Reaction of the platinum(III) dimeric complex [Pt-2(NH3)(4)((CH3)(3)CCONH)(2)(NO3)(2)](NO3)(2) (1), prepared in situ by the oxidation of the platinum blue complex [Pt-4(NH3)(8)((CH3)(3)CCONH)(4)](NO3)(5) (2) with Na2S2O8, with terminal alkynes CHdropCR (R = (CH2)(n)CH3 (n = 2-5), (CH2)(n)CH2OH (n = 0-2), CH2OCH3, and Ph), in water gave a series of ketonyl-Pt(III) dinuclear complexes [Pt-2(NH3)(4)((CH3)(3)CCONH)(2)(CH2-COR)](NO3)(3) (3, R = (CH2)(2)CH3; 4, R = (CH2)(3)CH3; 5, R = (CH2)(4)CH3; 6, R = (CH2)(5)CH3; 7, R = CH2OH; 8, R = CH2CH2OH; 9, R = (CH2)(2)CH2OH; 10, R = CH2OCH3; 11, R = Ph). Internal alkyne 2-butyne reacted with 1 to form the complex [Pt-2(NH3)(4)((CH3)(3)CCONH)(2)(CH(CH3)COCH3)](NO3)(3) (12). These reactions show that Pt(III) reacts with alkynes to give various ketonyl complexes, Coordination of the triple bond to the Pt(III) atom at the axial position, followed by nucleophilic attack of water and hydrogen shift from the enol to keto form, would be the mechanism. The structures of complexes 3.H2O, 7.0.5C(3)H(4)O, 9, 10, and 12 have been confirmed by X-ray diffraction analysis. A competitive reaction between equimolar 1-pentyne and 1-pentene toward 1 produced complex 3 and [Pt-2(NH3)(4)((CH3)(3)CCONH)(2)(CH2CH(OH)CH2CH2CH3)]-(NO3)(3) (14) at a molar ratio of 9:1, suggesting that alkyne is more reactive than alkene. The ketonyl-Pt(III) dinuclear complexes are susceptible to nucleophiles, such as amines, and the reactions with secondary and tertiary amines give the corresponding alpha-amino-substituted ketones and the reduced Pt(II) complex quantitatively. In the reactions with primary amines, the once formed alpha-amino-substituted ketones were further converted to the iminoketones and diimines. The nucleophilic attack at the ketonyl group of the Pt(III) complexes provides a convenient means for the preparation of alpha-aminoketones, alpha-iminoketones, and diinnines from the corresponding alkynes and amines.