MECHANISTIC STUDIES ON THE SUBSTITUTION-REACTIONS BETWEEN AQUATETRACYANOOXOTUNGSTATE(IV) AND HCN-CN- AND F-

The kinetics of the substitution reaction between aquatetracyanooxotun gstate (IV) ions and CN- and HCN was studied, and compared with the co rresponding molybdenum(IV) system. The aqua ligand in the [WO(OH2)(CN) (4)](2-) complex is substituted by both CN- ions and HCN via a dissoci ative mechanism according to the reaction: [WO(OH2)(CN)(4)](2-) + CN-- HCN (L) reversible arrow [WO(L)(CN)(4)](n-) + H2O. At 25 degrees C and l = 1 mol dm(-3) (KNO3), the equilibrium, forward and reverse rate co nstants of the reaction with CN- ions were determined as 1.0(2) x 10(3 ) dm(3) mol(-1), 1.0(3) dm(3) mol(-1) s(-1), 1.0(2) x 10(-3) s(-1), an d for HCN as 1.0(2) dm(3) mol(-1). 9(1) dm(3) mol(-1) s(-1), and 8.0(2 ) s(-1), respectively. Co-ordination of HCN to the metal centre result s in a decrease in the pK(a) value of hydrogen cyanide by approximatel y 3.2 pH units to 5.8(3). and the decrease is in direct correlation wi th 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 a ll the known [WO(L)(CN)(4)](n-) complexes was constructed. A dissociat ive mechanism is postulated for the reaction where L = pyridine. N-3(- ), NCS- and CN-. For the formation of [WO(F)(CN)(4)](3-), a volume of activation of -28(2) cm(3) mol(-1) was obtained. suggesting a more ass ociative activation for the reaction with F-, in agreement with the ob served deviation from the linear relationship obtained for the aforeme ntioned nucleophiles.