Thermodynamic studies on the addition of molecular oxygen to cobalt(II) complexes. Part 1. The cobalt(II)–ethylenediamine–oxygen system in aqueous solution at 25 °C

Abstract

The equilibrium constant for the formation of [Co₂(en)₄(O₂)(OH)]³⁺(en = ethylenediamine) in 1 mol dm^–3 KCI aqueous solution, at 25 °C, has been determined using polarographic oxygen analysis, pH metric, and spectrophotometric techniques. The enthalpy of formation of this complex has been obtained by direct calorimetric measurements. The thermodynamic functions for the addition of O₂ to [Co(en)₂]²⁺ in order to form the doubly bridged dicobalt complex [Co₂(en)₄(O₂)(OH)]³⁺ have been calculated. The enthalpy change is very large, but the stability of the binuclear species with respect to [Co(en)₂]²⁺ is strongly reduced by a large entropy effect which cannot be explained in terms of the value of the partial molar entropy of oxygen in 1 mol dm^–3 KCI solution.