The compound which does not exist is

Oxide and peroxides of sodium are stable (${\mathrm{Na}}_2\mathrm{O}$ and ${\mathrm{Na}}_2{\mathrm{O}}_2$) $\mathrm{K}, \mathrm{Rb}, \mathrm{Cs}$ form superoxides. Beryllium forms a hydride with molecular formula ${\mathrm{BeH}}_2$Organic lead (tetraethyl lead; TEL) is used as an antiknock agent in gasoline and jet fuels. The compound ${\left({\mathrm{NH}}_4\right)}_2{\mathrm{BeF}}_4$ on thermal decomposition produces ${\mathrm{BeF}}_2$ and ${\mathrm{NH}}_4\mathrm{F}$.