Low symmetry ligand field splittings in optical spectra of pseudotetrahedral Co(II) complexes with halogenide and chalcogenide ligands Original Research Article

Absorption spectra of A2CoX4 (A = Cs, ethyl4N; X = Cl, Br, I) and Na6CoL4 (L = S, Se) in the near infrared (2 000–7 000 cm−1) and in the visible region (14 000–28 000 cm−1) are recorded at low temperature (2 K). Some of the spectra are observed for the first time, for others band resolution has been obtained which allows identification of all Kramers components of 4T2(F),4T1(F) and 4T1(P) excited states which are split by spin–orbit coupling and due to lower symmetry in the crystal. Level assignments are obtained by comparison with the results from angular overlap model calculations which are performed for various sub-group symmetries. Toward higher energy, spin-forbidden bands of lower intensity belonging to transitions into doublet levels are considered as well. An optimal fit of model parameters to the experiment leads to valence angles in complex molecules anticipating corresponding low temperature data not available until now from crystal structure reports. The geometries obtained are in general more distorted from tetrahedral coordination than those reported for room temperature.