Which of the following complexes will show coordination isomerism?
A. $$[Ag(NH_3)_2][Ag(CN)_2]$$
B. $$[Co(NH_3)_6][Cr(CN)_6]$$
C. $$[Co(NH_3)_6][Co(CN)_6]$$
D. $$[Fe(NH_3)_6][Co(CN)_6]$$
E. $$[Co(NH_3)_6][Fe(CN)_6]$$
Choose the correct answer from the options given below :

In coordination isomerism both the cation as well as the anion must be complex ions and at least one ligand of the cationic complex must be able to exchange places with one ligand of the anionic complex to give a new pair of oppositely-charged complexes that differ from the original ones.

For such an interchange to give a new set of complexes the two metal centres must be different; otherwise the exchange merely reproduces the original species under a different name.

Let us examine the given salts one by one.

Case A: $$[Ag(NH_3)_2][Ag(CN)_2]$$

Both the cationic and anionic complexes contain the same metal $$Ag(I)$$ which is strictly linear (coordination number 2). If one attempts to interchange NH3 and CN−, the only possible product is the neutral molecule $$[Ag(NH_3)(CN)]$$, so after the exchange there would be no pair of oppositely charged complexes. Hence coordination isomerism is not possible.

Case B: $$[Co(NH_3)_6][Cr(CN)_6]$$

The cation has metal $$Co^{3+}$$ and the anion has metal $$Cr^{3+}$$, so the ligands can be swapped completely to give $$[Cr(NH_3)_6]^{3+}$$ and $$[Co(CN)_6]^{3-}$$. Both new species are genuine complex ions and constitute a coordination isomer of the original salt. Therefore this compound does show coordination isomerism.

Case C: $$[Co(NH_3)_6][Co(CN)_6]$$

Here both the cation and the anion contain the same metal ion $$Co^{3+}$$. Interchanging NH3 and CN− cannot give a different pair of oppositely charged complexes; it simply reproduces the original species (or names the same two ions in the opposite order). Hence no new salt is obtained and coordination isomerism is not shown.

Case D: $$[Fe(NH_3)_6][Co(CN)_6]$$

The cationic complex contains $$Fe^{3+}$$ while the anion contains $$Co^{3+}$$. Complete ligand exchange produces $$[Co(NH_3)_6]^{3+}$$ and $$[Fe(CN)_6]^{3-}$$, a brand-new pair of complex ions. Thus this compound shows coordination isomerism.

Case E: $$[Co(NH_3)_6][Fe(CN)_6]$$

Here again the metals in the two complex ions are different ($$Co^{3+}$$ and $$Fe^{3+}$$). Ligand interchange yields the new complex ions $$[Fe(NH_3)_6]^{3+}$$ and $$[Co(CN)_6]^{3-}$$. Therefore this salt does show coordination isomerism.

Summarising the above discussion:

• Case A : does not show
• Case B : shows
• Case C : does not show
• Case D : shows
• Case E : shows

Hence the complexes that exhibit coordination isomerism are B, D and E only.

Option B $$\longrightarrow\ $$B, D and E only