: 300
Stereoisomers have the same connectivity but differ in spatial arrangement. Two main types occur in coordination compounds.
Geometrical (cis-trans) isomerism arises when ligands can adopt different relative positions. In square planar [MA2B2]: cis (identical ligands adjacent, 90 degrees) and trans (identical ligands opposite, 180 degrees). Classic example: cis-[Pt(NH3)2Cl2] (cisplatin, anti-cancer) vs trans (inactive). Square planar [MABCD] gives 3 geometrical isomers. Tetrahedral complexes never show geometrical isomerism.
In octahedral complexes: [MA2B4] shows cis-trans; [MA3B3] shows facial (fac, three identical ligands on one triangular face) and meridional (mer, three identical ligands along a meridian). [MA2B2C2] gives 5 geometrical isomers — a frequently tested advanced topic.
Optical isomerism occurs when a complex lacks an improper axis of symmetry (including mirror planes and centre of inversion) and is non-superimposable on its mirror image. Key examples: cis-[Co(en)2Cl2]+ is optically active (exists as d and l forms) while the trans isomer is not. [M(en)3]n+ is always optically active. Tetrahedral [MABCD] is optically active. Square planar complexes are never optically active (molecular plane acts as mirror plane).
Critical JEE counting: [Co(en)2Cl2]+ has 3 total stereoisomers (cis-d, cis-l, trans). [Co(en)3]3+ has 2 (Delta, Lambda). The ability to rapidly enumerate stereoisomers is one of the most tested skills in this chapter.