The pair(s) of co-ordination complexes/ions exhibiting the same kind of isomerism is/are:
(A) \([Cr(NH_3)_5Cl]Cl_2\) and \([Cr(NH_3)_4Cl_2]Cl\)
(B) \([Co(NH_3)_4Cl_2]^+\) and \([Pt(NH_3)_2(H_2O)Cl]^+\)
(C) \([CoBr_2Cl_2]^{2-}\) and \([PtBr_2Cl_2]^{2-}\)
(D) \([Pt(NH_3)_3(NO_3)]Cl\) and \([Pt(NH_3)_3Cl]Br\)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (B) and (D) only
Isomers of complexes may be stereoisomers (geometrical, optical) or structural isomers (ionisation, linkage, coordination). Two complexes show the same kind of isomerism only if both show it.
(A) [Cr(NH3)5Cl]Cl2 and [Cr(NH3)4Cl2]Cl — Not the same kind. The first (MA5B) shows no geometrical isomerism, and swapping \( \ce{Cl-} \) for \( \ce{Cl-} \) gives no ionisation isomer. The second (MA4B2) shows cis-trans isomerism.
(B) [Co(NH3)4Cl2]+ and [Pt(NH3)2(H2O)Cl]+ — Same kind. The octahedral MA4B2 ion and the square planar MA2BC ion both show geometrical (cis-trans) isomerism.
(C) [CoBr2Cl2]2− and [PtBr2Cl2]2− — Not the same kind. The Co(II) ion is tetrahedral and shows no geometrical isomerism; the Pt(II) ion is square planar and shows cis-trans isomerism.
(D) [Pt(NH3)3(NO3)]Cl and [Pt(NH3)3Cl]Br — Same kind. In each, the counter ion can change places with a ligand: \( \ce{[Pt(NH3)3Cl]NO3} \) and \( \ce{[Pt(NH3)3Br]Cl} \) are their ionisation isomers.
NCERT (2026-27 edition):
Ionisation Isomerism – “This form of isomerism arises when the counter ion in a complex salt is itself a potential ligand and can displace a ligand which can then become the counter ion. An example is provided by the ionisation isomers [Co(NH3)5(SO4)]Br and [Co(NH3)5Br]SO4.”