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In this work, the reactivity of the nitrosyl complex [Ru(NO)(pybuS4)]Br towards different reducing agents is examined. The reaction with hydrazine hydrate in methanolic solution resulted in the formation of mono- and binuclear hydrazine complexes and one bimolecular, bridged hyponitrous acid complex. This hyponitrous acid complex was characterised by elemental analysis and by all spectroscopic means. It is the first complex of hyponitrous acid which exhibits the found structural motive. By variation of the reaction conditions, it was possible to gain this complex, as well as the mononuclear hydrazine complex, as the only reaction product. A mechanism which explains the found reactivity was postulated based on NMR spectroscopic analysis of the product distribution dependent on reaction conditions. Applying H3PO2 as non-nucleophilic reducing agent for reaction with the nitrosyl complex resulted in the formation of the mixed-valence hyponitrous acid complex. Prolonged reaction time resulted in the formation of a complex of the phosphorous acid tautomer P(OH)3 coordinated to the [Ru(pybuS4)] fragment. This complex was characterised structurally and is the first completely characterised, mononuclear P(OH)3 complex. Further reactivity studies showed that [Ru(NO)(pybuS4)] can be considered a model compound for the enzyme cytochrome c nitritereductase. Reactivity studies on the mononuclear hydrazine complex showed that the [Ru(pybuS4)] fragment is able to stabilise the labile diazene molecule. This is considered a prerequisite for nitrogenase model systems. The more electron rich [Ru(OMepybuS4)] fragment does not show a similar reactivity. It was found that the electron density of the complex fragment is raised but that this increase is not localised at the metal centre as expected but at the thiolate donor atoms of the ligand system. This unprecedented effect can give precious hints for the design of new potential nitrogenase model systems.