Synthesis, crystal structure and acidic properties in aqueous solution of phosphate ammine complexes of ruthenium nitrosyl

First examples of phosphate ammine complexes of ruthenium nitrosyl trans-[Ru(NO)(NH3)4(HxPO4)](NO3)x·(2 − x)H2O (x = 0, 1, 2) were obtained with good yields. Results of elemental analysis, X-ray, infrared, UV–Vis diffuse reflectance, and 31P NMR spectroscopies fit well with the formulas [Ru(NO)(NH3)4(PO4)]·2H2O (1), [Ru(NO)(NH3)4(HPO4)]NO3·H2O (2), and [Ru(NO)(NH3)4(H2PO4)](NO3)2 (3). In all the complexes, the phosphate group is in a trans-position to NO and is monodentate coordinated to ruthenium by one of the oxygen atoms. The pKa values 2.6 and 7.2 were calculated from potentiometric titration and 31P NMR for the reactions: trans-[Ru(NO)(NH3)4(H2PO4)]2+ + H2O ⇆ trans-[Ru(NO)(NH3)4(HPO4)]+ + H3O+ and trans-[Ru(NO)(NH3)4(HPO4)]+ + H2O ⇆ trans-[Ru(NO)(NH3)4(PO4)] + H3O+, respectively. The rate constant for the reaction trans-[Ru(NO)(NH3)4(H2O)]3+ + H2PO4− ⇆ trans-[Ru(NO)(NH3)4(H2PO4)]2+ + H2O was estimated as kobs = 7 × 10−8 s−1 at 70 °C in a dilute solution of H3PO4 (∼5 M).