Chemistry · Coordination Compounds · NEET
A chelate is a RING. It forms when one ligand grabs the central metal at two or more points instead of just one. Think of a crab holding a ball with both claws at the same time, not just poking it with one finger. That two-point grip closes a ring, and the metal now sits inside that ring. A ligand that can do this is called a chelating ligand.
Because a ring is hard to open. If one arm of the ligand lets go, the other arm is still holding the metal, so the ligand cannot float away. It just re-attaches. With single-point (monodentate) ligands like NH3, once one leaves, it is gone. There is also an entropy reason: one EDTA replacing six water molecules frees more particles into the solution, and nature likes more freedom. Both effects together are called the chelate effect. For NEET, remember: more rings = more stable.
Yes. en is H2N-CH2-CH2-NH2. It has TWO nitrogen atoms, each with a lone pair. Both nitrogens bind the same metal at the same time, so en is bidentate (two teeth). This closes a stable five-membered ring with the metal. So every en ligand you see in a complex like [Co(en)3]3+ is forming a chelate ring.
EDTA (ethylenediaminetetraacetate ion, EDTA 4-) has SIX donor atoms that can all bind one metal at once: 2 nitrogen atoms (from the en backbone) and 4 oxygen atoms (one from each of the four -COO- groups). 2 N + 4 O = 6 donor points, so it is hexadentate. It wraps fully around the metal and forms FIVE chelate rings. This is exactly what NEET 2021 asked.
They overlap but are not the same word. Bidentate means the ligand has two donor atoms. Chelate describes the RING that forms when those two (or more) atoms both grab the same metal. So en is a bidentate ligand that acts as a chelate. A ligand must be at least bidentate to chelate; a monodentate ligand (one donor) can never form a chelate.
Yes. Five-membered and six-membered chelate rings are the most stable. en forms a five-membered ring, which is why en complexes are very stable. Very small (3-membered) or very large rings have strain or are floppy, so they are less stable. For NEET the safe rule is: five- and six-membered chelate rings are best.
Look for chelating ligands (en, oxalate ox, EDTA). The complex with MORE chelate rings is more stable. A complex with only monodentate ligands (NH3, H2O, Cl-, Br-, NO3-) has no rings and is the least stable. NEET 2023 gave four Co complexes and the answer was the one with two en ligands, [CoCl2(en)2]NO3, because en makes chelate rings.
Which complex compound is the most stable?
Ethylenediaminetetraacetate (EDTA) ion is:
The order of energy absorbed which is responsible for the colour of complexes: (A) [Ni(H2O)2(en)2]2+, (B) [Ni(H2O)4(en)]2+, (C) [Ni(en)3]2+
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Chelate means a ring formed when one ligand grips the metal at two or more points. The word comes from the Greek 'chele', meaning a crab's claw, because the ligand holds the metal like a claw.
EDTA forms five chelate rings around one metal ion. It uses 2 nitrogen and 4 oxygen donor atoms, so it is hexadentate (six donor points).
Yes. Oxalate, C2O4 2-, is bidentate; it binds the metal through two oxygen atoms and forms a five-membered chelate ring. So oxalate complexes are also more stable, just like en complexes.
No. A ligand needs at least two donor atoms to close a ring. Monodentate ligands like NH3, H2O and Cl- have only one donor atom, so they can never chelate.
Because they are the clearest examples of the chelate effect, and the same fact answers both stability questions (most stable complex) and denticity questions (EDTA is hexadentate). Learning this one idea covers many marks.
