KINETICS AND MECHANISM OF OXYGEN EXCHANGE AND INVERSION ALONG THE M=O AXIS IN THE DIPROTONATED AND MONOPROTONATED DIOXOTETRACYANOMETALATE COMPLEXES OF RE(V), TC(V), W(IV), AND MO(IV)

摘要

Oxygen-17 NMR was utilized in aqueous medium (1.2-2.4 m KNO3) to study the oxygen exchange kinetics in the trans-dioxotetracyanometalate complexes of Re(V), Tc(V), W(IV), and Mo(IV). The kinetics are described by the two-term rate law R = (k(aq)[MOH(2)] + k(OH)[MOH])/2, where MOH(2) and MOH represent the di- and monoprotonated forms, [MO(OH2)(CN)(4)](n-) and [MO(OH)(CN)(4)]((n+1)-), of the dioxotetracyanometalate complexes, respectively. The aqua complexes are by far more reactive toward exchange (k(aq) values of (9.1 +/- 0.1) x 10(-2) and (137 +/- 5) s(-1) at 25.0 degrees C for Re(V) and W(IV), respectively), compared to the [MO(OH)(CN)(4)]((n+1)-) ions under the same conditions (k(OH): Re(V), (2.6 +/- 0.3) x 10(-3); W(IV), (6.5 +/- 0.1) x 10(-4) s(-1)), with the k(OH) value for the Tc(V) hydroxo oxo complex at 25 degrees C obtained as 13 +/- 1 s(-1). The activation parameters are as follows [Delta H-double dagger (kJ mol(-1)), Delta S-double dagger (J K-1 mol(-1))]. Re(V): k(aq), 84.00 +/- 1, 16.91 +/- 0.3; k(OH), 86.97 +/- 6, -2.71 +/- 20. W(IV): k(aq,) 79.98 +/- 2, 64.28 +/- 7; k(OH), 79.49 +/- 0.3, -39.30 +/- 1. Tc(V): k(OH), 75.95 +/- 4, 30.98 +/- 15. No evidence was found for the dioxo complexes undergoing exchange unless protonated. A variable-temperature coordinated aqua line width study was done on the [WO(OH2)(CN)(4)](2-) complex. The exchange data of the aqua complexes are in agreement with a dissociative activation, whereas the results obtained from the exchange of the hydroxo complexes point to a possible interchange/associative mechanism. This is the first case where data are obtained for substitution rates of the hydroxo Ligand in these types of complexes. The four metal systems are correlated as a function of pH with regard to the (i) rate of inversion of the metal complex along the M=O axis, as manifested by the proton exchange, and (ii) oxygen exchange. These pH dependent plots enable the prediction of the inversion and oxygen exchange rates over more than a 10 order of magnitude range. [References: 26]