The Discrete AlF 5 2– Fluoroaluminate Anion in the Structure of [Tetraethylammonium] 2 [AlF 5 ](H 2 O) 2

The first crystallographically defined example of the discrete fluoroaluminate anion (AlF 5 2– ) was elucidated by analysis of single crystals of [Et 4 N] 2 [AlF 5 ](H 2 O) 2 (Et 4 N + = tetraethylammonium) by X‐ray crystallography. Intentional preparation of a compound containing the Al 2 F 9 3– anion by the reaction of two equivalents of [Et 4 N][AlF 4 ] with [Me 4 N]F (Me 4 N + = tetramethylammonium) resulted in the formation of [Et 4 N][Me 4 N][AlF 5 ] with unreacted [Et 4 N][AlF 4 ]. Cation exchange between these species in the presence of water furnished [Et 4 N] 2 [AlF 5 ](H 2 O) 2 as a precipitate containing the trigonal bipyramidal AlF 5 2– anion. The data from the present study allows supplementing the ligand close packing (LCP) model for the third period of the periodic table. The Raman spectrum of [Et 4 N] 2 [AlF 5 ](H 2 O) 2 was analyzed on the basis of quantum chemical calculations.