Biological systems usually contain cysteine, glutathione or other sulfur-containing biomolecules. These S-nucleophiles were found to affect drastically the photolysis pathway generating products completely different from that of the neat cluster, which produces and NO and . The effect is interpreted in terms of formation of a pseudo-cubane adduct, , whose existence in equilibrium with the parent complex has no detectable influence on the spectral properties, whereas shifts the redox potential and induces photoconversion leading to the species and . Characteristic bond lengths, bond angles and atomic Mulliken charges were calculated using semi-empirical quantum chemical methods for the RBS anion and a series of pseudo-cubane complexes with S-donor or N-donor ligands. The results justify the hypothesis of the adduct formation and show that only in case of S-ligands the higher contribution of the components in adduct than in RBS is observed, which on excitation can undergo heterolytic cleavage yielding and , converted rapidly into . These results are crucial in understanding the physiological activity of RBS. Fe(III) formation can be detected only when the S-ligand enables formation of a stable compound; the effect was recorded in the presence of sulfide, thioglycolate, 2-mercaptopropionate, mercaptosuccinate, penicillamine, 2,3-dimercaptosuccinate, 2,3-dimercaptopropanol, and thiocyanate. For all these S-ligands the photoproducts were identified and characterised. In the case of other thiolates, their excess results in fast reduction of to , whereas can be still detected. Quantum yields of formation in the presence of the S-ligands are considerably higher than that of the photoproduction from neat .