Chemistry · Coordination Compounds · NEET
There are TWO main families, and each splits into sub-types. Structural isomerism (also called constitutional) = the atoms are bonded in a different order. Its four NEET sub-types are: ionisation, solvate (hydrate), linkage, and coordination isomerism. Stereoisomerism = the atoms are bonded in the same order but sit differently in 3D space. Its two sub-types are: geometrical (cis-trans, fac-mer) and optical (mirror images). So the full list you must know for NEET is 6 sub-types under 2 headings.
In STRUCTURAL isomers the connection order is different — something is inside the coordination sphere in one isomer and outside (or bonded through a different atom) in the other. In STEREOisomers the connection order is exactly the same; only the position in space changes. Quick test: if you can tell them apart by writing the formula differently (e.g. [Co(NH3)5Br]SO4 vs [Co(NH3)5SO4]Br), it is structural. If both have the same formula and the same bonds but a different shape (cis vs trans), it is stereo.
1) IONISATION isomerism — the ion inside and the ion outside the sphere swap. Example: [Co(NH3)5Br]SO4 and [Co(NH3)5SO4]Br. 2) SOLVATE / HYDRATE isomerism — water is inside the sphere in one, outside as crystal water in the other. Example: [Cr(H2O)6]Cl3 vs [Cr(H2O)5Cl]Cl2·H2O. 3) LINKAGE isomerism — an ambidentate ligand binds through a different atom. Example: [Co(NH3)5(NO2)]Cl2 (–NO2, N-bonded) vs [Co(NH3)5(ONO)]Cl2 (–ONO, O-bonded). 4) COORDINATION isomerism — ligands swap between two metal centres (both cation and anion are complexes). Example: [Co(NH3)6][Cr(CN)6] vs [Cr(NH3)6][Co(CN)6].
GEOMETRICAL isomerism: same ligands, but arranged cis (same side, 90 degrees apart) or trans (opposite side, 180 degrees apart). Octahedral MA3B3 complexes also show fac (three same ligands on one face) and mer (three in a line). Example: cis- and trans-[CoCl2(en)2], cis- and trans-[Pt(NH3)2Cl2]. OPTICAL isomerism: the molecule and its mirror image cannot be placed on top of each other (like your left and right hand). These non-superimposable mirror images are called enantiomers. Example: [Co(en)3]3+ and cis-[CoCl2(en)2].
Use this checklist. (1) Is there an ambidentate ligand like NO2, SCN, CN, NO2/ONO? -> LINKAGE. (2) Are there water molecules that could be inside or outside the sphere? -> SOLVATE (hydrate). (3) Are both the cation and the anion complex ions with different metals? -> COORDINATION. (4) Can an ion inside the sphere swap with the counter ion outside? -> IONISATION. (5) Is it square planar MA2B2 or octahedral MA4B2 / MA3B3 with no ambidentate/water tricks? -> GEOMETRICAL (cis-trans or fac-mer). (6) Is it a chelate like [M(en)3] or cis-[MCl2(en)2] with no mirror plane? -> OPTICAL. NEET usually gives 4 complexes to match; run each complex through this list.
No, and this is a common trap. Both look similar because something swaps between inside and outside the sphere. But in IONISATION isomerism a charged ion (like Br- and SO4^2-) swaps, so the two isomers give DIFFERENT ions in solution. In SOLVATE (hydrate) isomerism the thing that moves is a neutral solvent molecule (usually water), so the number of water molecules inside vs outside the sphere changes. If water is involved, it is solvate/hydrate; if two different ions swap, it is ionisation.
Match List-I (Complex) with List-II (Type of isomerism): (a) [Co(NH3)5(NO2)]Cl2 (b) [Co(NH3)5(SO4)]Br (c) [Co(NH3)6][Cr(CN)6] (d) [Co(H2O)6]Cl3 ; (i) Solvate isomerism (ii) Linkage isomerism (iii) Ionization isomerism (iv) Coordination isomerism.
Match List-I (Complex) with List-II (Type of isomerism): (a) [Pt(NH3)2Cl2] (b) [Co(en)3]^3+ (c) [Co(NH3)5(NO2)]Cl2 (d) [Cr(H2O)6]Cl3 ; (i) Optical (ii) Solvate (iii) Geometrical (iv) Linkage.
The type of isomerism shown by the complex [CoCl2(en)2] is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Isomerism means two or more compounds have exactly the same molecular formula but a different arrangement of their atoms, so they are different substances with different properties. In coordination compounds this arrangement can differ either in bonding order (structural) or in 3D shape (stereo).
Match-the-column questions that ask you to pair a complex with its isomerism type (linkage, ionisation, solvate, coordination, geometrical, optical) are extremely common — they appeared in 2024 and 2026. Also frequent: identifying cis-trans (geometrical) and whether a complex is optically active (chiral).
Geometrical isomers differ in the cis/trans (or fac/mer) position of ligands and are usually NOT mirror images. Optical isomers are non-superimposable mirror images (enantiomers) that rotate plane-polarised light in opposite directions. A complex can show both — for example cis-[CoCl2(en)2] is a geometrical isomer that is also optically active.
Yes, know all six: linkage, ionisation, solvate (hydrate), coordination, geometrical and optical. But do not just memorise — learn the one clue for each (ambidentate ligand, ion swap, water in/out, two metal complexes, cis-trans, mirror image) so you can classify any complex NTA gives you.
Ambidentate ligands cause LINKAGE isomerism, because they can bond to the metal through two different atoms — for example NO2 can bind through N (nitro, –NO2) or through O (nitrito, –ONO), and SCN through S (thiocyanato) or N (isothiocyanato).
