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

Abstract

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 (13C and 195Pt), IR, Raman, and EXAFS spectroscopy. The structure of 1 is rigid on the NMR time scale at room temperature. NMR: δ(13CA) 166.3, δ(13CB) 158.7, δ(195Pt) −211.0 ppm; 1J(Pt−CA) = 1281.5 Hz, 1J(Pt−CB) = 1595.7 Hz, 1J(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 D2d symmetry. ν(CO)IR: 2174 (E), 2187 (B2), and 2218 cm-1 (B2); ν(CO)Raman: 2173 (E), 2194 (B2), 2219 (B2), 2209 (A1) and 2233 cm-1 (A1). 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.