Several clusters Os-3(CO)(10)(alpha-diimine) (alpha-diimine = pyridine-2-carbaldehyde N-R-imine or 1,4-di-R-1,4-diazabutadiene) were synthesized and studied with respect to their spectroscopic and photochemical properties. According to the resonance Raman spectra the visible absorption band of these clusters belongs to electronic transitions having Os-to-a-diimine charge transfer (MLCT) character with a variable degree of pi-delocalization within the Os(alpha-diimine) moiety. Upon irradiation into these transitions zwitterions Os-(CO)(4)-Os(CO)(4)-Os+-(S)(CO)(2)(alpha-diimine) are formed in coordinating solvents (S) and biradicals *Os(CO)(4)-Os(CO)(4)-Os+(CO)(2)(alpha-diamine(.-)) in noncoordinating solvents and in THF at ambient temperature. The zwitterions live seconds in nitrile solvents and minutes in pyridine, and they largely regenerate the parent clusters. Quantum yields of zwitterion formation are wavelength independent and range from 10(-2) to 3 x 10(-2) with an activation energy varying from 440 to 720 cm(-1). For one of the clusters the quantum yield of zwitterion formation in pyridine was studied in dependence of applied pressure. The activation volume Delta V-double dagger = +7.0 +/- 0.5 cm(3) mol(-1) derived from these measurements indicates that the effect of bond cleavage is partially offset by coordination of the solvent. In apolar solvents biradicals are formed instead of zwitterions, which could be detected with nanosecond time-resolved absorption spectroscopy, while their adducts with nitrosodurene were observed with EPR spectroscopy. Their lifetimes vary from 5 ns to 1 mu s depending on the solvent and the alpha-diimine. The biradicals transform into zwitterions in the presence of a Lewis base. in addition, they produce with low efficiency an isomeric product in which the cr-diimine bridges between two Os atoms. The formation of very similar photoproducts (biradicals, cr-diimine-bridged isomeric products, charge-separated species) as in the case of binuclear metal-metal-bonded complexes such as (CO)(5)MnMn(CO)(3)(alpha-diimine) points to the occurrence of a primary photoprocess in which an Os-Os bond is broken homolytically. This reaction most likely occurs from a reactive (3) sigma pi* state after surface crossing from the unreactive but optically accessible MLCT states.