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).7H(2)O 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).20H(2)O 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 degreesC) on the 1:1 substitution equilibration Of [W3NiS4(H2O)(10)](4+) with NCS- ( > tenfold excess), give [H+]-independent formation k(f) (4.5 M-1 s(-1)) and aquation k(b) (7.5 x 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 10(3) and 10(4) 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)(5)OH](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 W-3(IV) component. (C) 2002 Elsevier Science B.V. All rights reserved.