Studies on the Reactions of Carbon Dioxide with Transition Metal Complexes. Preparation and Structure of μ-Carbonato-pentakis(triphenylphosphine)dirhodium-Benzene

The rhodium-hydrido complex RhH(PPh₃)₄ (Ph = C₆H₅) in toluene reacts with carbon dioxide at room temperature to give, on crystallization from benzene, red crystals of Rh₂(CO₃)(PPh₃)₅·C₆H₆, as characterized by diffraction methods. One mole of CO₂ per mole of the complex is evolved on thermolysis and acidolysis of the complex and by treatment with methyl iodide and triphenyl phosphite. The compound crystallizes in space group [formula omitted] with four formula units in the cell. The unit cell has dimensions of a = 22.518 (9) Å, b = 28.051 (12) Å, c = 12.841 (7) Å, α = 96.12 (3)°, β = 97.91 (3)°, γ = 92.66 (3)°, and V = 7972 ų. The observed density of 1.39 (1) g cm^-3 is in good agreement with that of 1.38 g cm^-3 calculated for four Rh₂(CO₃)(PPh₃)₅·C₆H₆ formula units per cell. There are thus two independent dimeric molecules in the cell. Refinement by full-matrix least-squares methods using 12330 observations for which F_o ² ≥ 3σ(F_o ²) yielded a conventional agreement index of 0.079. Each independent molecule of the compound was found to consist of two Rh atoms, both having square-planar coordination and linked by a planar carbonato ligand. Two oxygen atoms of the carbonate group are coordinated to one Rh atom while the remaining carbonato oxygen atom is bound to the second Rh atom. Triphenylphosphine ligands fill out the four-coordinate geometry of each Rh atom. The bidentate coordination of the carbonato group to the metal atom creates a four-membered ring. The average O-C-O angle involved in this ring is 115.9°, a slight decrease from the 120° value found in the free carbonate ion. The O-Rh-O angle of the four-membered ring averages 62.6°. No trend in the carbonato group C-O bond lengths is evident, the average value being 1.29 Å. The ³¹P[¹H] NMR spectrum of the complex in benzene is compatible with the unsymmetrical binuclear structure consisting of two rhodium(I) nuclei coordinated respectively with two and three triphenylphosphine ligands, as found in the solid state.