SYNTHESIS, ACID-HYDROLYSIS AND FORMATION OF THE [(H3N)(5)COOMOO3](-SOLUTION() ION IN AQUEOUS)

The complex [(H3N)(5)CoOMoO3]ClO4 has been synthesised and characteris ed by elemental analysis, UV/VIS and infrared spectroscopy. The kineti cs of its acid hydrolysis has been studied using the stopped-flow tech nique over the ranges 0.01 less than or equal to [H+] less than or equ al to 0.45 mol dm(-3), 24.9 less than or equal to theta less than or e qual to 35.6 degrees C, 0.04 less than or equal to l less than or equa l to 1.0 mol dm(-3) (NaClO4). The rate of reaction is inversely depend ent on [H+] due to the expansion of the co-ordination of the molybdenu m (VI) from four to six on protonation, along with resonance stabilisa tion. The rate constant (k(1)) for the hydrolysis is 5.09 +/- 0.02 s(- 1) at 24.9 degrees C for which Delta H double dagger = 82 +/- 1 kJ mol (-1) and Delta S double dagger = 43 +/- 5 J K-1 mol(-1). The kinetics of formation of the complex from pentaammineaquacobalt(III) and molybd ate(VI) ions has been studied using the stopped-flow method over the r ange pH 7.13-8.46 and at 25.0-35.9 degrees C, l = 1.0 mol dm(-3). Over this pH range both [Co(NH3)(5)(OH2)](3+) and [Co(NH3)(5)(OH)](2+) ion react with HMoO4- to give [(H3N)(5)CoOMoO3](+), and the rate constant s are (2.03 +/- 0.05) x 10(6) and (2.73 +/- 0.05) x 10(5) dm(3) mol(-1 ) s(-1), respectively at 25.0 degrees C. Both acid hydrolysis and comp lexation are very rapid, suggesting a mechanism involving cleavage of the Mo-VI-O not the Co-III-O bond. Details of both mechanisms are disc ussed.