Reactive p-block cations stabilized by weakly coordinating anions

The chemistry of the p-block elements is a huge playground for fundamental and applied work. With their bonding from electron deficient to hypercoordinate and formally hypervalent, the p-block elements represent an area to find terra incognita. Often, the formation of cations that contain p-block elements as central ingredient is desired, for example to make a compound more Lewis acidic for an application or simply to prove an idea. This review has collected the reactive p-block cations (rPBC) with a comprehensive focus on those that have been published since the year 2000, but including the milestones and key citations of earlier work. We include an overview on the weakly coordinating anions (WCAs) used to stabilize the rPBC and give an overview to WCA selection, ionization strategies for rPBC-formation and finally list the rPBC ordered in their respective group from 13 to 18. However, typical, often more organic ion classes that constitute for example ionic liquids (imidazolium, ammonium, etc.) were omitted, as were those that do not fulfill the - naturally subjective -"reactive"-criterion of the rPBC. As a rule, we only included rPBC with crystal structure and only rarely refer to important cations published without crystal structure. This collection is intended for those who are simply interested what has been done or what is possible, as well as those who seek advice on preparative issues, up to people having a certain application in mind, where the knowledge on the existence of a rPBC that might play a role as an intermediate or active center may be useful.

Trihalogenochalcogenonium-halogenometallates [EX3][MX n ] – Synthesis and crystal structures of [TeX3][MX6] (X=Cl, Br; M=W, Nb, Ta) and a suggestion for a structure designation code for the various structure types in this family of compounds

The reaction of Lewis acidic metal halides with chalcogen tetrahalides leads to double halides of the general formula [EX3][MX n ] (E=S, Se, Te; X=F, Cl, Br, I). α-[TeCl3][NbCl6] and α-[TeCl3][TaCl6] were obtained from Nb/Cl5/TaCl5 and TeCl4 in evacuated glass ampoules in a temperature gradient 423 → 403 K, [TeCl3][WCl6] was obtained from Te and WCl6 with WCl5 as by product. If the reaction of NbCl5/TaCl5, and TeCl4 was carried out in the presence of CCl4 a second polymorphic form β-[TeCl3][MCl6] (M=Nb, Ta) crystallized. [TeBr3][WBr6] and [TeBr3][TaBr6] could only be obtained by rapid cooling of the gasphase over solid WBr5/TaBr5–TeBr4 mixtures from 453 K to room temperature as orange colored crystals in low yields. The structures of the seven compounds have been determined by single crystal X-ray diffraction. All of them consist of trigonal-pyramidal [TeX3]+ and octahedral [MX6]− ions. The primary coordination sphere of the tellurium atom is completed by secondary Te–X interactions to a distorted TeX3+3 octahedron. The differences between the crystal structures lie in the different interconnections between cations and anions. [TeCl3][WCl6] and α-[TeCl3][MCl6] (M=Nb, Ta) are built of one-dimensional “ladder-type” connected polyhedra, the structures of β-[TeCl3][MCl6] (M=Nb, Ta) consist of discrete tetranuclear units and [TeBr3][MBr6] (M=W, Ta) are made up of face sharing double octahedral molecular units. Since there are several dozen representatives in the class of chalcogenonium-halogenometallates [EX3][MX n ] which are all known to contain trigonal-pyramidal [EX3]+ cations and tetrahedral, square-planar or octahedral anions a suggestion for a simplifying general structure designation code has been developed. The code gives the number of bridging halogen atoms of each anion, the number of connected cations and a ring index for the interconnection between the ions. The code is generally applicable and was assigned to more than 40 compounds of this class with the result that only a limited number of motifs of interconnections are realized.