Octahedral complexes charge transfer absorption

As shown, for both octahedral and tetrahedral complexes, three absorption bands are expected. Many complexes do, in fact, have absorption spectra that show three bands. However, charge transfer absorption makes it impossible in some cases to see all three bands, so spectral analysis must often be based on only one or two observed bands. 

FIGURE 18.9 Interpretation of M—charge transfer absorption in an octahedral complex using a modified molecular orbital diagram.The transitions are from e or t2g orbitals on the metal to orbitals on the ligands. 

It is extremely common for coordination compounds also to exhibit strong charge-transfer absorptions, typically in the ultraviolet and/or visible portions of the spectrum. These absorptions may be much more intense than d-d transitions (which for octahedral complexes usually have e values of 20 L moF cm or less) molar absorp-tivities of 50,000 L mole cm or greater are not uncommon for these bands. Such absorption bands involve the transfer of electrons from molecular orbitals that are primarily ligand in character to orbitals that are primarily metal in character (or vice versa). For example, consider an octahedral d complex with cr-donor ligands. The ligand electron pairs are stabilized, as shown in Figure 11-15. 

---