trans-[Ru(PTA)(4)Cl-2] (trans-1), (PTA = 1, 3,5-tri aza-7-phosphatricyclo[3.3.1.1(3,7)]decane) has been isolated and structurally characterized by X-ray crystallography. The structure reveals ruthenium in a slightly distorted-octahedral environment bound to two axial chlorides and four equatorial PTA ligands. In organic solvents, trans-1 undergoes a relatively clean isomerization to cis-1. In aqueous environments, trans-1 undergoes a more complicated transformation involving isomerization, protonation, and ligand substitution affording cis-1 and a series of structurally related molecules. From these results, we conclude that the synthesis of [Ru(PTA)(4)Cl-2] (1) affords trans-1, not cis-1, as earlier reports suggest. The water-soluble hydride cis-[Ru(PTA)4H2] (2) has also been synthesized from the reaction of trans-[Ru(PTA)(4)Cl-2] with excess sodium formate. Compound 2 is stable in deoxygenated water and undergoes H/D exchange with D2O (t(1/12) approximate to 120 min, at 25 degrees C). The solid-state structures of both trans-1 and 2 are described.