Preparation and properties of potassium µ-thio-bis[hexacyanomolybdate(IV)], K6[Mo2(CN)12S], the crystal structure of its double salt with potassium tetraoxomolybdate(VI), and a discussion of molybdenum–sulphur bond lengths and bond orders

Abstract

The complexes K₆[Mo₂(CN)₁₂S]·4H₂O (1a) and the double salt K_6.68[MoO₄]_0.34[Mo₂(CN)₁₂S]·5.32H₂O (1b) have been prepared by (a) reaction of molybdate(VI) with cyanide and hydrogen sulphide in aqueous solution in air and (b) aerial oxidation of an aqueous solution of K₆[Mo₂(CN)₈S₂]·4H₂O (2) and K₅[Mo(CN)₇]·H₂O (3). The overall reaction (a) involves intermediate formation of the complexes (2) and (3)via abstraction of sulphide by cyanide from thiomolybdate species to give thiocyanate and concomitant two- and four-equivalent reduction of Mo^VI. Crystals of the double salt (1b) are orthorhombic, a= 15.089(10), b= 11.753(9), c= 9.113(8)Å, Z= 2, space group Pmmn, and 1 926 independent above-background reflections have been refined to R= 0.066. The [Mo₂(CN)₁₂S]^6– ion (1) has imposed C_2v symmetry with both molybdenums in seven-co-ordinate pentagonal-bipyramidal environments and bridged via an axial sulphur atom [Mo–S 2.173(1)Å; Mo–S–Mo 169.5(2)°]. The metal–carbon bonds are equivalent, ranging from 2.173(8) to 2.190(11)Å. The Mo–S bond length is the same as in the [MoS₄]^2– ion. Molybdenum-sulphur bond lengths in (1b) and a series of MoS complexes may be correlated inversely with bond orders. The diamagnetism of complexes (1a) and (1b) and their electronic spectra are consistent with a S(p_π)→Mo(d_π) bonding interaction and a multiple Mo–S bond. A strong i.r. band at 623 cm_–1 is assigned ν_asym(Mo–S–Mo). The bridging sulphide in (1) is inert to H⁺, [OH]^–, and [CN]^–; there is no monomer–dimer equilibrium in aqueous solution. Abstraction of terminally bound (but not bridging) sulphur by cyanide raises the possibility that this may be the form of sulphur involved in cyanide deactivation of the enzyme xanthine oxidase.