VARIABLE-TEMPERATURE AND VARIABLE-PRESSURE STUDIES OF SOLVENT EXCHANGE, COMPLEX-FORMATION, AND DISSOCIATION OF VANADIUM(III) IN DMSO1,2

DMSO exchange on hexakis(dimethyl sulfoxide)vanadium(III) has been studied as a function of temperature and pressure by H-1 NMR spectroscopy at 200 and 400 MHz, in neat DMSO and in the diluent nitromethane. The following exchange parameters were obtained in neat solvent: k(ex)298/s-1 = 13.1; DELTA-H(ex)double-ended dagger/kJ mol-1 = 38.5; DELTA-S(ex)double-ended dagger/J K-1 mol-1 = -94.5; DELTA-V(ex)double-ended dagger/cm3 mol-1 = -10.1. In the diluent nitromethane a second-order rate law is observed with k2(298)/mol-1 kg s-1 = 2.7. The kinetics of formation and dissociation of V(DMSO)5NCS2+ (NCS = thiocyanate) have been studied in DMSO solution as a function of excess metal ion concentration, temperature, and pressure, by the stopped-flow technique. The thermodynamic stability of the complex was also determined spectrophotometrically. The kinetic and equilibrium data were submitted to a combined analysis. The rate constants and activation parameters for the formation (f) and dissociation (r) of the complex are as follows: k(f)298/M-1 s-1 = 153.8; k(r)298/s-1 = 0.93; DELTA-H(f)double-ended dagger/kJ mol-1 = 44.8; DELTA-H(r)double-ended dagger/kJ mol-1 = 42.0; DELTA-S(f)double-ended dagger/J K-1 mol-1 = -52.5; DELTA-S(r)double-ended dagger/J K-1 mol-1 = -104.6; DELTA-V(f)double-ended dagger (313 K)/cm3 mol-1 = -0.3; DELTA-V(r)double-ended dagger (313 K)/cm3 mol-1 = -11.8. The equilibrium constant for the formation of the monoisothiocyanato complex is K298/M-1 = 166.0, and the enthalpy and entropy of reaction are DELTA-H-degrees/kJ mol-1 = +2.9 and DELTA-S-degrees/J K-1 mol-1 = +52.1. The reaction volume is DELTA-V-degrees (318 K)/cm3 mol-1 = +11.5. The results for both processes are discussed and an associative interchange, I(a), mechanism is suggested for these substitution reactions on V(DMSO)6(3+) as for the analagous reactions in water.