We report the synthesis of Ru(II) and Os(II) trans hydrido–hydroxo complexes by reacting the unsaturated amido complexes MH(NHCMe₂CMe₂NH₂)(PPh₃)₂ (M = Ru, Os) with stoichiometric amounts of water. Proton exchange is rapid at room temperature between the amine/water/hydroxide moieties which leads to signal averaging of the NMR properties which can be slowed at low temperature in order to see resonances of separate complexes. These compounds can also be cleanly converted back to their starting complexes by dehydration in the presence of 3 Å molecular sieves. X-ray crystal structures of these Ru(II) and Os(II) trans hydrido–hydroxo complexes reveal that the unit cell contains an additional molecule of water trapped in the crystal lattice which hydrogen bonds with a neighbouring hydroxo ligand, forming a water bridged dimer in the solid state. Although there are many cases of oxidative addition of water to transition metal complexes, relatively few cases are well characterized where water addition occurs via metal–ligand cooperation (bifunctional addition) without altering the oxidation state of the metal center.