Silver-Ethene Complexes [Ag(η2-C2H4)n][Al(ORF)4] with n=1, 2, 3 (RF = Fluorine-Substituted Group)

Compounds including the free or coordinated gas-phase cations [Ag(eta(2)-C2H4)(n)](+) (n = 1-3) were stabilized with very weakly coordinating anions [A](-) (A = Al{OC(CH3)(CF3)(2)}(4), n = 1 (1); Al{OC(H)(CF3)(2)}(4), n = 2 (3); Al{OC(CF3)(3)}(4), n = 3 (5); {(F3C3)CO}(3)Al-F-Al{OC(CF3)(3)}(3), n = 3 (6)). They were prepared by reaction of the respective silver(I) salts with stoichiometric amounts of ethene in CH2Cl2 solution. As a reference we also prepared the isobutene complex [(Me2C=CH2)Ag(Al{OC(CH3)-(CF3)(2)}(4))] (2). The compounds were characterized by multinuclear solution-NMR, solid-state MAS-NMR, IR and Raman spectroscopy as well as by their single crystal X-ray structures. MAS-NMR spectroscopy shows that the [Ag(eta(2)-C2H4)(3)](+) cation in its [Al{OC(CF3)(3)}(4)](-) salt exhibits time-averaged D-3h-symmetry and freely rotates around its principal z-axis in the solid state. All routine X-ray structures (2 theta(max.) <55 degrees) converged within the 3 sigma limit at C=C double bond lengths that were shorter or similar to that of free ethene. In contrast, the respective Raman active C=C stretching modes indicated red-shifts of 38 to 45 cm(-1), suggesting a slight C=C bond elongation. This mismatch is owed to residual librational motion at 100 K, the temperature of the data collection, as well as the lack of high angular data owing to the anisotropic electron distribution in the ethene molecule. Therefore, a method for the extraction of the C=C distance in [M(C2H4)] complexes from experimental Raman data was developed and meaningful C=C distances were obtained. These spectroscopic C=C distances compare well to newly collected X-ray data obtained at high resolution (2 theta(max.) = 100 degrees) and low temperature (100 K). To complement the experimental data as well as to obtain further insight into bond formation, the complexes with up to three ligands were studied theoretically. The calculations were performed with DFT (BP86/TZVPP, PBE0/TZVPP), MP2/-FZVPP and partly CCSD(T)/AUG-cc-pVTZ methods. In most cases several isomers were considered. Additionally, [M(C2H4)31 (M=Cu+, Ag+, Au+, Ni-0, Pd-0, Pt-0, Na+) were investigated with AIM theory to substantiate the preference for a planar conformation and to estimate the importance of sigma donation and it back donation. Comparing the group 10 and 11 analogues, we find that the lack of pi back bonding in the group 11 cations is almost compensated by increased sigma donation.