ADDITIVE AND NONADDITIVE LIGAND EFFECTS IN MIXED HALIDE CARBONYL NITRILE OSMIUM COMPLEXES

In the series [OsX6]3-, [OsX5(CO)]2-, trans-[OsX4(CO)2]1- (X = Br) and the series trans-[OsX4(MeCN)2]1-, trans-[OsX4(MeCN)(CO)]1-, trans-[OsX4(CO)2]1-, osmium IV/III and III/II redox couples shift two to three times further for the first introduction of CO than for the second, contrary to recent predictions; the corresponding halide to Os(III) charge-transfer spectra (X = Cl and Br) confirm the strongly non-linear accumulation of ligand electronic effects, in accord with preliminary X-ray data and near-IR crystal-field spectra.