The water-soluble complexes cis-[PtX 2(PTA) 2] (X = Cl, Br, I; PTA = 1,3,5-triaza-7-phosphaadamantane) were prepared by the reaction of the aminophosphine PTA with [MX 2(COD)] (COD = 1,5-cyclooctadiene). The complexes were found to be active catalysts in the intramolecular hydroamination reaction in water. This is the first reported study of this metal-mediated transformation in aqueous media. Complexes of the general formula cis-[PtX 2(PTA) 2] (X = Cl, Br, I; PTA = 1,3,5-triaza-7-phosphaadamantane) were produced by the reaction of the water-soluble phosphine PTA with [MX 2(COD)] (COD = 1,5-cyclooctadiene) under a nitrogen atmosphere. Compound 1 has previously been prepared by a different means and crystallographically characterized. Therefore, in order to fully compare 1-3, compounds 2 and 3 were studied both in solution and the solid state. Cis-[PtBr 2(PTA) 2] (2) crystallized in the monoclinic space group C2/c with a = 19.992(4) Å, b = 9.741(2) Å, c = 9.510(2) Å, and β = 104.11(1)°while its iodide analog 3 crystallized as the HCl salt, cis-[PtI 2(PTAH) 2]Cl 2, in the P2/n space group with a = 8.9043(1) Å, b = 10.1131(1) Å, c = 25.2173(1) Å, and β = 90.245(1)°. Compounds 1, 2, and 3 were active catalysts in the intramolecular hydroamination of 4-pentyn-1-amine in water. This is the first reported study of this metal-mediated transformation in aqueous media.