Theoretical study of the electronic structure of a tetragonal chromium(III) complex

The low-temperature single-crystal polarized absorption and the luminescence spectra of Cs2[CrCl2(H2O)4]Cl3 are theoretically analyzed in order to determine the electronic structure of the trans-[CrCl2(H2O)4]+ complex. This study, based on the Racah theory, leads to a good agreement between the theoretical and experimental energy levels. The electronic-exited state 4T2g(Oh) is split into 4Eg and 4B2g components due to the lowering of the symmetry from Oh to D4h. The polarized absorption spectrum and the theoretical energies show that the 4Eg(D4h) state is lower in energy than the 4B2g(D4h) state. The resolved vibronic structure in the luminescence spectrum reveals a progression in a non-totally symmetric b1g mode, a manifestation of a Jahn–Teller effect in the emitting state 4T2g(Oh). Both Jahn–Teller and spin–orbit coupling in the orbitally degenerated 4Eg(D4h) state are necessary to account for the spectroscopic observations.