From weakly coordinating to non-coordinating anions? A simple preparation of the silver salt of the least coordinating anion and its application to determine the ground state structure of the Ag(η2-P4)2+ cation

The unexpected but facile preparation of the silver salt of the least coordinating [(RO)(3)Al-FA-I(OR)(3)](-) anion (R=C(CF3)(3)) by reaction of Ag[Al(OR)(4)] with one equivalent of PCl3 is described. The mechanism of the formation of Ag[(RO)(3)Al-F-Al(OR)(3)] is explained based on the available experimental data as well as on quantum chemical calculations with the inclusion of entropy and COSMO solvation enthalpies. The crystal structures of (RO)(3)Al<--OC4H8, Cs+ [(RO)(2)(Me)Al-F-Al(Me)(OR)(2)](-), Ag(CH2Cl2)(3)(+)[(RO)(3)Al-F-Al(OR)(3)] and Ag(eta(2)-P-4)(2)(+)[(RO)(3)Al-F-Al(OR)(3)](-) are described. From the collected data it will be shown that the [(RO)(3)Al-F-Al(OR)(3)](-) anion is the least coordinating anion currently known. With respect to the fluoride ion affinity of two parent Lewis acids Al(OR)(3) of 685 kJ mol(-1), the ligand affinity (441 kJ mol(-1)), the proton and copper decomposition reactions (-983 and -297 kJ mol(-1)) as well as HOMO level and HOMO-LUMO gap and in comparison with [Sb4F21](-), [Sb(OTeF5)(6)](-), [Al(OR)(4)](-) as well as [B(R-F)(4)](-) (R-F=CF3 or C6F5) the [(RO)(3)Al-F-Al(OR)(3)](-) anion is among the best weakly coordinating anions (WCAs) according to each value. In contrast to most of the other cited anions, the [(RO)(3)Al-F-Al(OR)(3)] anion is available by a simple preparation in conventional inorganic laboratories. The least coordinating character of this anion was employed to clarify the question of the ground state geometry of the Ag(eta(2)-P-4)(2)(+) cation (D-2h, D-2 or D-2d?). In agreement with computational data and NMR spectra it could be shown that the rotation along the Ag-(P-P-centroid) vector has no barrier and that the structure adopted in the solid state depends on packing effects which lead to an almost D2h symmetric Ag(eta(2)-P-4)(2)(+) cation (0 to 10.6degrees torsion) for the more symmetrical [AI(OR),]anion, but to a D, symmetric Ag(eta(2)-P-4)(2)(+) cation with a 44degrees twist angle of the two AgP2 planes for the less symmetrical [(RO)(3)Al-F-Al(OR)(3)](-) anion. This implies that silver back bonding, suggested by quantum chemical population analyses to be of importance, is only weak.