Kinetics of the complexing of Cl? NCS? to [Mo(H2O)6]3+ and assignment of an S N2 mechanism

Abstract

Details of the preparation and properties of hexa-aquomolybdenum(III) in aqueous p-toluene-sulphonic acid (HPTS) and HBF4 are reported. The kinetics of the 1 : 1 reactions of [Mo(H2O)6]3+ with Cl– and NCS– have been investigated spectrophotometrically in PTS media, I= 1·0 M(LiPTS), and found to be independent of [H+] in the range 0·17–1·0M. The reaction with chloride was studied with [Cl–] in at least ten-fold excess of [Mo(H2O)6]3+. Pseudo-first-order rate constants for the equilibration (i), [Mo(H2O)6]3++ Cl– [graphic omitted] [Mo(H2O)5Cl]2+(i) can be expressed as in (ii): keq=k1[Cl–]+k–1(ii) At 25 ° C k1= 4·6 × 10–3 l mol–1 s–1, ΔH1‡= 23·5 ± 0·6 kcal mol–1, ΔS–1‡= 9·6 ± 2·1 cal K–1 mol–1, and k–1= 4·26 × 10–4 s–1, ΔH–1‡= 12·9 ± 0·6 kcal mol–1, ΔS–1‡=–30·7 ± 2·1 cal K–1 mol–1. The formation constantk1/k–1= 10·8 l mol–1 at 25 °C. The reaction with thiocyanate is thermodynamically much more favourable, formation constant ca. 105 l mol–1 at 25 °C. The reaction (iii) was followed with [Mo(H2O)6]3+ in at least [Mo(H2O)6]3++ NCS– [graphic omitted] [Mo(H2O)5(NCS)]3+(iii) ten-fold excess. At 25 °C, k2= 0·28 l mol–1 s–1, ΔH2‡= 16·3 ± 0·4 kcal mol–1, and ΔS2‡=–6·4 ± 1·3 cal K–1 mol–1. An SN2 mechanism is suggested for these reactions. A comparison with aquation reactions of other trivalent aquo-metal ions is made.