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 Å. A different feature as compared to the sulfide analogues is the weak dimerisation of two trimer units giving short Se · · · Se contacts (≈3.5 Å). 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 ^- .