${\mathrm{XeF}}_6+\mathrm{MF}\to {\mathrm{M}}^{+}{\left[{\mathrm{XeF}}_7\right]}^{-}$. Here ' $\mathrm{M}$ ' is

         ${\mathrm{XeF}}_6+\mathrm{MF}\to {\mathrm{M}}^{+}{\left[{\mathrm{XeF}}_7\right]}^{-}$

• In this reaction ${\mathrm{XeF}}_6$ acts as a fluoride ion acceptor as it accepts a fluoride ion. And $\mathrm{MF}$ acts as a fluoride ion donor.
• The ${\mathrm{M}}^{+}$formed is a unipositive metal. And unipositive metals are formed by alkali metals. Thus, here ' $\mathrm{M}$ ' is alkali metals.