Q.Which one of the following complexes is not expected to exhibit isomerism?

Structural (constitutional) isomers share one molecular formula but differ in how the atoms are actually connected -- four types are recognised. Linkage isomers arise from an ambidentate ligand (like NO₂⁻) bonding through a DIFFERENT donor atom in each isomer: [Co(NH₃)₅(NO₂)]²⁺ bound through nitrogen (nitro) versus through oxygen (nitrito/ONO) is the textbook pair.

Coordination isomers arise only when BOTH the cation and the anion of a salt are themselves complex ions, and ligands are interchanged between them: [Co(NH₃)₆][Cr(CN)₆] versus [Cr(NH₃)₆][Co(CN)₆].

Ionisation isomers arise when an ionisable counter ion can itself act as a ligand, and is swapped with a ligand already inside the coordination sphere -- [Co(NH₃)₅Cl]SO₄ (sulfate free, gives a precipitate with BaCl₂ but not AgNO₃) versus [Co(NH₃)₅SO₄]Cl (chloride free, the reverse reactivity); the two are told apart experimentally by exactly which precipitate test fires. …