The unexpected but facile prepn. of the Ag salt of the least coordinating [(RO)3Al-F-Al(OR)3]- anion (R = C(CF3)3) by reaction of Ag[Al(OR)4] with one equiv. of PCl3 is described. The mechanism of the formation of Ag[(RO)3Al-F-Al(OR)3] is explained based on the available exptl. data as well as on quantum chem. calcns. with the inclusion of entropy and COSMO solvation enthalpies. The crystal structures of (RO)3Al(OC4H8), Cs[(RO)2(Me)AlFAl(Me)(OR)2], Ag(CH2Cl2)3[(RO)3AlFAl(OR)3] and Ag(h2-P4)2[(RO)3AlFAl(OR)3]- are described. From the collected data the [(RO)3AlFAl(OR)3]- anion is the least coordinating anion currently known. With respect to the F- 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 Cu decompn. 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(RF)4]- (RF = CF3 or C6F5) the [(RO)3Al-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)3AlFAl(OR)3] anion is available by a simple prepn. in conventional inorg. labs. The least coordinating character of this anion was employed to clarify the question of the ground state geometry of the Ag(h2-P4)2+ cation (D2h, D2 or D2d). 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 sym. Ag(h2-P4)2+ cation (0 to 10.6 Deg torsion) for the more sym. [Al(OR)4]- anion, but to a D2 sym. Ag(h2-P4)2+ cation with a 44 Deg twist angle of the two AgP2 planes for the less sym. [(RO)3AlFAl(OR)3]- anion. This implies that Ag back bonding, suggested by quantum chem. population analyses to be of importance, is only weak. [on SciFinder (R)]