Types of Ligands: Monodentate, Bidentate and Polydentate
Chemistry · Coordination Compounds · NEET
A ligand is a molecule or ion that donates a lone pair to the central metal. We classify ligands by "denticity" = how many donor atoms of that ONE ligand grip the metal at the same time. Monodentate grips with 1 donor atom (like NH3, Cl-, H2O), bidentate with 2 (like en and oxalate), and polydentate with many (EDTA uses 6). Memory hook: "dent" = tooth, so count the teeth that bite the metal.
Denticity is just counting donor atoms: NH3 uses 1 (monodentate), en uses 2 N and forms a ring (bidentate), and EDTA uses 2 N + 4 O = 6 sites (hexadentate polydentate). More donor atoms binding one metal means a chelate ring and higher stability.
Your doubts, answered
How do I actually find the denticity of a ligand?
Count the number of donor atoms in that ONE ligand that can point their lone pair at the metal at the same time. Look for atoms with lone pairs: N, O, S, P, and the negative atoms of ions. In en (H2N-CH2-CH2-NH2) there are two N atoms with lone pairs, so denticity = 2 (bidentate). In EDTA there are 2 N and 4 O donors, so denticity = 6 (hexadentate). Just find the lone-pair atoms that can reach the metal and count them.
Is ethylenediamine (en) monodentate or bidentate?
en is BIDENTATE. Its full structure is H2N-CH2-CH2-NH2. It has two nitrogen atoms, each with a lone pair, and they are far enough apart to both bind the same metal at once. So one en molecule uses 2 coordination positions. This is why [Co(en)3]3+ has coordination number 6, not 3 (3 en x 2 = 6). NEET loves this counting trap.
Why is oxalate bidentate and not monodentate?
Oxalate is C2O4^2-. It has four oxygen atoms, but only two of them (one from each -COO- end) point at the metal and form a stable 5-membered ring. So it binds through 2 donor oxygens at once = bidentate. The other two oxygens are double-bonded and do not grip the metal. Denticity counts donor atoms that actually bond, not all atoms present.
What is the difference between bidentate and ambidentate?
They sound similar but mean opposite things. BIDENTATE = the ligand binds through TWO donor atoms at the SAME time (like en, oxalate) and uses 2 positions. AMBIDENTATE = the ligand has two possible donor atoms but binds through only ONE at a time (like SCN- through S or N, and NO2- through N or O). Ambidentate is still monodentate in how many positions it uses. Do not confuse them; NEET sets this as a distractor.
Is NH3 monodentate or bidentate? It has 3 hydrogens.
NH3 is MONODENTATE. The hydrogens are NOT donor atoms. Only the nitrogen has a lone pair, and only that one N binds the metal. Number of H atoms does not decide denticity; number of donor atoms with a free lone pair does. So NH3, H2O, Cl-, and CN- are all monodentate (one donor atom each).
What is a polydentate ligand and which one comes in NEET most?
A polydentate ligand has many (more than two) donor atoms that all bind the same metal. The star example is EDTA4- (ethylenediaminetetraacetate). It is HEXADENTATE: 2 nitrogen + 4 oxygen donors = 6 sites, wrapping fully around one metal ion. NEET has directly asked 'EDTA is a ...dentate ligand with how many N and O donors?' The answer is hexadentate, 4 O and 2 N.
Does a chelating ligand and a bidentate ligand mean the same thing?
Not exactly. A CHELATING ligand is any di- or polydentate ligand that binds the same metal at two or more points and forms a RING (a chelate). So every bidentate/polydentate ligand that forms a ring is a chelate, but the word 'chelate' describes the ring effect, while 'bidentate/polydentate' describes how many donor atoms it uses. en and EDTA are both chelating; the ring is why their complexes are extra stable (chelate effect).
⚠️ The NEET trap ✗ Students see SCN- or NO2- has two different atoms (S and N, or N and O) and mark it as bidentate. ✓ SCN- and NO2- are AMBIDENTATE, and they act as MONODENTATE (they bind through only ONE donor atom at a time). Bidentate means two donors bind together at once, like en and oxalate. Different donor CHOICE is ambidentate; two donors used TOGETHER is bidentate. 🧠 Ambidentate = a choice of one door. Bidentate = two doors used at the same time.
Real NEET questions
NEET 2021
Ethylenediaminetetraacetate (EDTA) ion is:
A · Bidentate ligand with two "N" donor atoms
B · Tridentate ligand with three "N" donor atoms
C · Hexadentate ligand with four "O" and two "N" donor atoms ✓
D · Unidentate ligand
Solution: Denticity = number of donor atoms of one ligand that bind the metal at the same time. EDTA4- has 2 amine nitrogen atoms (from the ethylenediamine backbone) and 4 carboxylate oxygen atoms (one from each -COO- group). All 6 can bind one metal ion at once, wrapping around it and forming five chelate rings. So EDTA is HEXADENTATE with 4 O and 2 N donors. Answer: (C).
NEET 2023 Phase 1
Which complex compound is the most stable?
A · [Co(NH3)4(H2O)Br](NO3)2
B · [Co(NH3)3(NO3)3]
C · [CoCl2(en)2]NO3 ✓
D · [Co(NH3)5](SO4)3
Solution: This tests the chelate effect, which follows from denticity. In [CoCl2(en)2]NO3 the two en ligands are BIDENTATE and each forms a stable five-membered chelate ring with cobalt. Ring formation gives a large positive entropy and a high stability constant. The other options have only monodentate ligands (NH3, H2O, Br-, NO3-) and no rings, so they are less stable. Answer: (C). Lesson: bidentate/polydentate (chelating) ligands make complexes more stable than monodentate ones.
NEET 2026 Phase 1
Which one of the following is an ambidentate ligand?
A · Ethane-1,2-diamine
B · Ethylenediaminetetraacetate ion
C · Thiocyanate ✓
D · Oxalate
Solution: First sort the options by type. Ethane-1,2-diamine (en) = bidentate. EDTA = hexadentate (polydentate). Oxalate = bidentate. Only thiocyanate SCN- can coordinate through EITHER sulphur (M-SCN) or nitrogen (M-NCS), one at a time = AMBIDENTATE. Answer: (C). Note ambidentate is about a choice of donor atom, not about denticity.
Solved Coordination Compounds NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What decides if a ligand is monodentate, bidentate, or polydentate?
The number of donor atoms (atoms with a lone pair) that bind the same metal at the same time. 1 donor = monodentate, 2 donors = bidentate, more than 2 = polydentate.
How many coordination positions does a bidentate ligand use?
Two. That is why [Co(en)3]3+ has coordination number 6 (3 en x 2 donor atoms each = 6), even though there are only 3 ligand molecules.
Is EDTA monodentate or polydentate?
EDTA4- is polydentate, specifically hexadentate. It binds a metal through 2 nitrogen and 4 oxygen donor atoms (6 sites total).
Why does denticity matter for NEET?
It is asked directly (EDTA denticity, en counting) and it also controls coordination number, isomerism, and stability (the chelate effect). Getting denticity right lets you solve many other coordination-compound questions.
