Origin of the Anomalous Color of Egyptian and Han Blue Historical Pigments: Going beyond the Complex Approximation in Ligand Field Theory

• Published in: Journal of chemical education 2016-01, Vol.93 (1), p.111-117
• Main Authors: García-Fernández, Pablo, Moreno, Miguel, Aramburu, José Antonio
• Format: Article
• Language: English
• Subjects: Approximation; Beryl; Chemical compounds; Chemistry; Chromophores; Cobalt oxides; College Science; Color; Complexity; Coordination compounds; Crystal structure; Crystallinity; Electric fields; Field theory; Graduate Study; Ligands; Mathematical analysis; Metal compounds; Misconceptions; Optical properties; Optics; Perovskites; Pigments; Ruby; Science Instruction; Scientific Concepts; Spectroscopy; Theory; Transition metal compounds; Undergraduate Study
• ISSN: 0021-9584; 1938-1328
• DOI: 10.1021/acs.jchemed.5b00288

The complex approximation is widely used in the framework of the Ligand Field Theory for explaining the optical properties of crystalline coordination compounds. Here, we show that there are essential features of these systems that cannot be understood with the usual approximation that only considers an isolated complex at the correct equilibrium geometry. We also show that a quantitative understanding of such optical transitions cannot, in general, be reached unless the internal electric field, E R(r), created by the whole crystal on active electrons confined in the complex, is also taken into consideration. Seeking to prove the key role played by this internal field, usually ignored in crystalline transition metal compounds, we focus on the origin of the color displayed by the Egyptian Blue pigment (CaCuSi4O10), the first ever synthesized by humans. This pigment, together with Han Blue (BaCuSi4O10), are chosen as model systems because the anisotropic E R(r) field produces huge shifts, up to ∼0.9 eV, in their d–d transitions, which are unusual compared to the majority of compounds containing the same square-planar CuO4 6– chromophore. The relevance of the internal field for explaining phenomena such as the distinct color of ruby and emerald or the optical spectrum of CuF6 4– complexes in layered perovskites is also emphasized.