Transition element complexes electronic absorption spectra

In lanthanide elements, the 5s and 5p shells are on the outside of the 4f shell. The 5s and 5p electrons are shielded, any force field (the crystal field or coordinating field in crystals or complexes) of the surrounding elements in complexes have little effect on the electrons in the 4f shell of the lanthanide elements. Therefore, the absorption spectra of lanthanide compounds are line-like spectra similar to those of free ions. This is different from the absorption spectra of d-block compounds. In d-block compounds, spectra originate from 3d 3d transitions. The nd shell is on the outside of the atoms so no shielding effect exists. Therefore, the 3d electrons are easily affected by crystal or coordinating fields. As a result, d-block elements show different absorption spectra in different compounds. Because of a shift in the spectrum line in the d-block, absorption spectra change from line spectra in free ions to band spectra in compounds. 

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