Alkali Metal (Li+–Cs+) Salts with Hexafluorochromate(V), Hexafluorochromate(IV), Pentafluorochromate(IV), and Undecafluorodichromate(IV) Anions

Abstract

AbstractThe compounds ACrF6 (A = Li–Cs) were prepared by photochemical reactions of AF/CrF3 mixtures in anhydrous HF with elemental F2 at ambient temperature. The crystal structures of compounds ACrF6 (A = K–Cs) are analogous to that of KOsF6, and NaCrF6 exhibits polymorphism. The trigonal phase (II) can be classified to have the well‐known LiSbF6 type of structure, while the crystal structure of the orthorhombic modification (I) appears to be a new structure‐type. Thermal decomposition of the ACrF6 salts produce ACrF5(A = Rb, Cs), ACrF5/A2CrF6 (A = K), or A2CrF6 (A = Li, Na). These compounds undergo partial solvolysis in anhydrous HF with precipitation of CrF4. From the remaining solutions of the [CrF6]2– anions and dissolved AF (A = Li–Cs), single crystals of ACrF5 (A = K–Cs), A2CrF6·2HF (A = Na, K), A2CrF6·4HF (A = Rb, Cs), Li2CrF6, and K3Cr2F11·2HF were grown, and their crystal structures determined. The main structural feature of the ACrF5 compounds is the infinite zig‐zag [CrF5]nn– chain of distorted [CrF6] octahedra joined by cis vertices. The crystal structures of A2CrF6·2HF (A = Na, K) and A2CrF6·4HF (A = Rb, Cs) consist of distorted [CrF6]2– octahedra involved in moderate to strong hydrogen bonding with HF molecules, while two A+ cations compensate the negative charge of each octahedron. In Na2CrF6·2HF, two neighboring HF molecules are involved in moderate to strong hydrogen bonding with each other. (HF)2 dimers with a parallelogram structure are formed. The mutual interactions in the crystal structure of K2CrF6·2HF differ from those found in Na2CrF6·2HF. In the former, each HF molecule interacts with the [CrF6]2– anion and three K+ cations. A2CrF6·4HF compounds of Rb and Cs are isostructural. Their structures consist of A+ cations and [CrF6]2– anions involved in hydrogen bonding with two sets of HF molecules in the trans position. The crystal structure of K3Cr2F11·2HF reveals a rare case of the [M2F11]3– anion. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)