This Review gives a comprehensive overview of the most topical weakly coordinating anions (WCAs) and contains information on WCA design, stability, and applications. As an update to the 2004 review, developments in common classes of WCA are included. Methods for the incorporation of WCAs into a given system are discussed and advice given on how to best choose a method for the introduction of a particular WCA. A series of starting materials for a large number of WCA precursors and references are tabulated as a useful resource when looking for procedures to prepare WCAs. Furthermore, a collection of scales that allow the performance of a WCA, or its underlying Lewis acid, to be judged is collated with some advice on how to use them. The examples chosen to illustrate WCA developments are taken from a broad selection of topics where WCAs play a role. In addition a section focusing on transition metal and catalysis applications as well as supporting electrolytes is also included.

An effective route to stable, almost-"naked" Cu(I) salts of weakly coordinating anions (WCAs) of the type [Al(OR(F))4]- has been developed. Born-Fajans-Haber cycles and theoretical calculations suggest that this methodology is useful for the generation of Cu(I) salts regardless of the larger WCA used. The first homoleptic Cu(I)-arene complex [Cu(1,2-F2C6H4)2](+)[Al{OC(CF3)3}4]- (1), the first Cu(I)-methylenechloride complex [Cu(CH2Cl2)Al{OC(CH3)(CF3)2}4] (2), and the donor-free dimer [CuAl{OCH(CF3)2}4]2 (3) were synthesized in quantitative yields by sonicating Li[Al(OR(F))4] (R(F)=C(CF3)3, C(CH3)(CF3)2, or CH(CF3)2), AgF, and a three-fold excess of CuI in 1,2-F2C6H4 (1) or CH2Cl2 (2, 3). Substances 1-3 are good starting materials for further Cu(I) chemistry, and the reaction of 1-3 with the weak Lewis base cyclooctasulfur gave the first Cu(I)-sulfur complexes of type [Cu(S12)(S8)](+)[Al{OC(CF3)3}4]- (4), [Cu(CH2Cl2)(S12)](+)[Al{OC(CF3)3}4]- (5), [A1Cu(1,5-eta1,eta1-S8)CuA1] (6; A1=[Al{OC(CH3)(CF3)2}4]-), and a Cu(I)-S8 1D coordination polymer with [Cu2(S8)2A(2)(2)] (7; A2=[Al{OCH(CF3)2}4]-), as a monomeric repeat unit. Complexes 4 and 5 are the first example of any metal coordinated to cyclo-S12 and 4 is the first example of a complex having an element in two allotropic modifications as a ligand.

We have unambiguously demonstrated the binding of the silver(I) ion in the central cavity of [7]-helicene, given that the counterion is weakly coordinating. In such a binding mode, the helicene is functioning as a chiral molecular tweezer.

The reaction of Cu(I), Ag(I), and Au(I) salts with carbon monoxide in the presence of weakly coordinating anions led to known and structurally unknown non-classical coinage metal carbonyl complexes [M(CO)n][A] (A = fluorinated alkoxy aluminates). The coinage metal carbonyl complexes [Cu(CO)n(CH2Cl2)m](+)[A](-) (n = 1, 3; m = 4-n), [Au2(CO)2Cl](+)[A](-), [(OC)nM(A)] (M = Cu: n = 2; Ag: n = 1, 2) as well as [(OC)3Cu⋅⋅⋅ClAl(OR(F))3] and [(OC)Au⋅⋅⋅ClAl(OR(F))3] were analyzed with X-ray diffraction and partially IR and Raman spectroscopy. In addition to these structures, crystallographic and spectroscopic evidence for the existence of the tetracarbonyl complex [Cu(CO)4](+)[Al(OR(F))4](-) (R(F) = C(CF3)3) is presented; its formation was analyzed with the help of theoretical investigations and Born-Fajans-Haber cycles. We discuss the limits of structure determinations by routine X-ray diffraction methods with respect to the C-O bond lengths and apply the experimental CO stretching frequencies for the prediction of bond lengths within the carbonyl ligand based on a correlation with calculated data. Moreover, we provide a simple explanation for the reported, partly confusing and scattered CO stretching frequencies of [Cu(I)(CO)n] units.

An einen Übersichtsartikel aus dem Jahre 2004 anknüpfend, wird hier über den strukturellen Aufbau, die Stabilität und die Anwendungen moderner schwach koordinierender Anionen (weakly coordinating anions, WCAs) berichtet, aber auch über Fortschritte der “klassischen” WCAs. In tabellarischer Form werden sowohl Ausgangsverbindungen zur Einführung einer Vielzahl von WCAs als auch deren physikochemische Eigenschaften zusammengefasst. Damit lässt sich für mögliche Anwendungen gut beurteilen, wie WCAs am besten in vorgegebene Systeme inkorporiert werden können. Die angeführten Anwendungsbeispiele für WCAs sind breit gestreut, eigene Abschnitte bilden aber die Schwerpunkte Übergangsmetallkomplexe, katalytische Anwendungen und Leitsalze für elektrochemische Anwendungen.

The simple metathesis reaction of the silver(I) weakly coordinating anion (WCA) salt Ag[Al(ORF)4] {RF = C(CF3)3} with gold(I) chloride and ethene led to the formation of the second isolable tris(ethene)gold(I) complex, the last missing entry in the triad [M(C2H4)3]+[Al(ORF)4]– (M = Cu, Ag, Au). The Au atom is coordinated by three ethene ligands in a distorted trigonal‐planar manner in a so‐called spoke‐wheel arrangement. The formation of the tris(ethene) complex instead of the more expected bis(ethene) complex was analyzed by theoretical investigations and by a Born–Fajans–Haber cycle for [Au(C2H4)3]+[Al(ORF)4]– as well as the only other known example [Au(C2H4)3]+[SbF6]–.

Ion-like ethylzinc(II) compounds with weakly coordinating aluminates [Al(OR(F))4](-) and [(R(F)O)3Al-F-Al(OR(F))3](-) (R(F)=C(CF3)3) were synthesized in a one-pot reaction and fully characterized by single-crystal X-ray diffraction, NMR and vibrational spectroscopy, and by quantum chemical calculations. The catalytic activity of ion-like Et-Zn[Al(OR(F))4] in intermolecular hydroamination and in the unusual double hydroamination of anilines and alkynes was investigated. Favorable performance was also found in comparison to the Et2Zn/[PhNMe2H](+)[B(C6F5)4](-) system generated in situ at lower catalyst loadings of 2.5 mol %.

Dedicated to Professor Dieter Fenske on the occasion of his 70th birthdayThe chemical properties of the homologues sulfur and selenium are closely related owing to their similar covalent radii and electronegativities. However, whereas sulfur is the element with the most allotropes and forms numerous cyclic and molecular modifications (S n ; n = 6-15, 18, 20), for selenium only three Se n homocycles are known (n = 6-8) and a Se 12 ring was observed as a co-crystal. [1][2][3] Such selenium and sulfur rings serve as neutral ligands in Ag + and Cu + complexes with weakly coordinating anions. Examples include {[AgSe 6 ] + } 1 , [4] [Ag 2 Se 6 ] 2+ , [5] [Ag(S 8 ) 2 ] + , [6] [Cu-(S 12 )(S 8 )] + , [Cu(S 12 )(CH 2 Cl 2 )] + , [7] and also the dicationic complex [Ag 2 Se 12 ] 2+ Angewandte Chemie 6009