Kinetics of anation of cis-diaquo-bis-(1,3-diaminopropane) cobalt(iii) by oxalate

Abstract

The kinetics of anation of cis-diaquobis(1,3-diaminopropane) cobalt(III) by oxalic acid and bioxalate anion have been studied at 35°, 40° and 45°C, I = 1.0 M(NaClO₄). The observed rate law for the formation of Co(tmd)₂C₂O⁺₄, - d in[Co(III)]_T/dr = k_obs = k₁Q₁[H₂C₂O₄] + k₂Q₂[HC₂O^-₄]/1 + Q₁[H₂C₂O₄] + Q2[HC₂O^-₄], is valid in the range of [Ox]_T = 0.03-0.50 M, pH=0.3-2.03. The anation rate constants, k₁ and k₂, for cis-{Co(tmd)₂(OH₂)³⁺₂, H₂C₂O₄) and cis-{Co(tmd)₂(OH₂)³⁺₂, HC₂O₄^-] ion pairs respectively are found to be virtually the same (k₁ = 10.7 × 10^-4 sec^-1 and k₂ = 11.0 ×10^-4 sec^-1 at 40°C). Temperature independent oxalic acid and bioxalate ion-pair association constants (Q₁ and Q₂) are 1.7(±0.3)M^-1 and 11.5(±0.7)M^-1, respectively. The activation enthalpy and entropy for k₁ and k₂ paths are 122.6 kJ mol^-1 89.5 JK^-1 mol^-1 and 103.8 kJ mol^-1, 29.3 JK^-1 mol^-1, respectively. The anation of cis-{Co(tmd)₂(OH₂)³⁺₂, X} ion pairs (X = H₂C₂O₄ or HC₂O^-₄) is believed to involve Id mechanism. The reaction is catalysed by NO^-₃.