 4 anion . These unstable salts easily disproportionate into xenon gas and perxenate salts , containing the XeO4 − 
 4 anion . These unstable salts easily disproportionate into xenon gas and perxenate salts , containing the XeO4 − 
4 anion . These unstable salts easily disproportionate into xenon gas and perxenate salts , containing the XeO4 −