Kinetics of substitution of H2O by NCS– on µ-selenido incomplete cuboidal MIV3 clusters [Mo3OxSe4–x(H2O)9]4+ and on [Mo4Se4(H2O)12]5+

Abstract

Rate constants (k_f/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 Mo^IV₃ cluster complexes [Mo₃O_xSe_4–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 Se^2– and O^2– 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 [Mo₃O_xS_4–x(H₂O)₉]⁴⁺ series. Labilising effects of electron-rich µ-Se^2– > µ-S^2– > µ-O^2– 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 K_aM 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 k_eq.