Stable Chloro- and Bromoxenate Cage Anions; [X 3 (XeO 3 ) 3 ] 3- and [X 4 (XeO 3 ) 4 ] 4- (X = Cl or Br).

The number of isolable compounds which contain different noble-gas-element bonds is limited for xenon and even more so for krypton. Examples of Xe-Cl bonds are rare, and prior to this work, no Xe-Br bonded compound had been isolated in macroscopic quantities. The syntheses, isolation, and characterization of the first compounds to contain Xe-Br bonds and their chlorine analogues are described in the present work. The reactions of XeO3 with [N(CH3 )4 ]Br and [N(C2 H5 )4 ]Br have provided two bromoxenate salts, [N(C2 H5 )4 ]3 [Br3 (XeO3 )3 ] and [N(CH3 )4 ]4 [Br4 (XeO3 )4 ], in which the cage anions have Xe-Br bond lengths that range from 3.0838(3) to 3.3181(8) Å. The isostructural chloroxenate anions (Xe-Cl bond lengths, 2.9316(2) to 3.101(4) Å) were synthesized by analogy with their bromine analogues. The bromo- and chloroxenate salts are stable in the atmosphere at room temperature and were characterized in the solid state by Raman spectroscopy and low-temperature single-crystal X-ray diffraction, and in the gas phase by quantum-chemical calculations. They are the only known examples of cage anions that contain a noble-gas element. The Xe-Br and Xe-Cl bonds are very weakly covalent and can be viewed as σ-hole interactions, similar to those encountered in halogen bonding. However, the halogen atoms in these cases are valence electron lone pair donors, and the σ*Xe-O orbitals are lone pair acceptors.