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] (ci s) and (trans), Cs-2[OsBr5Cl], and Cs-2[OsBr4Cl2] (cis) are measured from m icrocrystaIs 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 co upling limit band assignments to orbital transitions are obtained. The spec tra in the region up to 33 000 cm(-1) are explained by ligand to metal char ge-transfer transitions starting from metal-ligand pi-bonded orbitals t(1g) , t(1u), and t(2u) (and t(2g)). Corresponding sigma orbitals and intermixin g with metal-ligand pi orbitals could be neglected due to energy reasons. B and splittings and assignments to irreducible representations of low symmet ry point groups are obtained from parameter relations. Due to the large num ber of orbital interaction parameters which exceeds the number of electroni c transitions observable, numerical eigenvalue calculations from pertubatio n matrixes have not been carried out.