In which of the following pairs the two species are not isostructural?

Solution:

 Except in the pair $({\mathrm{PF}}_{5 }\mathrm{and} {\mathrm{BrF}}_5 )$, the number of paired of electrons around the central atoms of the two compounds in each pair are identical. Hence, the compounds in each pair have identical structures. In  , $({\mathrm{PF}}_{5 }\mathrm{and} {\mathrm{BrF}}_5 )$the number of valence electrons are 40 and 42, respectively. These are distributed as

Five paired of electrons around$\mathrm{P}, {\mathrm{dsp}}^3$ hybridization of$\mathrm{P}$orbitals

Six paired of electrons around Br, sp 3d 2 hybridization of Br orbitals