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

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

There are nine electrons in the d-orbital of the Cu⁺² ion (d⁹ system). Therefore, the spectroscopic term for the ground state of the Cu⁺² ion is ²D. In the ground state, six electrons occupy the lower t_2g orbital and three electrons reside in the higher e_g orbital (i.e., t_2g⁶ e_g³), and a hole may be considered to exist in the e_g orbitals. Upon absorption of radiation, the electron gets excited to the e_g orbital from the t_2g orbital (i.e., t_2g⁵ e_g⁴), meaning a hole can now be considered in the t_2g orbital.

The hole in the t_2g orbitals may be placed in any of the three degenerate configurations. Therefore, the configuration is triply degenerate and the term corresponding to this state is ²T_2g. Thus, the energy state of ²E_g is lower than that of ²T_2g. In other words, the ²E_g state acts as the ground state and ²T_2g serves as the excited state. Therefore, there is a hole transition from ²E_g to ²T_2g which is analogous to an electron transition and can be represented as ²T_2g ← ²E_g. This sequence is inversely related to the typical order of individual orbital configurations.

Consequently, the absorption band of the [Cu(H₂O)₆]⁺² complex in the electronic spectrum is observed as a characteristically broad band centered near 12,500 cm⁻¹ due to substantial Jahn-Teller (J-T) distortion.

Because of this asymmetric J-T distortion, the ²E_g state splits into a lower-energy B_1g state and a higher-energy A_1g state, while the ²T_2g state splits into a lower B_2g state and an upper E_g state as illustrated below. Four distinct electronic transitions can theoretically occur from the B_1g base to the A_1g, B_2g, and E_g levels, along with a baseline ²T_2g ← ²E_g configuration transition that is very weak. As a result, three prominent absorption transitions overlap directly and resolve experimentally as a single broad band.

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