Read Which Complexes Can Actually Be Chiral
Geometrical isomerism needs at least two different ligand types arranged so cis and trans are distinguishable; optical isomerism needs the added condition that the specific arrangement has no internal mirror plane.
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Spotting geometrical isomerism
[Cr(H2O)4Cl2]+ shows geometrical isomerism (cis/trans Cl arrangement), while [Co(NH3)6]3+ (all six ligands identical) and [Pt(NH3)3Cl]+ (only one Cl, nothing to arrange against) cannot. Two different 2D drawings of an octahedral M(en)2A2-type complex can sometimes represent the exact same isomer viewed from different angles — check carefully before calling two drawings 'different'; they may be identical.
[Co(en)3]3+ and fully-chelated complexes are always chiral
[Co(en)3]3+ has no plane of symmetry once its three chelate rings are arranged in 3D — it exists as a pair of non-superimposable mirror images, so it's optically active. Contrast [Co(NH3)3Cl3], which retains an internal mirror plane in every possible arrangement (fac or mer) — it shows NO optical isomerism at all.
Cis is often chiral; trans usually isn't
For [Co(en)2Cl2]+: the trans isomer has an internal mirror plane (achiral), but the cis isomer does not — cis-[Co(en)2Cl2]+ is the chiral one. This cis-active/trans-inactive pattern repeats across the whole en-containing family, including [CrCl2(ox)2]3-, where only the cis form is chiral. Choose D.
Square planar MA2B2 is never chiral
Square planar geometry is flat, so it always retains a mirror plane in its own molecular plane — square planar complexes show geometrical isomerism but never optical isomerism, regardless of how many different ligand types are present.
OlympIQ exam lens
- ✓Geometrical isomerism needs at least 2 different ligand types positioned so cis/trans differ.
- ✓A fully chelated complex like [Co(en)3]3+ is always chiral — no internal mirror plane is possible.
- ✓Within a cis/trans pair, the cis form is usually the chiral one; trans usually keeps a mirror plane.
- ✓[Co(NH3)3Cl3] has zero optical isomers in any geometrical form — don't assume 'octahedral + 2 ligand types' guarantees chirality.
- ✓Square planar complexes never show optical isomerism, only geometrical.
Don't assume 'has cis and trans forms' automatically means 'both forms are chiral' — check the cis and trans arrangements separately for a mirror plane; very often only the cis form is optically active, and square planar complexes are never chiral at all.
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