Superelectrophilic tetrakis(carbonyl)palladium(II)- and -platinum(II) undecafluorodiantimonate(V), [Pd(CO)(4)][Sb2F11](2) and [Pt(CO)(4)][Sb2F11](2):Syntheses, physical and spectroscopic properties, their crystal, molecular, and extended structures, and density functional calculation: An experimental, computational, and comparative study

The salts [M(CO)(4)][Sb2F11](2), M = Pd, Pt, are prepared by reductive carb onylation of Pd[Pd(SO3F)(6)], Pt(SO3F)(4) or PtF6 in liquid SbF5, or HF-SbF 5. The resulting moisture-sensitive, colorless solids are thermally stable up to 140 degreesC (M = Pd) or 200 degreesC(M = Pt). Their thermal decompos itions are studied by differential scanning calorimetry (DSC). Single cryst als of both salts are suitable for an X-ray diffraction study at 180 K. Bot h 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 devi ations from D-4h symmetry. The interatomic distances and bond angles for bo th 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 sa lt 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 va n der Weals radii by 0.58 - 0.37 Angstrom. In addition, structural, and spe ctroscopic details of recently synthesized [Rh(CO)(4)][Al2Cl7] are reported . The cations [Rh(CO)(4)](+) and [M(CO)(4)](2+), M = Pd, Pt, are characteri zed by IR and Raman spectroscopy. Of the 16 vibrational modes (13 observabl e, 3 inactive) 10 (Pd, Pt) or 9 (Rh), respectively, are found experimentall y. The vibrational assignments are supported by DFT calculations, which pro vide in addition to band positions also intensities of IR bands and Raman s ignals as well as internal force constants for the cations. C-13 NMR measur ements complete the characterization of the square planar metal carbonyl ca tions. The extensive characterization of [M(CO)(4)][Sb2F11](2), M = Pd, Pt, reported here, allows a comparison to linear and octahedral [M(CO)(n)][Sb2 F11](2) salts [M = Hg (n = 2); Fe, Ru, Os (n = 6)] and their derivatives, w hich permit a deeper understanding of M-CO bonding in the solid state for s uperelectrophilic cations with [Sb2F11](-) or [SbF6](-) as anions.