Hexacarbonyldiplatinum(I). Synthesis, Spectroscopy, and Density Functional Calculation of the First Homoleptic, Dinuclear Platinum(I) Carbonyl Cation, [{Pt(CO)₃}₂]²⁺, Formed in Concentrated Sulfuric Acid

The dissolution of PtO 2 in concentrated H 2 SO 4 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 ( 13 C and 195 Pt), IR, Raman, and EXAFS spectroscopy. The structure of 1 is rigid on the NMR time scale at room temperature. NMR: δ ( 13 C A ) 166.3, δ ( 13 C B ) 158.7, δ ( 195 Pt) −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 ( ρ = 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 2 d symmetry. ν (CO) IR: 2174 ( E ), 2187 ( B 2 ), and 2218 cm - 1 ( B 2 ); ν (CO) Raman: 2173 ( E ), 2194 ( B 2 ), 2219 ( B 2 ), 2209 ( A 1 ) and 2233 cm - 1 ( A 1 ). EXAFS measurements show that the Pt−Pt bond is 2.718 Å and the mean length of the Pt−C bonds is 1.960 Å. 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° with respect to each other.