Crystal structures and magnetic behaviour of the two forms of tetra-n-butylammonium tetraisothiocyanatochromate(II)

Abstract

From single-crystal X-ray studies, the red form of [NBu₄]₂[Cr(CNS)₄], which has a temperature-independent magnetic moment of 4.76 µ_Bover the range 85–300 K, has been found to contain planar [Cr(NCS)₄]^2–ions in which the Cr–N bond distances are essentially equal [2.010(3) and 2.012(2)Å]. On the other hand, the blue form, which exhibits antiferromagnetic behaviour, has been found to contain dinuclear anions [Cr₂(NCS)₈]^4–. In these there are two thiocyanate bridges, and the distorted square-pyramidal co-ordination of each Cr is completed by three isothiocyanate ligands, the nitrogen atom of a bridging thiocyanate group, and the sulfur atom of the other bridging thiocyanate at the apex. The Cr–N distances are similar to those in the red salt, but the Cr–S distance at 2.72(2)Å is long. The magnetic behaviour can be reproduced by the substitution of the values J= 8.76 cm^–1, g= 1.92 and N(α)= 100 × 10^–6 cm³ mol^–1 in the standard expression for a dinuclear species with S= 2. The Cr ⋯ Cr separation is 5.837(3)Å, and the structure is unrelated to that of quadruply bonded dinuclear chromium(II) complexes.