Count Isomers Systematically, Ligand Set by Ligand Set
Counting 'total isomers' rewards a system: first fix the geometrical (cis/trans or fac/mer) arrangements, then check each one separately for a non-superimposable mirror image.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
MA4B2-type octahedral complexes: 2 geometrical forms
Compare [Co(en)2(NH3)2]3+: cis and trans again, but here the cis form IS chiral once en's chelate rings are drawn in 3D — so this system gives 2 geometrical, 2 optical (both from the cis form), 3 total distinct isomers.
Mixed bidentate + monodentate ligand sets go higher
A complex combining one bidentate ligand (like en) with two different pairs of monodentate ligands allows the richest isomer count — more cis/trans arrangements survive because the chelate ring constrains geometry less symmetrically than an all-monodentate set. This kind of mixed set consistently scores highest among the options in 'which has the most isomers' questions.
Square planar MA2B2 rarely goes beyond geometrical — until linkage isomerism enters
Square planar complexes like [Pd(NH3)2(SCN)2] can show geometrical isomerism (cis/trans) plus linkage isomerism from an ambidentate ligand like SCN- (S-bound vs N-bound)
Choose D here.
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- ✓Always separate the count into geometrical forms first, then check each geometrical form for chirality.
- ✓A cis form is more likely to be chiral than a trans form, because trans often retains an internal mirror plane.
- ✓Mixed bidentate + monodentate ligand sets typically allow more total isomers than purely monodentate sets.
- ✓An ambidentate ligand (like SCN-) can multiply the isomer count further through linkage isomerism.
- ✓MA3B3 octahedral complexes can show 3 stereoisomeric forms — check fac/mer and whether each is chiral.
Don't stop counting after geometrical isomers — always re-check each geometrical form individually for a non-superimposable mirror image before reporting a final 'total isomers' number.
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