CRYSTAL-STRUCTURES AND MAGNETIC-BEHAVIOR OF THE 2 FORMS OF TETRA-N-BUTYLAMMONIUM TETRAISOTHIOCYANATOCHROMATE(II)

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 mu(B) over the range 85-300 K, has been found to contain planar [Cr(NCS)4]2- ion s in which the Cr-N bond distances are essentially equal [2.010(3) and 2.012(2) angstrom]. On the other hand, the blue form, which exhibits antiferromagnetic behaviour, has been found to contain dinuclear anion s [Cr 2(NCS)8]4-. In these there are two thiocyanate bridges, and the distorted square-pyramidal co-ordination of each Cr is completed by th ree isothiocyanate ligands, the nitrogen atom of a bridging thiocyanat e 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) angstrom is long. The magnetic behaviour can be reproduced by the substitution of the values J = 8.76 cm-1, g = 1. 92 and N(alpha) = 100 X 10(-6) cm3 mol-1 in the standard expression fo r a dinuclear species with S = 2. The Cr ... Cr separation is 5.837(3) angstrom, and the structure is unrelated to that of quadruply bonded dinuclear chromium(II) complexes.