The XeCl+ Ion: [XeCl]+ [Sb2 F11 ].

Chlorine-fluorine exchange in XeF+ leads to the orange crystalline salt [XeCl]+ [Sb2 F11 ]- , which is stable below -10°C. Thus, the number of known noble gas monohalogen cations is now three and, as calculations show, is approaching the theoretically possible limit. The synthesis of other such compounds-with the exception of ArF+ , which remains a formidable task-seems unlikely.