Ligand Field Theory and the Fascination of Colours: Oxidic Iron(III) Solids as the Omnipresent Examples

The treatment of the high-spin d5-configurated iron(III) cation in 6- and 4-coordinate ligand fields is a highly complex matter. The 6A1-ground state allows only spin-forbidden transitions, here of relevance to ten spin-reduced 4A1-, 4A2-, 4E(2x)-, 4T1(3x)-, 4T2(3x)-states, which are not easy to handle. Though the literature offers a series of carefully prepared solids with beautifully resolved ligand field spectra, the philosophy of utilising these in terms of their binding character, particularly in respect to the d-electron cloud density between cation and the anions, diverges. Accordingly, the magnitudes of reported ligand field parameters Δ and of the Racah parameters of interelectronic repulsion B and C, which parameterise the mentioned effects, differ, and comparisons become difficult. In this review we propose a well-founded and comprehensible calculational procedure, in order to, as the main matter of concern, convince the readers that the ligand field spectra also sensibly reflect finer perturbational details of local or even cooperative binding quality. The origin of the latter effects is from the chemical environment beyond the first coordination sphere of a central, say FeIIIL n -complex (n = 6,4) in an extended solid. Already subtle disturbances of this kind will modify the shade of colour. An essential point of the discussion is the symmetry analysis, which provides rigorous constraints and sets strict conditions on what can be experimentally observed. We will first discuss manganese(II) in oxidic solids. Because a divalent cation is associated with rather ionic binding properties towards oxygen, crystal field theory is the appropriate analytical instrument. Iron(III) provides a situation, which requires more sophistication and a refinement of the theory by taking also bond covalency into account. A symmetry-based reinterpretation of the additional absorptions in the d–d-spectra of FeIII and CrIII in corundum-type solids is presented. This treatment sheds new light on the finer roots of the impressive red-to-green colour change of Cr3+ in mixed crystals Al2−x Cr x O3 with increasing x. Particular examples are discussed, where absorptions due to octahedral and tetrahedral iron(III) overlap in the ligand field spectra of spinel- and garnet-type solids, which model the hue in a predictable way. Finally, though not directly related to the primary topic, the charge-transfer properties of oxidic iron(III) are briefly examined. These absorptions often stray far into the visible region, with a very frequently significant influence on the apparent hue.