Chemistry · Coordination Compounds · NEET
Only complexes where the shape lets ligands sit in two different fixed patterns. For square planar (4 ligands): MA2B2 (like [Pt(NH3)2Cl2]) and MA2BC show cis-trans. For octahedral (6 ligands): MA4B2, MA2B4, MA4BC, MA3B3, and complexes with bidentate ligands like [CoCl2(en)2] show it. Simple MA6, MA5B, and most tetrahedral complexes do NOT show geometrical isomerism because every position is equivalent.
Look at the two identical ligands you care about. If they are next to each other (bond angle 90 degrees), that is the cis isomer. If they are on exactly opposite sides (bond angle 180 degrees), that is the trans isomer. Example: cis-[Pt(NH3)2Cl2] has the two Cl close together; trans has the two Cl opposite. Same formula, different arrangement, different properties.
Fac and mer appear only in octahedral MA3B3 complexes (three of one ligand, three of another). In the FAC (facial) isomer, the three same ligands sit on one triangular face, so all three are cis to each other (90 degrees apart). In the MER (meridional) isomer, the three same ligands lie in a plane through the metal (a meridian line), so two are trans and one is cis. Example: [Cr(py)3Cl3] and [Co(NH3)3(NO2)3].
In a tetrahedron all four corners are next to each other at equal 109.5 degree angles. There is no 'opposite' position, so you cannot make a trans arrangement. That is why MA2B2 tetrahedral complexes have only one form. Square planar is different: it is flat, so ligands can be adjacent (cis) or across (trans).
No. [Co(en)3]3+ has three identical bidentate 'en' ligands, so there is no way to make a different geometrical (cis-trans) arrangement. But it DOES show optical isomerism (it is chiral, it has non-superimposable mirror images). Do not mix these up: geometrical isomerism needs different ligand positions; optical needs a non-superimposable mirror image.
Yes. It is octahedral with two bidentate 'en' ligands and two Cl ligands. The two Cl can be next to each other (cis, 90 degrees) or opposite (trans, 180 degrees). So it shows geometrical isomerism. Note: the cis form is also optically active, but the question 'which isomerism' is answered as geometrical. NEET 2018 asked exactly this.
The type of isomerism shown by the complex [CoCl2(en)2] is:
The complex which has facial and meridional isomers is: (py = pyridine, en = H2N-CH2-CH2-NH2)
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
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is when a complex has the same central metal and the same ligands, but the ligands are placed in different positions in space. The two forms (like cis and trans) have the same formula but different shapes and often different colour, melting point, or reactivity. This matters for NEET because match-the-column questions test whether you can name it.
Cis = close (same ligands next to each other, 90 degrees). Trans = across/total opposite (same ligands on opposite sides, 180 degrees). For square planar MA2B2 there is exactly one cis and one trans form.
Fac and mer come only from octahedral MA3B3 complexes (three of one ligand, three of another). Fac = three same ligands on one face (all cis). Mer = three same ligands in a line (two trans, one cis). If the complex is MA2B2 or MA4B2, you say cis-trans, not fac-mer.
Yes. For example, cis-[CoCl2(en)2] is geometrically the cis form AND is also optically active (chiral). The trans form is not chiral. But when a NEET question just asks 'type of isomerism shown by [CoCl2(en)2]', the expected answer is geometrical.
