Absorption of light by inorganic complexes

The phasing of the molecular orbitals (shown as +/-) is a result of the wavefunctions describing the orbitals. + shows that the wavefunction is positive in a particular region in space and - shows that the wavefunction is negative. 

according to the symmetry selection rule, n — n transitions are allowed but n — ti transitions are forbidden. However, in practice the n — it transition is weakly allowed due to coupling of vibrational and electronic motions in the molecule (vibronic coupling). Vibronic coupling is a result of the breakdown of the Born-Oppenheimer approximation. 

The metal complexes of the d-block elements absorb light due to electronic transitions occurring between d-orbitals of the metal species, or because of charge transfer within the complex. 

In a free d-block atom, all five d-orbitals are degenerate (all five have the same energy) but this is not the case in d-block metal complexes. In the octahedral complex [Ti(H20)6]2+, the five d-orbitals on the titanium are split into two sets a triply-degenerate, lower-energy set (t2g) and a doubly-degenerate, higher-energy set (eg). 

As with organic compounds, the intensity of a transition is governed by selection rules. For inorganic complexes there are three selection rules to consider  

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