The structure of ${\mathrm{Cl}}_2{\mathrm{O}}_7$ is ${\mathrm{O}}_3\mathrm{Cl}-\mathrm{O}-{\mathrm{ClO}}_3$. The $\mathrm{Cl}-\mathrm{O}-\mathrm{Cl}$ bond angle is :

Due to strict crowding and also due to back-bonding, the $\mathrm{Cl}-\mathrm{O}-\mathrm{Cl}$ bond angle In ${\mathrm{O}}_3\mathrm{Cl}-\mathrm{O}-{\mathrm{ClO}}_3$ is ${118}^{\circ }{37}^{\mathrm{\prime }}$ .