Complex OsH6((PPr3)-Pr-i)(2) (1) deprotonates cytosine to give molecular hydrogen and the d(4)-trihydride derivative OsH3(cytosinate)((PPr3)-Pr-i)(2) (2), which in solution exists as a mixture of isomers containing kappa(2)-N1,O (2a) and kappa(2)-N3,O (2b) amino-oxo and kappa(2)-N3,N4 (2c) imino-oxo tautomers. The major isomer 2b associates with the minor one 2c through N-H center dot center dot center dot N and N-H center dot center dot center dot O hydrogen bonds to form [2b.2c](2) dimers, which crystallize from saturated pentane solutions of 2. Complex 1 is also able to perform the double deprotonation of cytosine (cytosinate') to afford the dinuclear derivative ((PPr3)-Pr-i)(2)H3Os(cytosinate')OsH3((PPr3)-Pr-i)(2) (3), where the anion is coordinated kappa(2)-N1,O and kappa(2)-N3,N4 to two different OsH3((PPr3)-Pr-i)(2) metal fragments. The deprotonation of deoxycytidine and cytidine leads to OsH3(deoxycytidinate)((PPr3)-Pr-i)(2) (4) and OsH3(cytidinate)((PPr3)-Pr-i)(2) (5), respectively, containing the anion kappa(2)-N3,N4 coordinated Dimer [2b.2c)(2) and dinuclear complex 3 have been characterized by X-ray. diffraction analysis.