The Correlation between Structure and Color of Iron Oxide‐type Solids, Sustainable Pigments with Gentle Hues

Oxidic solids with hexaccordinate iron(III) appear as colored species only, if charge-transfer bands are present in the visible region, which originate from transitions, occurring between low-lying mainly oxygen-centered electronic states and antibonding, predominantly metal-3d states. Transitions within the 3d-shell are expected to be only very weak in intensity, because they are not only parity- but also spinforbidden, and hence don't usually contribute signifycantly to the coloration. It is shown by various examples, how one may steer the hue by the aimed manipulation of certain structural and chemical parameters, which effectively alter the location and, in particular, the band-width of the charge transfer transitions. The spectral properties are compared with those of the isoelectronic hexaoxo-coordinate manganese(II). The subsequent discussion of the color properties of 3d6-configured iron(II) in oxidic ligand fields helps to analyze the spectral behavior of mixed-valence FeII/FeIII solids. The herein additionally observed intervalence bands may lend a specific blue color to the solid, as in the case of Prussion Blue, and possibly also when inspecting the deeply blue coloration of iron gallic ink – the extreme case of a long-range electronic delocalization being black Fe3O4. Iron is, in mostly oxidic solids, omnipresent on earth and lends characteristic red and yellowish hues with brownish tinges – occasionally even black colors – to the rocks, overwhelming impressions of color compositions, which – as in Armenia, for example – link nature with culture. It is furthermore known in its role in the process of respiration – as the cationic constituent of red haemoglobin – as well as to be the main component of steel, an essential in modern life. Iron and its oxidic solids are sustainable materials and become increasingly important.