${\mathrm{C}}_6{\mathrm{H}}_5{ \mathrm{N}}_2{{}^{+}\mathrm{Cl}}^{-}+{\mathrm{HBF}}_4\to {\mathrm{C}}_6{\mathrm{H}}_5{ \mathrm{N}}_2{{}^{+}\mathrm{BF}_4}^{-}$

$C_6{H}_5{ N}_2{}^{+}BF_4{ }^{-}$ is known as benzene diazonium fluoro borate.

${\mathrm{C}}_6{\mathrm{H}}_5{ \mathrm{N}}_2{}^{+}\mathrm{BF}_4{}^{-}\stackrel{\mathrm{\Delta }}{\to }{\mathrm{C}}_6{\mathrm{H}}_5 \mathrm{F}+{\mathrm{N}}_2+{\mathrm{BF}}_3$

So, on heating it yields fluorobenzene.