Identification of CT Bands
Charge-transfer complexes are identified by
- Color: The color of CT complexes is reflective of the relative energy balance resulting from the transfer of electronic charge from donor to acceptor.
- Solvatochromism: In solution, the transition energy and therefore the complex color varies with variation in solvent permittivity, indicating variations in shifts of electron density as a result of the transition. This distinguishes it from the π* ← π transitions on the ligand.
- Intensity: CT absorptions bands are intense and often lie in the ultraviolet or visible portion of the spectrum. For inorganic complexes, the typical molar absorptivities, ε, are about 50000 L mol−1 cm−1, that are three orders of magnitude higher than typical ε of 20 L mol−1 cm−1 or lower, for d-d transitions (transition from t2g to eg). This is because the CT transitions are spin-allowed and Laporte-allowed. However, d-d transitions are only spin-allowed; they are Laporte-forbidden.
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