Kinetics of the complexing of Cl– NCS– to [Mo(H2O)6]3+ and assignment of an SN2 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): k_eq=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 S_N2 mechanism is suggested for these reactions. A comparison with aquation reactions of other trivalent aquo-metal ions is made.