Isomerism in Coordination Compounds 🤹♀️
Isomers share a formula but differ in how their atoms line up or point in space.
We meet two big families:
Stereoisomers – same bonds, new 3-D pose;
Structural isomers – bonds themselves swap around. :contentReference[oaicite:0]{index=0}
1 · Stereoisomerism 🎭
1.1 Geometrical (cis–trans) Isomerism
- Shows up in square-planar and octahedral scenes where ligands can sit side-by-side (cis) or opposite (trans). Example: \( \text{[Pt(NH}_3)_2\text{Cl}_2} \) gives cis- and trans-twins. :contentReference[oaicite:1]{index=1}
- Octahedral sets like \( \text{[Co(NH}_3)_4\text{Cl}_2]^+ \) and \( \text{[CoCl}_2(\text{en})_2] \) behave the same way. :contentReference[oaicite:2]{index=2}
- With three of each kind, the ligands can hug one face (fac) or stretch around the middle line (mer). Think of fac-[Co(NH3)3(NO2)3] vs. mer-… 🟡 🟢 🟠 :contentReference[oaicite:3]{index=3}
1.2 Optical Isomerism 🔦
Mirror-image (d ↔ l) pairs that never overlap, just like left and right hands. Common in octahedral complexes with chelating (bite-sized) ligands. Key stars: \( \text{[Co(en)}_3]^{3+} \) and the cis (only!) form of \( \text{[PtCl}_2(\text{en})_2]^{2+} \). They twist plane-polarised light right (d) or left (l) — pretty cool! 😎 :contentReference[oaicite:4]{index=4}
2 · Structural Isomerism 🏗️
2.1 Linkage Isomerism 🔗
Ambidentate ligands connect through different donor atoms. Classic scene: thiocyanate \( \text{SCN}^- \) latches via S (M–SCN) or N (M–NCS). Another memory hook: red \( \text{[Co(NH}_3)_5(ONO)]^{2+} \) vs. yellow \( \text{[Co(NH}_3)_5(NO_2)]^{2+} \). :contentReference[oaicite:5]{index=5}
2.2 Coordination Isomerism 🔄
In salts with two metal ions, ligands can trade places. Compare \( \text{[Co(NH}_3)_6][\text{Cr(CN)}_6] \) with its partner \( \text{[Cr(NH}_3)_6][\text{Co(CN)}_6] \). :contentReference[oaicite:6]{index=6}
2.3 Ionisation Isomerism ⚡
Here, a ligand leaves the inner circle and becomes the counter-ion (or vice-versa). Twin pair: \( \text{[Co(NH}_3)_5(SO_4)]\text{Br} \) ↔ \( \text{[Co(NH}_3)_5Br]\text{SO}_4 \). :contentReference[oaicite:7]{index=7}
2.4 Solvate (Hydrate) Isomerism 💧
A solvent molecule may sit snugly on the metal or float free in the lattice. Example: violet \( \text{[Cr(H}_2\text{O)}_6]\text{Cl}_3 \) vs. grey-green \( \text{[Cr(H}_2\text{O)}_5\text{Cl]Cl}_2·\text{H}_2\text{O} \). :contentReference[oaicite:8]{index=8}
High-Yield NEET Nuggets 🚀
- The cis–trans trick for square-planar \( \text{[MX}_2\text{L}_2] \) and octahedral \( \text{[MX}_2\text{L}_4] \) is a frequent MCQ favourite.
- Only cis chelate complexes such as \( \text{[PtCl}_2(\text{en})_2]^{2+} \) can show optical activity — remember that! 🔍
- Linkage flip (ONO ↔ NO2) questions often test colour changes and naming rules.
- The fac/mer labels in octahedra appear regularly as pictorial questions.
- Ionisation vs. solvate isomers test whether you can track counter-ions and water molecules. 💡
✨ Keep practising drawing those isomers — sketching builds super-speed in exams! ✨
