Kinetics of the 1 : 1 complexing of oxalate to hexa-aquamolybdenum(III)

Abstract

The kinetics of the 1 :1 equilibration of oxalate with [Mo(H₂O)₆]³⁺ have been investigated spectrophotometrically at 350 nm in aqueous p-toluenesulphonic acid (Hpts) at 25 °C, I= 1.0 M [Li(pts)]. For the range of [H⁺] values studied, 0.35–0.90 M, protonated forms of oxalate are present. The [H⁺] dependence, equation (i), is consistent with HC₂O₄^– and not H₂C₂O₄(acid dissociation k_eq=k₁K_a[Mo³⁺]//_[H⁺]+Ka+k_–1[H⁺](i) constant K_a= 0.084 M at 25 °C) as the dominant reactant, where k₁ and k_–1 are as defined in equation (ii). At 25 °C, k₁= 0.49 ± 0.1 M^–1 s^–1 and k_–1=(4.2 ± 0.2)× 10^–4 M^–1 s^–1, giving an Mo³+ HC₂O₄^– [graphic omitted] Mo(C₂O₄)⁺+ H⁺(ii) equilibrium constant of 1 170 ± 100, which is in agreement with an independently determined value of 1 230 ± 460. Using trifluoromethanesulphonic acid instead of Hpts, k₁ is 15% higher but k_–1 remains unchanged. The results are discussed in terms of a model in which the oxalate chelates to the metal.