Chemistry · Coordination Compounds · NEET
Draw the complex and look for a plane of symmetry (a mirror plane that cuts the molecule into two matching halves) or a centre of symmetry. If you find EITHER one, the complex is achiral (not optically active). If NEITHER exists, the complex is chiral and shows optical isomerism. You do not need to test superimposability by hand; the symmetry check is faster and is what NEET expects.
[Co(en)3]3+ has three bidentate 'en' ligands that wind around the cobalt like the threads of a screw. This spiral shape has no plane of symmetry, so its mirror image is different, making it chiral. [Co(NH3)6]3+ has six identical simple ligands placed symmetrically, so it has many planes of symmetry and its mirror image is identical. That is why only the en complex is optically active.
Usually NO. In most cases the trans arrangement is more symmetric and has a plane of symmetry, so it is achiral. For example, trans-[Cr(H2O)2(ox)2]- has the two water ligands directly opposite each other, which creates a plane of symmetry, so it is NOT chiral. The cis isomer of the same complex is chiral. This exact trap was tested in ReNEET 2026.
Often yes, especially when the complex has two or more bidentate (chelate) ligands. In cis-[Co(en)2Cl2]+, the two en ligands and the arrangement leave no plane of symmetry, so the cis form is chiral and optically active. The trans form of the same complex is achiral. So the cis form is more likely to be optically active than the trans form.
For NEET, treat this as very rare. Chirality in coordination compounds almost always comes from bidentate chelate ligands like en (ethylenediamine) or oxalate (ox) wrapping around the metal. If the complex has only simple monodentate ligands like NH3, Cl, or H2O, assume it is achiral unless a special case is given. Focus your energy on the chelate examples, because those are what the exam tests.
Geometrical isomerism (cis/trans) is about WHERE ligands sit around the metal, and the two forms have different physical properties. Optical isomerism is about the mirror image: the two forms (called enantiomers or d and l) are non-superimposable mirror images that have identical physical properties but rotate plane-polarised light in opposite directions. A single geometrical isomer, like the cis form, can itself be further split into two optical isomers.
Statement-I: [Fe(ox)3]3- is chiral. Statement-II: trans-[Cr(H2O)2(ox)2]- is chiral. (oxH2 = HOOC-COOH). Choose the most appropriate answer.
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. List-II: (i) Optical (ii) Solvate (iii) Geometrical (iv) Linkage.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
An optically active substance can rotate the plane of plane-polarised light. One mirror image (called d or +) rotates it clockwise, and the other (l or -) rotates it anticlockwise by the same amount. Only chiral complexes are optically active.
An enantiomer is one of the two non-superimposable mirror-image forms of a chiral complex. The pair is called d and l (or + and -). They have identical physical properties except that they rotate polarised light in opposite directions.
Yes. [Co(en)3]3+ is the classic example asked repeatedly in NEET. Its three bidentate en ligands form a propeller-like shape with no plane of symmetry, so it exists as two enantiomers and is optically active.
In the trans arrangement, identical ligands sit opposite each other, which usually creates a plane of symmetry passing through the molecule. Any complex with a plane of symmetry has an identical mirror image, so it is achiral and not optically active.
Isomerism is a high-yield topic in Coordination Compounds, and optical isomerism appears almost every year, often as statement or matching questions like ReNEET 2026 and NEET 2026. Knowing the fast symmetry check lets you answer these in seconds without drawing full 3D structures.
