Potassium tetrakis(isothiocyanato)cobaltate(II) trihydrate: a redetermination

Abstract

K2Co(NCS)4.3H20, M=423.51, orthorhombic, a = l l . l 1 6 (5), b=12.981 (7), c=5-354 (5) A, U = 771.2/k 3, Z = 2 , de= 1.82, din= 1.80 (2) g cm -3, Mo K~ radiation 2=0.7107 A, /z=21.7 cm -1. Space group P21212 from systematic absences h00, h = 2 n + 1, 0k0, k = 2 n + 1. The discrete anion [Co(NCS)4] 2has CoN C angles of 164.6 (7) and 166.8 (7) ° and not 111 ° as reported earlier by Zhdanov & Zvonkova [Zh. Fiz. Chim. (1950). 24, 1339-1344]. The cobalt atom has a distorted tetrahedral environment and the bond lengths are C o N 1.967 (7), 1.949 (7); N C 1.15 (1), 1.14 (1); C-S 1.627 (9), 1.657 (8) A. 886 independent reflexions, measured by counter methods, have been refined to R =0.054. Introduction. Deep-blue needles of K2Co(NCS)4.3H20 were obtained by reacting excess potassium thiocyanate with cobalt nitrate. A crystal with dimensions 1 .00×0.25x 1.7 mm was mounted with the c axis parallel to the instrument axis of a General Electric X R D 5 apparatus which was used to measure diffraction intensities and cell dimensions. It was equipped with a manual goniostat, scintillation counter and pulse-height discriminator. Zirconium-filtered molybdenum X-radiation was used. 1076 independent reflexions with 20 a(/) were used in subsequent calculations. No extinction or absorption corrections were applied. The positions of the cobalt atoms were determined from a Patterson function and Fourier syntheses were calculated to determine the positions of the remaining atoms. The anion has crystallographic C2 symmetry with the cobalt atoms in special positions, 2(a), on the twofold axis. The structure was refined by full-matrix least-squares calculations, all atoms anisotropically to R 0.054. (Refinement with the opposite enantiomorph gave R 0.059.) The weighting scheme was chosen to give average values of wzl 2 for groups of reflexions independent of the values of Fo and sin 0/2, and was I/w= 1 for Fo 15. Calculations were made on a C.D.C. 7600 computer at the University of London Computer Centre using the X-RAY set of programs (Stewart, 1972). Atomic scattering factors for all atoms were taken from International Tables for X-ray Crystallography (1962) as were the corrections for the real and imaginary parts of the anomalous dispersion for the cobalt, potassium and sulphur atoms. The artisotropic temperature factor is defined as exp [-0.25rc2~,~Z~h~hlU~jbtbj ( i , j=1,2,3)] , where b~ is the ith reciprocal cell dimension. The 190 zero reflexions showed no large discrepancies. A difference Fourier synthesis showed no extraneous peaks, and the hydrogen atoms of the water molecules were not discernible. In the final cycle of refinement all shifts were < 0.05tr. The final list of positions is given in Table 1, thermal parameters in Table 2 and bond lengths and