Transition-group complexes, forbidden transitions electronic energies

The value of this energy spacing between the groups of d-orbitals can be calculated (19B) from the spectra of the ions and the free atoms. In fact the visible spectra of the simple complexes of the transdtion metal ions is largely due to excitation of these d-electrons. The extinction coefficients are often very small since the transitions are forbidden by symmetry and electron pairing selection rules. 

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