Match List-I with List-II: (a) [Co(NH3)6][Cr(CN)6], (b) [Co(NH3)3(NO2)3], (c) [Cr(H2O)6]Cl3, (d) cis-[CrCl2(ox)2]3-…

🧠 Four Isomerism Fingerprints

Match-the-isomerism problems collapse to a fingerprint scan:

• Two complex ions in the same formula ($[\mathrm{ML_n}][\mathrm{M'L'_m}]$) → coordination isomerism (ligands swap between cation and anion).
• Ambidentate ligand present (NO₂, SCN, CN) → linkage isomerism.
• Water both inside and outside the bracket possible → solvate (hydrate) isomerism.
• No plane of symmetry, multiple chelates → optical isomerism.

🗺️ Tag Each Complex

(a) $[\mathrm{Co(NH_3)_6}][\mathrm{Cr(CN)_6}]$ — two complex ions, both have ligands they could in principle swap. Coordination → (iii).

(b) $[\mathrm{Co(NH_3)_3(NO_2)_3}]$ — contains ambidentate $\mathrm{NO_2^-}$. Linkage → (i).

(c) $[\mathrm{Cr(H_2O)_6}]\mathrm{Cl_3}$ — water can be inside or outside; e.g. $[\mathrm{Cr(H_2O)_5Cl}]\mathrm{Cl_2 \cdot H_2O}$ is a hydrate isomer. Solvate → (ii).

(d) cis-$[\mathrm{CrCl_2(ox)_2}]^{3-}$ — cis with two chelating oxalates lacks a mirror plane. Optical → (iv).

So a-iii, b-i, c-ii, d-iv → option (1).

⚡ The "What Word Triggers Which Isomerism" Cheat

| Trigger word | Isomerism | |---|---| | $\mathrm{NO_2^-}$, $\mathrm{SCN^-}$, $\mathrm{CN^-}$ | Linkage | | Bracket + bracket together | Coordination | | $\mathrm{H_2O}$ + halide swap possible | Solvate / Hydrate | | Two chelates, no mirror plane | Optical | | Cis vs trans | Geometrical |

⚠️ Trans vs Cis Optical

The trans isomer of $[\mathrm{CrCl_2(ox)_2}]^{3-}$ is not optically active — it has a mirror plane through the Cl–Cr–Cl axis. Only the cis form is chiral. Always check the cis/trans label before pinning optical isomerism.

$\boxed{\text{Answer: (1) a-iii, b-i, c-ii, d-iv}}$