Isostructural compounds are those that have same structure as well as same hybridisation. (a) PF6−(6 bond pairs) and SF6 (6 bond pairs) are isostructural. (b) IO3−(4 pairs, i.e. 3 bond pair +1 lone pair) and XeO3 (4 pairs, i.e. 3 bond pairs +1 lone pair) are isostructural. (c) BH4−(4 bond pairs) and NH4+(4 bond pairs) are isostructural. (d) BrF5 ( 5 bond pairs) and XeF4 (4 bond pairs and 1 lone pair) are not isostructural.