Kinetics of substitution of H2O by NCS–on µ-selenido incomplete cuboidal MIV3clusters [Mo3OxSe4–x(H2O)9]4+and on [Mo4Se4(H2O)12]5+

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Rate constants (kf/M–1 s–1) have been determined at 25 °C, l= 2.00 M (LiClO4 and in one case Lipts; pts–= toluene-p-sulfonate) for the 1:1 substitution of H2O by NCS– at Mo on the trinuclear MoIV3 cluster complexes [Mo3OxSe4–x(H2O)9]4+, x= 0–4. The dominant process is substitution of H2O at the transµ-Se positions with rate constants for [Mo3(µ3-Se)(µ-Se)3(H2O)9]4+(480) > [Mo3(µ3-Se)(µ-O)(µ-Se)2(H2O)9]4+(131 and 52) > [Mo3(µ3-Se)(µ-O)2(µ-Se)(H2O)9]4+(13.5 and 2.8) > [Mo3(µ3-Se)(µ-O)3(H2O)9]4+(0.19), alongside the previously studied [Mo3(µ3-O)(µ-O)3(H2O)9]4+(2.13) giving a 2530-fold spread of values at [H+]=l= 2.00M. The complexes with x= 1 and 2 have non-identical molybdenum centres according to the number of core Se2– and O2– ligands attached, and give biphasic kinetics. Statistical factors of 1, 2 and 3 corresponding to the number of identical centres are defined for the different reactions. The slowness of substitution at positions trans to the µ3-Se ligands is confirmed by studies on cuboidal [Mo4Se4(H2O)12]5+, which has all µ3-Se core ligands. Comparisons are made with results obtained for the [Mo3OxS4–x(H2O)9]4+ series. Labilising effects of electron-rich µ-Se2– > µ-S2– > µ-O2– at the trans H2O ligand are noted. In contrast replacement of the µ3-O of [Mo3O4(H2O)9]4+ by µ3-Se and µ3-S in turn produces 14- and 6-fold retardation effects. Substitution at the H2O ligands trans to µ-Se ligands is increased on decreasing [H+] consistent with the involvement of conjugate-base forms Mo3(OH)3+(k2) alongside Mo34+(k1). The kinetics give H2O ligand acid dissociation constants KaM of 0.32 and 0.44 M for [Mo3Se4(H2O)9]4+ and [Mo3O3Se(H2O)9]4+ respectively. Similar patterns are observed for the aquation rate constants keq.