The paramagnetic anisotropies and ligand fields of the tetrahedral cobaltous chlorides and thiocyanate

The temperature dependence of the paramagnetic susceptibilities of single crystals of Cs$_3$CoCl$_5$, Cs$_2$CoCl$_4$ and K$_2$Co(CNS)$_4\ldots$4H$_2$O have been measured over the range 80 to 300 $^\circ$K. The splitting of the excited $^4$T$_2$ term by low symmetry crystal fields is of the order of 1000 cm$^{-1}$, and the extent of t$_{2g}$ electron delocalization in each complex corresponds to a delocalization parameter k of approximately 0$\cdot$9. The magnitudes of the observed $^4$T$_2$ term splittings are calculated in a semi-quantitative way through a comparison of the details of the molecular geometry with crystal field theory for a distorted tetrahedron. Magnetic exchange effects in the complexes, corresponding to exchange integrals of several degrees, are sufficiently large that the zero field splitting effect is relatively unimportant. The geometrical arrangement of adjacent (CoCl$_4$)$^{2-}$ tetrahedra taken with the delocalization parameters allows an order-of-magnitude calculation of anisotropic magnetic exchange in crystals of Cs$_3$CoCl$_5$ and Cs$_2$CoCl$_4$. A new X-ray analysis of the co-ordination geometry of Cs$_3$CoCl$_5$ is also summarized.