A series of half-sandwich Ru-cyclopentadienyl derivatives containing the cage-like water soluble monodentate phosphine 1,3,5-triaza-7-phosphaadamantane (PTA) was synthesized and tested in aqueous phase or biphasic homogeneous hydrogenation of activated olefins via hydrogen transfer or under hydrogen pressure at mild conditions. The hydrogen transfer results show higher activity for the Cp* derivatives (Cp* = C5Me5) than for the corresponding Cp complexes (CP = C5H5). Under H, pressure, all complexes show high selectivity to C=C double bond hydrogenation. The X-ray crystal structure featuring a protonated 2-aminoethylcyclopentadienyl Ru complex was also determined. (C) 2004 Elsevier B.V. All rights reserved.