For polyatomic species to be considered isoelectronic in this context, they must have the same number of valence electrons. For them to be isostructural, they must possess the same geometry.

• $\text{XeF}_2$: The total number of valence electrons is $8 \text{ (from Xe)} + 2 \times 7 \text{ (from F)} = 22$. The central Xenon atom forms 2 bond pairs and has 3 lone pairs (steric number = 5, $sp^3d$ hybridisation). According to VSEPR theory, the 3 lone pairs occupy equatorial positions to minimise repulsion, resulting in a linear shape.
• $\text{IBr}_2^-$: The total number of valence electrons is $7 \text{ (from I)} + 2 \times 7 \text{ (from Br)} + 1 \text{ (from negative charge)} = 22$. Similar to $\text{XeF}_2$, the central Iodine atom forms 2 bond pairs and has 3 lone pairs (steric number = 5, $sp^3d$ hybridisation), which also gives it a linear shape. Because both species have 22 valence electrons and a linear structure, they are isoelectronic and isostructural.