A series of new water soluble and stable organometallic compounds is reported, and the peculiar role of water in their formation and stabilization is documented together with their catalytic properties in aqueous solution. The complex fac-Ru(OCOCF3)(2)(CO)(3)(H2O) (1) constitutes the entry to a new type of aqueous organometallic chemistry. The substitution of trifluoroacetato ligands by H2O yields [fac-RU(CO)(3)(H2O)(3)](2+) (2), isolated as the tetrafluoroborate derivative, the first structurally characterized complex bearing CO and H2O ligands only. In water, 2 undergoes nucleophilic attack by the solvent yielding [fac-Ru(COOH)(CO)(2)(H2O)(3)](+) (3), followed by CO2 elimination to give [fac-RuH(CO)(2)(H2O)(3)](+) (4), a ruthenium(II) hydride devoid of group-15-donor coligands and stable toward strong acids. 4 inserts ethene in water to give [fac-Ru(C2H5)(CO)(2)(H2O)(3)](+) (5), an exceptionally inert alkyl complex which inserts CO yielding [fac-Ru(C(O)C2H5)(CO)(2)(H2O)(3)](+) (6). Attempts to isolate the mononuclear cationic acyl complex gave the tetranuclear Ru-4(C(O)C2H5)(4)(OH)(2)(CF3SO3)(2)(CO)(8) (7) At 140 degrees C the mononuclear organometallic complexes become labile intermediates of a Reppe hydrocarboxylation of ethene in fully aqueous solvent.