Preparation, structure and properties of trinuclear [M3Se4(CN)(9)](5-) (M=Mo or W) complexes obtained from M3Se7 core compounds and related studies

The preparations of [Mo3Se4(CN)(9)](5-) and [W3Se4(CN)(9)](5-) by reacting polymeric {M3Se7Br4}(x) or the derivative [M3Se7Br6](2-) ions (M = Mo or W) with CN- are described. Both products were isolated as the Cs-5[M3Se4(CN)(9)]. CsCl . 4H(2)O salts. The crystal structures are essentially the same with corresponding cell dimensions within 0.3% of each other, and those for the W compound slightly the larger. The M3Se4 cores (approximate symmetry C-3v) can be described as distorted incomplete cubes with, in the tungsten case, d(ave)(W-W) 2.829, d(ave)(W-mu-Se) 2.449, d(ave)(W-mu(3)-Se) 2.397, d(ave)(W-C) 2.179 and d(ave)(C-N) 1.16 Angstrom. A different feature as compared to the sulfide analogues is the weak dimerisation of two trimer units giving short Se ... Se contacts (approximate to 3.5 approximate to). The UV/VIS spectra of brown [Mo3Se4(CN)(9)](5-) and green [W3Se4(CN)(9)](5-) indicate a red shift on replacing Mo by W, and a similar shift on exchanging S for Se. In electrochemical studies using cyclic, square-wave and differential pulse voltammetric techniques reduction potentials for the [M3Se4(CN)(9)](5-16-) couple (vs. normal hydrogen electrode) of -0.63 (Mo) and -0.97 V (W) were obtained. Using a Hg-Au electrode two additional reduction steps were observed, and there is evidence for the formation of Hg-containing heterometallic clusters. Properties of the two aqua ions [M3Se7(H2O)(6)](4+) were also studied, where these are similarly converted to [M3Se4(CN)(9)](5-) by Se-abstraction and substitution of H2O by CN-.