Spin density and bonding in the CoCl42– ion in Cs3CoCl5. Part 2. Valence electron distribution in the CoCl42– ion

Abstract

We present an interpretation of the polarised neutron diffraction data for Cs₃CoCl₅ in terms of a simple molecular-orbital description of the CoCl₄^2– ion, with direct modelling of the spin-density features. A direct observation of the overlap spin density in each Co–Cl bond has been obtained, and a probable overlap population of –0.13 ± 0.01 spins has been deduced.

We have found that the cobalt atom 3d orbital populations are essentially t₂ 2.71 ± 0.15, e 0.0 ± 0.1 spins, no statistically significant differentiation of the t₂ orbital populations on account of departure from cubic symmetry being observed. There is a further significant centrosymmetric spin population (0.50 ± 0.10) on the cobalt atom, but in a diffuse orbital (maximising at ca. 100 pm from the nucleus) whose angular variation cannot be specified. The spin population of each chlorine atom is, as expected, mainly in the p_σ orbital (0.062 ± 0.006 spins) with a total p_π contribution less by a factor approaching three. Fairly minor changes in Co²⁺ 3d and Cl⁰ 3p free-ion orbital wavefunctions are deduced for the atoms in the complex ion.

In general, it is found that our data are sufficiently numerous and accurate that we are able to test the validity of simple ligand-field and molecular-orbital models. A ligand-field treatment of CoCl₄^2– ion describes the data well, but not completely, even if we allow for the occurrence of the diffuse cobalt-centred spin distribution. In terms of a simple molecular-orbital model a mixing parameter A_σ 0.15 ± 0.05 is deduced. The model is further qualitatively successful in accounting for the overlap spin population provided the ‘diffuse’ orbital is considered to contribute significantly to the metal–ligand overlap integrals. However, it is not possible to quantify this aspect nor to exclude spin-polarisation effects as a major contribution to the observed apparent overlap population of –0.13 spins. The total cobalt spin population is greater than 3.0 (3.18 ± 0.06) which is evidence for spin-polarisation effects. In terms of usual fractional spin-transfer coefficients, f, we have f_σ 6.2, f_π 1.1, and f₈ 0.3% for spin transferred to the bonded chlorine atoms.