The salts [M(CO)(4)][Sb2F11](2), M = Pd, Pt, are prepared by reductive carbonylation of Pd[Pd(SO3F)(6)], Pt(SO3F)(4) or PtF6 in liquid SbF5, or HF-SbF5. The resulting moisture-sensitive, colorless solids are thermally stable up to 140 degreesC (M = Pd) or 200 degreesC(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)][Sb2F11](2) is smaller than that of [Pd(CO)(4)][Sb2F11](2) by about 0.4%, The cations [M(CO)(4)](2+), M = Pd, Pt, are square planar with only very slight angular and but-of-plane deviations from D-4h symmetry. The interatomic distances and bond angles for both cations are essentially identical. The [Sb2F11](-) anions in [M(CO)(4)][Sb2F11](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 Weals radii by 0.58 - 0.37 Angstrom. In addition, structural, and spectroscopic details of recently synthesized [Rh(CO)(4)][Al2Cl7] 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. C-13 NMR measurements complete the characterization of the square planar metal carbonyl cations. The extensive characterization of [M(CO)(4)][Sb2F11](2), M = Pd, Pt, reported here, allows a comparison to linear and octahedral [M(CO)(n)][Sb2F11](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 [Sb2F11](-) or [SbF6](-) as anions.