Spin density and bonding in the CoCl4 2? ion in Cs3CoCl5. Part 1. Magnetic structure factors from polarised neutron diffraction

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Abstract
The magnetic structure factors of 101 unique hkl reflections of Cs3CoCl5 have been measured at 4.2 K with a magnetic field of 4.6 T along the crystal c axis. A further 110 unique reflections with a magnetic field of 1.5 T along the a axis have also been obtained. A statistical analysis of the errors in the data sets has been made so that the reliability of their interpretation in models of chemical interest may be assessed. Ten of the c-data reflections, all based on low nuclear structure factors, were rejected on statistical grounds. The major sources of error in the data were found to be extinction and multiple scattering, with counting statistics and machine instability playing a lesser role. The two data sets agree well, no reflection exceeding 3σ from its scaled value. Using a free-ion Co2+ magnetic form factor, the data are reproduced to within 5% by a single spherical model of spin occupation 3d2.4x0.6, where x represents orbitals much more diffuse than Co2+ 3d functions. That is, the data indicate that ca. 80% of the spin is located in Co2+ 3d-like orbitals and 20% is in more diffuse orbitals, whether cobalt-centred or delocalised onto the chlorine ligand atoms. Covalence is indicated by the non-zero intensities of the l-odd data. Small, but statistically significant deviations from the above model are expected to be correlated with, for example, the concentration of the Co2+ 3d spin into the t2 orbitals and the details of the distribution of the diffuse ‘x’ population.

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