Mechanism of substitution on trinuclear incomplete cuboidal [M3X4(OH2)9]4+ ions: kinetic studies of water exchange and substitution by Cl– on [Mo3S4(OH2)9]4+

Abstract

The complex [Mo₃S₄(OH₂)₉]⁴⁺ possesses non-equivalent H₂O ligands with those (d, two per Mo) which are trans to µ-S undergoing water exchange 10³ faster than those (c, one per Mo) which are trans to µ₃-S. The former arises from a labilising pathway involving the monohydroxo conjugate base [Mo₃S₄-(OH₂)₈(OH)]³⁺(K_aM≈ 0.2 mol dm^–3). The chloride anation reaction has also been studied and comparisons are made with data previously reported for the thiocyanate anation. Together the results suggest an l_d mechanism for substitution at the d sites on [Mo₃S₄(OH₂)₈(OH)]³⁺. For water exchange on [Mo₃S₄(OH₂)₉]⁴⁺ at both the d and c sites, an l_a mechanism is tentatively suggested from comparisons with complex-formation data.