The water-soluble phosphine complex of ruthenium(II), cis-RuCl2(PTA)(4) (3), has been prepared by the reduction of RuCl3 in ethanol in the presence of the air-stable phosphine 1,3,5-triaza-7-phosphaadamantane (2). Complex 3 is an effective catalyst for the regioselective conversion of unsaturated aldehydes to unsaturated alcohols using a biphasic aqueous/organic medium with sodium formate as the source of hydrogen, thus affording facile catalyst recovery and recycle. Both aromatic and aliphatic aldehydes were reduced to the corresponding alcohols. The formate ion was shown to directly be the hydrogen source by deuterium labeling experiments. The rate of hydrogenation of benzaldehyde was found to be first order in catalyst concentration, first order in substrate concentration (in the low-concentration regime), and independent of formate concentration at concentrations > 2.5 M. The reduction process was greatly retarded by the presence of excess phosphine ligand. The apparent activation energy determined was 23.9 kcal mol(-1). A catalytic cycle was proposed which involves the rapid formation of a ruthenium hydride formate complex followed by. phosphine dissociation and aldehyde addition in a slower step. Subsequent formation of a metal alkoxide intermediate and protonation completes the cycle. Recycling experiments demonstrated the catalyst to be quite robust. Complex 3 crystallized in the monoclinic system, space group P2(1)/n, with a = 11.399(7) Angstrom, b = 19.281(8) Angstrom, c = 15.068(7) Angstrom, beta = 110.85(4)degrees, V = 3095(3)Angstrom(3), and D-calcd = 1.718 g cm(-3) for Z = 4 from a water solution. Complex 4, RuCl2(PTA)(4).2HCl, crystallized in the orthorhombic space group Fdd2, with a = 23.403(5) Angstrom, b = 16.317(4) Angstrom, c = 19.588(5) Angstrom, V = 7480 Angstrom(3), and D-calcd = 1.679 g cm(-3) for Z = 8 from a dilute HCl solution of 3. In addition a small quantity of RuCl3(PTA)(2).2HCl (5) crystallized in the triclinic space group P ($) over bar 1, with a = 7.496(2) Angstrom, b = 8.028(2) Angstrom, c = 11.593(4) Angstrom, alpha = 72.73(3)degrees, beta = 72.98(2)degrees, gamma = 78.08(2)degrees, V = 631.5(3) Angstrom(3), and D-calcd = 1.753 g cm(-3) for Z = 1.