Color of coordination Compounds

Relate the colors of coordination compounds to their crystal field splitting energies and CFSE (Section 8.5, Problems 27-36). 

Bonding theories for coordination compounds should be able to account for structural features, colors, and magnetic properties. The earliest accepted theory was the valence bond theory (Chapter 8). It can account for structural and magnetic properties, but it offers no explanation for the wide range of colors of coordination compounds. The crystal field theory gives satisfactory explanations of color as well as of structure and magnetic properties for many coordination compounds. We will therefore discuss only this more successful theory in the remainder of this chapter. 

In this section, we will investigate the relationship between structure and isomerism. In the following sections, we will look into explanations of the paramagnetism and color of coordination compounds. 

FIGURE 24-18 Effect of ligands on the colors of coordination compounds... 

Reactions in which isomerization of coordination compounds occur in solutions are common, and some reactions of this type in solid complexes have been studied. Generally, there is a change in color of the complex as the crystal field environment of the metal ion changes. Accordingly, some of the color changes that occur when complexes are heated may indicate isomerization, but very few geometrical isomerization reactions in solid complexes have been studied in detail. One such reaction is... 

Weroer. in formulating his ideas about the structure of coordination compounds, had before hint facts such as the following. Four complexes of cobaltflll) chloride with ammonia had been discovered and named according to their colors ... 

Y" h Laurence Poncini and Franz L. >UI Wimmer, "Color Classification of Coordination Compounds/ J. 

The chemistry of cobalt involves mainly its +2 and +3 oxidation states, although compounds containing cobalt in the 0, +1, or +4 oxidation states are known. Aqueous solutions of cobalt(II) salts contain the Co(H20)62+ ion, which has a characteristic rose color. Cobalt forms a wide variety of coordination compounds, many of which will be discussed in later sections of this chapter. Some typical cobalt compounds are listed in Table 20.8. 

Crystal field theory was developed, in part, to explain the colors of transition-metal complexes. It was not completely successful, however. Its failure to predict trends in the optical absorption of a series of related compounds stimulated the development of ligand field and molecular orbital theories and their application in coordination chemistry. The colors of coordination complexes are due to the excitation of the d electrons from filled to empty d orbitals d-d transitions). In octahedral complexes, the electrons are excited from occupied t2g levels to empty Cg levels. The crystal field splitting Ao is measured directly from the optical absorption spectrum of the complex. The wavelength of the strongest absorption is called Amax and it is related to Ao as follows. E = hv, so Ao = hv = Because en-... 

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