A comparison of substitution and redox properties of [W3NiS4(H2O)10]4+ and [Mo3NiS4(H2O)10]4+

Differences in properties of [M3NiS4(H2O)10]4+ (M=Mo, W), in aqueous acidic solutions are reported. From previous X-ray crystallography [Mo3NiS4(H2O)10](pts)4·7H2O exists as a single cube (pts−=p-toluene-sulfonate), while the corresponding [W3S4(H2O)4]4+ derivative gives the edge-linked double cube [{W3NiS4(H2O)9}2](pts)8·20H2O from 4 M Hpts. However, using Dowex cation-exchange chromatography the 4+ single cubes are eluted with 2.0 M acids (M=Mo, W), and are the dominant species in solution. Stopped-flow kinetic studies (25 °C) on the 1:1 substitution equilibration of [W3NiS4(H2O)10]4+ with NCS− (>tenfold excess), give [H+]-independent formation kf (4.5 M−1 s−1) and aquation kb (7.5×10−3 s−1) rate constants, which are approximately tenfold smaller than those reported for [Mo3NiS4(H2O)10]4+. With [Co(dipic)2]− (dipic=2,6-dicarboxylatopyridine) and [Fe(H2O)6]3+ as oxidants, rate constants for the rate determining first stages with [W3NiS4(H2O)10]4+ are unusually large and are, respectively, approximately 103 and 104 times greater than those for [Mo3NiS4(H2O)10]4+. The [Fe(H2O6]3+ oxidation of [W3NiS4(H2O)9]4+ is unusual in that no [H+]−1 dependence is observed in the range 0.5–2.0 M investigated. This is explained by the relatively slow interaction of [Fe(H2O)5OH]2+ with [W3NiS4(H2O)9]4+ to give an inner-sphere complex. The much stronger reducing properties of [W3NiS4(H2O)10]4+ are attributed to the W3 IV component.