Synthesis, Acid Hydrolysis and Formation of the Molybdatopentaamminechromium(III) Ion in Aqueous Solution

Abstract

The title complex has been synthesised for the first time and characterised by elemental analysis and spectroscopic analyses. The kinetics of the acid hydrolysis and formation of the title complex have been studied using the stopped-flow technique. The acid hydrolysis experiments were carried out over the ranges, 0.01 < [H'] < 0.14 mol dm 3 , 22.4° C < θ < 30.1 C and at ionic strength 1.0 mol dm 3 (NaNO 3 ), whereas the formation kinetics were studied over the ranges 7.22 < pH < 8.18 and 25.0 C < θ < 33.9 C, with ionic strength 1.0 mol dm 3 (NaClO 4 ). The rate of acid hydrolysis is inversely dependent on [Η ' ] which has been ascribed to the expansion of the coordination number of the molybdenum(VI) from four to six on protonation, along with resonance stabilisation. The rate constant, k f o r the hydrolysis is 22.6 ± 0.3 s 1 at 25.1 C for which ΔΗ' = 89±5 kJ mol 1 and ΔS'= 81 ± 2 1 J mol 1 Κ 1 The rate constants for the reactions of (NH 3 ) 5 CrOH 3 and (NH 3 ) 5 CrOH 2 to form the title complex are (1.18 ±0.07) x 10 7 mol 1 dm 3 s 1 and (1.13 ± 0.05) x 105 mol 1 dm 3 s 1 respectively at 25.0 C. The results indicate that both processes are occurring via a mechanism involving Mo VI -O bond breakage.