Spin density and bonding for the CoBr 2– 4 ion in Cs 3 CoBr 5 . A polarized neutron diffraction study

Abstract

Magnetization density data have been obtained for Cs 3 CoBr 5 . A chemically based model for the spin density in the CoBr 2– 4 ion has been fitted to 160 observed magnetic structure factors by a least-squares method ( X = 1.04). Most of the spin resides in the 3d(t 2 ) orbitals as expected (2.77 (4)) but 12 % is delocalized onto the ligand bromine atoms in a bond of mainly σ symmetry, but having a significant small π -symmetry component. On the cobalt atom, there is spin in 4p z orbitals, and substantial mixing of 4p z and 3d xy orbitals. This mixing, and other parameters, show substantial deviation from cubic symmetry. While the covalence is 17 % more than in the CoCl 2– 4 ion in the isomorphous compound Cs 3 CoCl 5 , the deviation from cubic symmetry is much less . This suggests that the non-cubic distortions arise from outside the CoX 2– 4 ions, that is a ‘crystal field’ effect on the whole CoX 2– 4 ion.