Optical Properties and the Spectrochemical Series

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- 

Experimental measurements of the optical absorption spectra and magnetic properties of transition-metal complexes provide a critical test of the validity of crystal field theory. The theory makes specific predictions about the strengths of crystal fields produced by different ligands. Charged ligands such as the halides 

FIGURE 8.29 The colors of the hexa-aqua complexes of metal Ions (from left) Mn +, Fe +, Co +, NF+, Cu +, and Zn, prepared from their nitrate salts. Note that the Zn complex Is colorless. The green color of the Is due to absorption of both red and blue light that passes through the solution. The yellow color of the solution containing Fe(H20)6 Is caused by hydrolysis of that Ion to form Fe(0H)(H20)i If this reaction Is suppressed, the solution Is pale violet. 

TABLE 8.6 Absorption Wavelengths for Selected Octahedral Transition-Metal Complexes 

Octahedral Complexes Amax (nm) Octahedral Complexes Amax (nm) 

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