Existence of Dichlorine Pentoxide
The crux of this (Russian) article is that $\ce{Cl2O5}$ is not stable enough to be isolated in a pure form, and if it were, it would be strongly acidic oxidizer.  

This web site is representative of several others that simply give example reactions of the compound (the second reaction being the most common exampe):  

$$\ce{NiCl2 + 3O2 -&gt; NiO + Cl2O5}$$
$$\ce{H2O + Cl2O5 -&gt; 2HClO3}$$  

The statement in the first article and the simple fact that there is such a dearth of information about this compound suggest that it really only exists as a transient, unstable, oxidizing acid that apparently has not been isolated as a stable compound.  Furthermore, information can be found in Wikipedia and/or PubChem for $\ce{Cl2O_x}$, where $x \in 1, 2, 3, 4, 6, 7$.