Thiocyanate substitution on the incomplete cuboidal clusters [Mo3S4(H2O)9]4+ and [W3S4(H2O)9]4+
- 1 January 1992
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in J. Chem. Soc., Dalton Trans.
- No. 2,p. 325-329
- https://doi.org/10.1039/dt9920000325
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 MoIV 3 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 WIV 3 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 Ka 2O 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.Keywords
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