The dissolution of PtO2 in concentrated H2SO4 under an atmosphere of CO results in the formation of hexacarbonyldiplatinum(I), [{Pt(CO)(3)}(2)](2+) (1), the first homoleptic, dinuclear, cationic platinum carbonyl complex, of which a prolonged evacuation leads to reversible disproportionation to give cis-[Pt(CO)(2)](2+)((solv)) (2) and Pt(0). 1 has been completely characterized by NMR (C-13 and Pt-195), IR, Raman, and EXAFS spectroscopy. The structure of 1 is rigid on the NMR time scale at room temperature. NMR: delta(C-13(A)) 166.3, delta(C-13(B)) 158.7, delta(Pt-195) -211.0 ppm; (1)J(Pt-C-A) = 1281.5 Hz, (1)J(Pt-C-B) 1595.7 Hz, (1)J(Pt-Pt') = 550.9 Hz. The strongly polarized, sharp Raman band at 165 cm(-1) (rho ca. 0.25) indicates the presence of a direct Pt-Pt bond. The IR and Raman spectra in the CO stretching region are entirely consistent with the presence of only terminal CO's on a nonbridged Pt-Pt bond with D-2d symmetry. v(CO)(IR): 2174 (E), 2187 (B-2), and 2218 cm(-1) (B-2); v(CO)Raman: 2173 (E), 2194 (B-2), 2219 (B-2)r 2209 (A(1)) and 2233 cm(-1) (A(1)). EXAFS measurements show that the Pt-Pt bond is 2.718 Angstrom and the mean length of the Pt-C bonds is 1.960 Angstrom. The geometric optimization for 1 by a density functional calculation at the B3LYP level of theory predicts that the dinuclear cation contains two essentially planar tricarbonyl platinum(I) units that are linked via a Pt-Pt bond about which they are twisted by exactly 90.0 degrees with respect to each other.