Electronic Spectra of [Cu(H₂O)₆]⁺² Complex Ion

Absorption Spectra of [Cu(H₂O)₆]⁺² Complex Ion

There is nine electrons in d-orbital of Cu⁺² ion (d⁹ system). Therefore, the spectroscopic term for the ground state of Cu⁺² ion is ²D. In the ground state, six electrons occupy the lower t_2g orbital and three electrons in higher e_g orbital (i.e. T_2g⁶ e_g³) and a hole may be considered in e_g orbitals. on absorption of radiation, the electron gets excited to e_g orbital from t₂g orbital (i.e. T_2g⁵ e_g⁴) and now a hole may be considered in T_2g orbital.
The hole in the T_2g orbitals may be placed in any of the three orbitals. Therefore, the configuration is triply degenarate and the term corresponding to this configuration is ²T_2g. Thus, the energy of ²e_g is lower than that of ²T_2g. In other words, ²e_g state is ground state and ²T_2g is excited state. Therefore, There is a hole transition from ²E_g to ²T_2g which is similar to electron transition and can be represented as ²T_2g ← ²E_g. This is opposite of the order of energies of the orbitals (T_2g lower energy than E_g energy).

Therefore, absorption band of [Cu(H₂O)₆]⁺² complex in electronic spectra is observed as broad band at 12,500cm⁻ due to J-T-distortion.
Due to J-T distortion, ²E_2g state splits into B_1g lower energy and A_1g higher energy and ²T_2g state splits into B_2g lower energy and E_g higher energy as shown below. There are four electronic transition may occur from B_1g to A_1g, B_2g and E_g state and also from ²T_2g ← ²E_g but this transition is very poor. So, only three absorption bands overlap together and becomes a broad band.

Electronic Spectra of [Ti(H₂O)₆]⁺³ Complex Ion