Covalent bonding and spin density in cis-[Fe(bipy)2Cl2][FeCl4](bipy = 2,2′-bipyridyl) studied by polarised neutron diffraction

Abstract

Polarised neutron-diffraction experiments on the Pccn polymorph of deuteriated cis-[Fe^III-(bipy)₂Cl₂][Fe^IIICl₄](bipy = 2,2′-bipyridyl) gave 402 unique magnetic structure factors collected with magnetic fields almost parallel to the c and b crystal directions. These data were analysed using a valence-orbital model involving anisotropic spin populations on all non-hydrogen atoms. The model refined to R′= 0.052 and χ= 1.56. Most of the spin resides on the iron atoms. In the cation the populations are Fe 3d 4.38(15), 4p–0.60(15), in the anion Fe 3d 4.13(15), 4p 0.00(17). Approximately 0.23 spins reside on each chlorine atom and 0.34 on each bipyridyl molecule of the cation. In the [FeCl₄]^– anion the division between 3d t₂ and 3d e populations on the iron(III) atom, 3p_σ and 3p_π populations on the chlorine atoms, and overlap density in the Fe–Cl bonds show the spin delocalised onto the chlorine atoms via both e* and t₂* molecular orbitals with approximately equal amounts of σ and π spin transfer. There is significant 4p participation in the t₂* molecular orbital. In the [Fe(bipy)₂Cl₂]⁺ cation the Fe–Cl bonding is similar to that in the anion. The bidentate 2,2′-bipyridyl ligand has much more spin in the ring bonded by the shorter Fe–N bond, 0.25(3), than on the other ring, 0.09(3), with σ donation into 3d_σ and π back-donation from the iron 3d t_2g orbitals. The distribution agrees qualitatively, but not quantitatively, with the charge-density results and theoretical predictions. Comparison with charge-density results shows that besides covalent spin transfer there are spin-polarisation effects, arising from electron–electron correlation, of similar size. These effects complicate simple molecular orbital descriptions of the bonding in both cation and anion of the complex, but agree with theoretical conclusions developed for the [CoCl₄]^2– ion.