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

Absorption spectra of A(2)CoX(4) (A = Cs, ethyl(4)N; X = Cl, Br, I) and Na6 CoL4 (L = S, Se) in the near infrared (2 000-7 000 cm(-1)) and in the visib le region (14 000-28 000 cm(-1)) are recorded at low temperature (2 K). Som e of the spectra are observed for the first time, for others band resolutio n has been obtained which allows identification of all Kramers components o f T-4(2)(F),T-4(1)(F) and T-4(1)(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 calc ulations which are performed for Various sub-group symmetries. Toward highe r energy, spin-forbidden bands of lower intensity belonging to transitions into doublet levels are considered as well. An optimal fit of model paramet ers to the experiment leads to valence angles in complex molecules anticipa ting corresponding low temperature data not available until now from crysta l structure reports. The geometries obtained are in general more distorted from tetrahedral coordination than those reported for room temperature. (C) 1999 Elsevier Science B.V. All rights reserved.