The salts [M(CO)(4)][Sb(2)F(11)](2), M = Pd, Pt, are prepared by reductive carbonylation of Pd[Pd(SO(3)F)(6)], Pt(SO(3)F)(4) or PtF(6) in liquid SbF(5), or HF-SbF(5). The resulting moisture-sensitive, colorless solids are thermally stable up to 140 degrees C (M = Pd) or 200 degrees C (M = Pt). Their thermal decompositions are studied by differential scanning calorimetry (DSC). Single crystals of both salts are suitable for an X-ray diffraction study at 180 K. Both isostructural salts crystallize in the monoclinic space group P2(1)/c (No. 14). The unit cell volume of [Pt(CO)(4)][Sb(2)F(11)](2) is smaller than that of [Pd(CO)(4)][Sb(2)F(11)](2) by about 0.4%. The cations [M(CO)(4)](2+), M = Pd, Pt, are square planar with only very slight angular and out-of-plane deviations from D(4)(h)() symmetry. The interatomic distances and bond angles for both cations are essentially identical. The [Sb(2)F(11)](-) anions in [M(CO)(4)][Sb(2)F(11)](2,) M = Pd, Pt, are not symmetry-related, and both pairs differ in their Sb-F-Sb bridge angles and their dihedral angles. There are in each salt four to five secondary interionic C- -F contacts per CO group. Of these, two contacts per CO group are significantly shorter than the sum of the van der Waals radii by 0.58 - 0.37 A. In addition, structural, and spectroscopic details of recently synthesized [Rh(CO)(4)][Al(2)Cl(7)] are reported. The cations [Rh(CO)(4)](+) and [M(CO)(4)](2+), M = Pd, Pt, are characterized by IR and Raman spectroscopy. Of the 16 vibrational modes (13 observable, 3 inactive) 10 (Pd, Pt) or 9 (Rh), respectively, are found experimentally. The vibrational assignments are supported by DFT calculations, which provide in addition to band positions also intensities of IR bands and Raman signals as well as internal force constants for the cations. (13)C NMR measurements complete the characterization of the square planar metal carbonyl cations. The extensive characterization of [M(CO)(4)][Sb(2)F(11)](2), M = Pd, Pt, reported here, allows a comparison to linear and octahedral [M(CO)(n)()][Sb(2)F(11)](2) salts [M = Hg (n = 2); Fe, Ru, Os (n = 6)] and their derivatives, which permit a deeper understanding of M-CO bonding in the solid state for superelectrophilic cations with [Sb(2)F(11)](-) or [SbF(6)](-) as anions.

Dimeric rhodium(I) bis(carbonyl) chloride, [Rh(CO)(2)(mu-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 AlCl(3) or GaCl(3) produces in a CO atmosphere at room temperature the salts [Rh(CO)(4)][M(2)Cl(7)] (M = Al, Ga), which are characterized by Raman spectroscopy and single-crystal X-ray diffraction. Crystal data for [Rh(CO)(4)][Al(2)Cl(7)]: triclinic, space group Ponemacr; (No. 2); a = 9.705(3), b = 9.800(2), c = 10.268(2) A; alpha = 76.52(2), beta = 76.05(2), gamma = 66.15(2) degrees; V = 856.7(5) A(3); Z = 2; T = 293 K; R(1) [I > 2sigma(I)] = 0.0524, wR(2) = 0.1586. Crystal data for [Rh(CO)(4)][Ga(2)Cl(7)]: triclinic, space group Ponemacr; (No. 2); a = 9.649(1), b = 9.624(1), c = 10.133(1) A; alpha = 77.38(1), beta = 76.13(1), gamma = 65.61(1) degrees; V = 824.4(2) A(3); Z = 2; T = 143 K; R(1) [I > 2sigma(I)] = 0.0358, wR(2) = 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)(2)Cl](2) in HSO(3)F in a CO atmosphere allows formation of [Rh(CO)(4)](+)((solv)). Oxidation of [Rh(CO)(2)Cl](2) by S(2)O(6)F(2) in HSO(3)F results in the formation of ClOSO(2)F 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 S(2)O(6)F(2) produces fac-Rh(CO)(3)(SO(3)F)(3) in about 95% yield. This Rh(III) complex can be reduced by CO at 25 degrees C in anhydrous HF to give [Rh(CO)(4)](+)((solv)); addition of SbF(5) at -40 degrees C to the resulting solution allows isolation of [Rh(CO)(4)][Sb(2)F(11)], which is found to have a highly symmetrical (D(4)(h)()) [Sb(2)F(11)](-) anion. Oxidation of [Rh(CO)(2)Cl](2) in anhydrous HF by F(2), followed in a second step by carbonylation in the presence of SbF(5), is found to be a simple, straightforward route to pure [Rh(CO)(5)Cl][Sb(2)F(11)](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.