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

Abstract

The preparations of [Mo 3 Se 4 (CN) 9 ] 5- and [W 3 Se 4 (CN) 9 ] 5- by reacting polymeric {M 3 Se 7 Br 4 } x or the derivative [M 3 Se 7 Br 6 ] 2- ions (M = Mo or W) with CN - are described. Both products were isolated as the Cs 5 [M 3 Se 4 (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 M 3 Se 4 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–µ-Se) 2.449, d ave (W–µ 3 -Se) 2.497, d ave (W–C) 2.179 and d ave (C–N) 1.16 A. A different feature as compared to the sulfide analogues is the weak dimerisation of two trimer units giving short Se · · · Se contacts (≈3.5 A). The UV/VIS spectra of brown [Mo 3 Se 4 (CN) 9 ] 5- and green [W 3 Se 4 (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 [M 3 Se 4 (CN) 9 ] 5-/6- 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 [M 3 Se 7 (H 2 O) 6 ] 4+ were also studied, where these are similarly converted to [M 3 Se 4 (CN) 9 ] 5- by Se-abstraction and substitution of H 2 O by CN - .