Charge-transfer band splittings in electronic spectra of mixed ligand halogeno osmium(IV) complexes

The charge-transfer absorption spectra of Cs-2[OsCl5Br], Cs-2[OsCl4Br2] (cis) and (trans), Cs-2[OsBr5Cl], and Cs-2[OsBr4Cl2] (cis) are measured from microcrystaIs in KBr disks at 2 K and compared to corresponding Cs-2[OsCl6] and Cs-2[OsBr6] spectra recorded with the same technique. The observed band shifts and band splittings due to lower molecular symmetry are interpreted using perturbation methods on the basis of molecular orbital theory which considers interligand orbital interactions as most important. For complexes containing more bromide than chloride ligands, spin-orbit coupling on the ligands must be included. Since coupling conditions are close to the j-j coupling limit band assignments to orbital transitions are obtained. The spectra in the region up to 33 000 cm(-1) are explained by ligand to metal charge-transfer transitions starting from metal-ligand pi-bonded orbitals t(1g), t(1u), and t(2u) (and t(2g)). Corresponding sigma orbitals and intermixing with metal-ligand pi orbitals could be neglected due to energy reasons. Band splittings and assignments to irreducible representations of low symmetry point groups are obtained from parameter relations. Due to the large number of orbital interaction parameters which exceeds the number of electronic transitions observable, numerical eigenvalue calculations from pertubation matrixes have not been carried out.