidaioleazo) benzene (lAB) and 2-(2-aminoethyl)benzimidazole (AEB) has been studied at <em>l</em>=0.3 mol dm^-3. At 25&deg;C the values of log <em>K</em>_M (<em>K</em>_M is the stability constant), ∆<em>H⁰</em>/KJ mol^-1, and ∆<em>S⁰</em>/JK^-1 mol'! are 2.09&plusmn;0.03, 14.4&plusmn;2.4, 88&plusmn;8 for Co(IAB)2+ and 2.65&plusmn;0.01, 20.0&plusmn;7.2, 120&plusmn;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&plusmn;0.4 (lAB), 1.01&plusmn;0.03 (AEB) at 10&deg;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⁺.