Mechanistic studies on the substitution reactions between aquatetracyanooxotungstate(IV) and HCN–CN– and F–

Abstract

The kinetics of the substitution reaction between aquatetracyanooxotungstate(IV) ions and CN^– and HCN was studied, and compared with the corresponding molybdenum(IV) system. The aqua ligand in the [WO(OH₂)(CN)₄]^2– complex is substituted by both CN^– ions and HCN via a dissociative mechanism according to the reaction: [WO(OH₂)(CN)₄]^2–+ CN^––HCN (L)⇌[WO(L)(CN)₄]^n–+ H₂O. At 25 °C and l= 1 mol dm^–3(KNO₃), the equilibrium, forward and reverse rate constants of the reaction with CN^– ions were determined as 1.0(2)× 10³ dm³ mol^–1, 1.0(3) dm³ mol^–1 s^–1, 1.0(2)× 10^–3 s^–1, and for HCN as 1.0(2) dm³ mol^–1, 9(1) dm³ mol^–1 s^–1, and 8.0(2) s^–1, respectively. Co-ordination of HCN to the metal centre results in a decrease in the pK_a value of hydrogen cyanide by approximately 3.2 pH units to 5.8(3), and the decrease is in direct correlation with the stability constants for monodentate substitution reactions for the molybdenum(IV) and tungsten(IV) systems. A free-energy correlation between the hydrolysis rate constant and the stability constant for all the known [WO(L)(CN)₄]^n– complexes was constructed. A dissociative mechanism is postulated for the reaction where L = pyridine, N₃^–, NCS^– and CN^–. For the formation of [WO(F)(CN)₄]^3–, a volume of activation of –28(2) cm³ mol^–1 was obtained, suggesting a more associative activation for the reaction with F^–, in agreement with the observed deviation from the linear relationship obtained for the aforementioned nucleophiles.