When a solution of is kept for a period of months, it breaks down and crystals of are formed, which contain tetrahedral For most purposes, perrhenic acid and rhenium (VII) oxide are used interchangeably.
When a solution of Re 2 O 7 is kept for a period of months, it breaks down and crystals of HReO 4. H 2 O are formed, which contain tetrahedral ReO 4 − For most purposes, perrhenic acid and rhenium (VII) oxide are used interchangeably.