Concurrent NCS– substitution at non-equivalent molybdenum(IV) centres of the incomplete cuboidal aqua ion [Mo3(µ3-S)(µ-O)(µ-S)2(H2O)9]4+

Abstract

The kinetics of NCS^– substitution on the unsymmetrical (crystallographically defined) molybdenum(IV) aqua ion, Mo₃SOS₂⁴⁺, have been studied, under conditions in which substitution is confined to 1 : 1 reaction at each Mo^IV. Biphasic behaviour was observed with first Mo₃SOS₂⁴⁺(> 10-fold) and then NCS^–(> 10-fold) in excess, and two equilibration rate constants were obtained. These correspond to concurrent substitution at the molybdenum(IV) centres, two of one kind and one of another. A statistical factor of 2 applies to the slower process, which is therefore assigned to substitution at the two µ-oxo co-ordinated molybdenum(IV) centres. Formation rate constants are 82 and 23 M^–1 s^–1 respectively at 25 °C in 2 M HClO₄.