Dichlorine heptoxide

Dichlorine heptoxide is the chemical compound with the formula Cl2O7. This chlorine oxide is the anhydride of perchloric acid. It is produced by the careful distillation of perchloric acid in the presence of the dehydrating agent phosphorus pentoxide:
 * 2 HClO4 +  P4O10   →  Cl2O7  +  H2P4O11

It slowly hydrolyzes back to perchloric acid, which is also hazardous when anhydrous.

Cl2O7 is an endothermic molecule, which means that it is intrinsically unstable.
 * Cl2O7 →  Cl2  +  3.5 O2  ΔH = 135 kJ/mol

Cl2O7 is bent with Cl-O-Cl angle of 118.6° giving the molecule C2 symmetry. The terminal Cl-O distances are 1.709 Å and the Cl=O distances are 1.405 Å. In this compound, chlorine exists in its highest formal oxidation state of 7+, although the bonding in this hypervalent molecule is significantly covalent.

Safety
Cl2O7 is strong oxidizer as well as an explosive that can be set off with flame or mechanical shock.