2 ClO 2 + 2 O 3 → 2 ClO 3 + 2 O 2 → Cl 2O 6 + 2 O 2
It was originally reported to exist as the monomeric chlorine trioxide ClO3 in gas phase, but was later shown to remain an oxygen-bridged dimer after evaporation and until thermal decomposition into chlorine perchlorate, Cl2O4, and oxygen. The compound ClO3 was then rediscovered.
It is a dark red fuming liquid at room temperature that crystallizes as a red ionic compound, chloryl perchlorate, [ClO 2]+ [ClO 4]− . The red color shows the presence of chloryl ions. Thus, chlorine's formal oxidation state in this compound remains a mixture of chlorine (V) and chlorine (VII) both in the gas phase and when condensed; however by breaking one oxygen-chlorine bond some electron density does shifts towards the chlorine (VII).
Cl2O6 is diamagnetic and is a very strong oxidizing agent. Although stable at room temperature, it explodes violently on contact with organic compounds and reacts with gold to produce the chloryl salt [ClO 2]+ [Au(ClO 4) 4]− . Many other reactions involving Cl2O6 reflect its ionic structure, [ClO 2]+ [ClO 4]− , including the following:
Nevertheless, it can also react as a source of the ClO3 radical:
2 AsF5 + Cl2O6 → 2 ClO3AsF5
^C. F. Goodeve, F. A. Todd (1933). "Chlorine Hexoxide and Chlorine Trioxide". Nature. 132 (3335): 514–515. doi:10.1038/132514b0.
^Lopez, Maria; Juan E. Sicre (1990). "Physicochemical properties of chlorine oxides. 1. Composition, ultraviolet spectrum, and kinetics of the thermolysis of gaseous dichlorine hexoxide". J. Phys. Chem. 94 (9): 3860–3863. doi:10.1021/j100372a094.