Thiocyanate substitution on the incomplete cuboidal clusters [Mo3S4(H2O)9]4+ and [W3S4(H2O)9]4+

Abstract

Absorbance changes at ∼300 nm corresponding to the second stage in the reaction of excess of thiocyanate (millimolar amounts) with [Mo₃S₄(H₂O)₉]⁴⁺ and [W₃S₄(H₂O)₉]⁴⁺ have been studied for the first time. The Mo^IV₃ reaction, which is [NCS^–] dependent and independent of [H⁺]= 0.70–2.00 M, corresponds to substitution of a second H₂O at Mo. In the W^IV₃ case no [NCS^–] dependence is observed, and the dominant process is assigned as isomerisation of S- to N-bonded thiocyanate or a positional change. The kinetics exhibit an [H⁺]^–1 dependence from which it can be concluded that an acid dissociation process with K_a <0.008 M applies. The latter is assumed to be at an H₂O on the same W to which thiocyanate is attached. Spectrophotometric measurements on the unsubstituted reactants give acid dissociation constants of 0.18 and 0.26 M for [Mo₃S₄(H₂O)₉]⁴⁺ and [W₃S₄(H₂O)₉]⁴⁺ respectively. The kinetic results obtained illustrate differences in the behaviour of Mo and W, and are rationalised in terms of hard–soft acid–base theory.