Cationic Carbonyl Complexes of Rhodium(I) and Rhodium(III):  Syntheses, Vibrational Spectra, NMR Studies, and Molecular Structures of Tetrakis(carbonyl)rhodium(I) Heptachlorodialuminate and -gallate, [Rh(CO)4][Al2Cl7] and [Rh(CO)4][Ga2Cl7]

Dimeric rhodium(I) bis(carbonyl) chloride, [Rh(CO)2(μ-Cl)]2, is found to be a useful and convenient starting material for the syntheses of new cationic carbonyl complexes of both rhodium(I) and rhodium(III). Its reaction with the Lewis acids AlCl3 or GaCl3 produces in a CO atmosphere at room temperature the salts [Rh(CO)4][M2Cl7] (M = Al, Ga), which are characterized by Raman spectroscopy and single-crystal X-ray diffraction. Crystal data for [Rh(CO)4][Al2Cl7]: triclinic, space group P1̄ (No. 2); a = 9.705(3), b = 9.800(2), c = 10.268(2) Å; α = 76.52(2), β = 76.05(2), γ = 66.15(2)°; V = 856.7(5) Å3; Z = 2; T = 293 K; R1 [I > 2σ(I)] = 0.0524, wR2 = 0.1586. Crystal data for [Rh(CO)4][Ga2Cl7]: triclinic, space group P1̄ (No. 2); a = 9.649(1), b = 9.624(1), c = 10.133(1) Å; α = 77.38(1), β = 76.13(1), γ = 65.61(1)°; V = 824.4(2) Å3; Z = 2; T = 143 K; R1 [I > 2σ(I)] = 0.0358, wR2 = 0.0792. Structural parameters for the square planar cation [Rh(CO)4]+ are compared to those of isoelectronic [Pd(CO)4]2+ and of [Pt(CO)4]2+. Dissolution of [Rh(CO)2Cl]2 in HSO3F in a CO atmosphere allows formation of [Rh(CO)4]+ (solv). Oxidation of [Rh(CO)2Cl]2 by S2O6F2 in HSO3F results in the formation of ClOSO2F and two seemingly oligomeric Rh(III) carbonyl fluorosulfato intermediates, which are easily reduced by CO addition to [Rh(CO)4]+ (solv). Controlled oxidation of this solution with S2O6F2 produces fac-Rh(CO)3(SO3F)3 in about 95% yield. This Rh(III) complex can be reduced by CO at 25 °C in anhydrous HF to give [Rh(CO)4]+ (solv); addition of SbF5 at −40 °C to the resulting solution allows isolation of [Rh(CO)4][Sb2F11], which is found to have a highly symmetrical (D 4 h ) [Sb2F11]- anion. Oxidation of [Rh(CO)2Cl]2 in anhydrous HF by F2, followed in a second step by carbonylation in the presence of SbF5, is found to be a simple, straightforward route to pure [Rh(CO)5Cl][Sb2F11]2, which has previously been structurally characterized by us. All new complexes are characterized by vibrational and NMR spectroscopy. Assignment of the vibrational spectra and interpretation of the structural data are supported by DFT calculations.