JEE Mains Chemistry · Coordination Compounds

Ligands, Denticity and Nomenclature

A ligand is classified by how many donor atoms it binds with and whether it has a choice of donor atom; the name of a complex lists its ligands alphabetically, ends an anionic complex in -ate and gives the metal's oxidation state in Roman numerals.

All Coordination Compounds notes JEE Mains Chemistry strategy

Why this matters

Twenty-three PYQs, sixteen of them multiple choice, and two from 2026. Ten classify ligands as ambidentate, chelating, homoleptic or σ-donor and π-acceptor; seven are about the nickel–dimethylglyoxime complex and copper sulphate pentahydrate; six ask for an IUPAC name or the metal's oxidation state and d-electron count.

Concept 1 of 3: Denticity and types of ligands

Count the atoms a ligand actually bonds with. One donor atom makes it monodentate, two make it bidentate, six make EDTA⁴⁻ hexadentate. A ligand that bonds through two or more atoms at once closes a ring with the metal (a chelate). An ambidentate ligand is different: it has two possible donor atoms but uses only one of them at a time.

Definition

Monodentate: one donor atom ( $\mathrm{NH_3}$ , $\mathrm{H_2O}$ , $\mathrm{Cl^-}$ , CO).
Chelating (polydentate): two or more donor atoms at once, forming a ring: en, $\mathrm{C_2O_4^{2-}}$ , dmgH⁻, biuret (all bidentate), EDTA⁴⁻ (hexadentate).
Ambidentate: one donor atom at a time, chosen from two: $\mathrm{NO_2^-}$ (N or O), $\mathrm{SCN^-}$ (S or N), $\mathrm{CN^-}$ (C or N).
Homoleptic: only one kind of ligand, as in $\mathrm{[Ni(CN)_4]^{2-}}$ . Heteroleptic: more than one kind, as in $\mathrm{[Co(NH_3)_4Cl_2]^+}$ .
$\mathrm{N(CH_3)_3}$ is only a σ-donor. $\mathrm{P(CH_3)_3}$ and $\mathrm{PPh_3}$ are σ-donors and also π-acceptors, through empty orbitals on phosphorus.

Ligand	Donor atom(s)	Denticity	Type
$\mathrm{NH_3}$ , $\mathrm{H_2O}$	N; O	1	Monodentate, neutral
$\mathrm{NO_2^-}$	N (nitro) or O (nitrito)	1	Ambidentate
$\mathrm{SCN^-}$	S (thiocyanato) or N (isothiocyanato)	1	Ambidentate
$\mathrm{CN^-}$	C (cyanido) or N (isocyanido)	1	Ambidentate
en, $\mathrm{H_2NCH_2CH_2NH_2}$	Two N	2	Chelating, neutral
Oxalate, $\mathrm{C_2O_4^{2-}}$	Two O	2	Chelating, not ambidentate Oxalate uses both oxygens at once; it has no choice of donor atom, so it is not ambidentate.
Biuret, $\mathrm{H_2NCONHCONH_2}$	Two carbonyl O, or two deprotonated amide N in alkali	2	Chelating
EDTA⁴⁻	Two N and four O	6	Chelating; octahedral even around Ca²⁺
$\mathrm{PPh_3}$ (in Wilkinson's catalyst)	P	1	σ-donor and π-acceptor

Chelating and ambidentate are different ideas: a chelate uses two atoms together, an ambidentate ligand uses one of two.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2024 · 9 April 2024 · Q58Moderate

Example 1 · Coordination Compounds · Ligands, Denticity and Nomenclature

Number of ambidentate ligands among the following is______

NO_{2}^{-},SCN^{-},C_{2}O_{4}^{2 -},NH_{3},CN^{-},SO_{4}^{2 -},H_{2}O

A chelating ligand is not ambidentate

Oxalate and ethane-1,2-diamine bind through two atoms together, so they chelate. Ambidentate means one bond through either of two different atoms, as in

\mathrm{NO_2^-}

\mathrm{SCN^-}

and

\mathrm{CN^-}

Nitrogen and phosphorus ligands bond differently

\mathrm{N(CH_3)_3}

and

\mathrm{P(CH_3)_3}

both donate a lone pair, but only phosphorus can also accept π electrons back from the metal. A statement that their bonding is always the same is false.

Wilkinson's catalyst carries triphenylphosphine

Wilkinson's catalyst is

\mathrm{[RhCl(PPh_3)_3]}

. Its ligand is triphenylphosphine, a phosphine, not

\mathrm{PH_3}

itself. A 2024 key treated 'phosphine acts as a ligand in Wilkinson catalyst' as false, reading phosphine as

\mathrm{PH_3}

Drill 9 more on denticity and types of ligands

Concept 2 of 3: Nickel dimethylglyoximate and copper sulphate pentahydrate

Two named compounds come up again and again. Dimethylglyoxime (dmgH₂) loses one oxime proton, and two of the anions hold Ni²⁺ in a flat square: the red precipitate that detects nickel. In blue vitriol, four waters sit on copper and the fifth is held only by hydrogen bonds.

Definition

dmgH₂ is $\mathrm{CH_3C(=NOH)C(=NOH)CH_3}$ , $\mathrm{C_4H_8N_2O_2}$ . Each loses one H to give dmgH⁻, $\mathrm{C_4H_7N_2O_2^-}$ , a bidentate anion that binds through two N.
$\mathrm{[Ni(dmgH)_2]}$ is neutral, red, square planar and diamagnetic (d⁸ with dsp²). It is insoluble in the ammoniacal solution (pH about 9) where the test is done.
Its 14 H atoms: 12 in four methyl groups and 2 in the O–H···O hydrogen bonds that link the two ligands.
Rings: two five-membered chelate rings (Ni, N, C, C, N) and two six-membered hydrogen-bonded rings (Ni, N, O, H, O, N).
$\mathrm{CuSO_4\cdot 5H_2O}$ is written $\mathrm{[Cu(H_2O)_4]SO_4\cdot H_2O}$ : four Cu–O bonds to water; the fifth water is hydrogen-bonded to sulphate and to coordinated water. No ligand binds through sulphur.

Feature	Value	Reason
Colour of $\mathrm{[Ni(dmgH)_2]}$	Red (rosy red precipitate)	Used to detect Ni²⁺ in ammoniacal solution
Geometry and magnetism of $\mathrm{[Ni(dmgH)_2]}$	Square planar, diamagnetic	d⁸ Ni²⁺ with dsp² hybridisation; N–Ni–N angles close to 90°
H atoms in $\mathrm{[Ni(dmgH)_2]}$	14	Two ligands of $\mathrm{C_4H_7N_2O_2^-}$
H atoms in hydrogen bonds	2	One O–H···O bridge on each side; the other 12 H are in methyl groups
Five-membered rings in $\mathrm{[Ni(dmgH)_2]}$	2	One chelate ring per dmgH⁻; the other two rings are six-membered
Charge of the ligand as bound	−1 (dmgH⁻)	One oxime proton is lost; dmgH₂ itself is neutral
Waters bonded to Cu in $\mathrm{CuSO_4\cdot 5H_2O}$	4	Secondary valency 4 in the textbook formula $\mathrm{[Cu(H_2O)_4]SO_4\cdot H_2O}$
Hydrogen-bonded water in $\mathrm{CuSO_4\cdot 5H_2O}$	1	Held between sulphate and coordinated water, not bonded to Cu

Nickel dimethylglyoximate and blue vitriol are asked as facts, so the numbers in this table are worth memorising.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2025 · 22 Jan 2025 · Q123Moderate

Example 2 · Coordination Compounds · Ligands, Denticity and Nomenclature

The complex of

Ni^{2 +}

ion and dimethyl glyoxime contains____ number of Hydrogen

(H)

atoms.

dmgH⁻ is an anion when it binds

Dimethylglyoxime loses one proton before it binds nickel, so the ligand is dmgH⁻ and the complex is neutral. Calling it a neutral bidentate ligand is the error in a common statement question.

Blue vitriol has a longer story than four waters

JEE keys follow the textbook formula

\mathrm{[Cu(H_2O)_4]SO_4\cdot H_2O}

: four waters on copper, secondary valency 4. In the real crystal two sulphate oxygens also sit on copper at longer distances, so copper is six-coordinate. Answer with 4 unless the question describes the crystal.

Drill 6 more on nickel dimethylglyoximate and copper sulphate pentahydrate

Concept 3 of 3: IUPAC names, oxidation state and d-electron count of complexes

A name is built in a fixed order: the cation first, then the anion; inside the complex, ligands in alphabetical order, then the metal with its oxidation state in Roman numerals. The same oxidation state then gives the d-electron count, which the rest of the chapter depends on.

Definition

Ligands are listed alphabetically, ignoring the multiplying prefixes: di-, tri-, tetra- for simple ligands; bis-, tris-, tetrakis- for ligands whose names already contain a number or are in brackets, such as bis(ethane-1,2-diamine) and bis(trimethylphosphine).
Anionic ligands end in -ido: chlorido, bromido, cyanido, oxido, hydroxido. Neutral ligands keep special names: aqua, ammine, carbonyl, nitrosyl.
An anionic complex ends in -ate: cobaltate, manganate, platinate, and ferrate for iron, cuprate for copper, argentate for silver.
The number of counter-ions is not stated: $\mathrm{K_3[Fe(CN)_6]}$ is potassium hexacyanidoferrate(III), not tripotassium.
d-electron count = (group number of the metal) − (oxidation state). In the nitroprusside ion NO is counted as NO⁺.

Oxidation state and d count

x + \sum q_{\text{ligands}} = q_{\text{complex}} \qquad n_d = \text{group number} - x

Worked example

Name

\mathrm{K_3[Fe(CN)_6]}

and give the d-electron count of iron.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2024 · 29 January 2024 · Q126Moderate

Example 3 · Coordination Compounds · Ligands, Denticity and Nomenclature

The correct IUPAC name of

K_{2}MnO_{4}

Oxido, not oxo; manganate, not permanganate

Current IUPAC names call the oxide ligand oxido, and an anionic manganese complex is a manganate. Permanganate is the common name of

\mathrm{MnO_4^-}

only, and the charge fixes the oxidation state:

\mathrm{K_2MnO_4}

is manganese(VI).

Bis and tris for ligands that already carry a number

Two trimethylphosphine ligands are bis(trimethylphosphine), not di(trimethylphosphine). Both trioxalatocobaltate(III) and tris(oxalato)cobaltate(III) are accepted for

\mathrm{[Co(C_2O_4)_3]^{3-}}

; pick the one the options offer.

Drill 5 more on iupac names, oxidation state and d-electron count of complexes

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

IUPAC names, oxidation state and d-electron count of complexes
Oxidation state and d count
$x + \sum q_{\text{ligands}} = q_{\text{complex}} \qquad n_d = \text{group number} - x$

Reference tables (2)

Denticity and types of ligands9 rows

Ligand	Donor atom(s)	Denticity	Type
$\mathrm{NH_3}$ , $\mathrm{H_2O}$	N; O	1	Monodentate, neutral
$\mathrm{NO_2^-}$	N (nitro) or O (nitrito)	1	Ambidentate
$\mathrm{SCN^-}$	S (thiocyanato) or N (isothiocyanato)	1	Ambidentate
$\mathrm{CN^-}$	C (cyanido) or N (isocyanido)	1	Ambidentate
en, $\mathrm{H_2NCH_2CH_2NH_2}$	Two N	2	Chelating, neutral
Oxalate, $\mathrm{C_2O_4^{2-}}$	Two O	2	Chelating, not ambidentate Oxalate uses both oxygens at once; it has no choice of donor atom, so it is not ambidentate.
Biuret, $\mathrm{H_2NCONHCONH_2}$	Two carbonyl O, or two deprotonated amide N in alkali	2	Chelating
EDTA⁴⁻	Two N and four O	6	Chelating; octahedral even around Ca²⁺
$\mathrm{PPh_3}$ (in Wilkinson's catalyst)	P	1	σ-donor and π-acceptor

Chelating and ambidentate are different ideas: a chelate uses two atoms together, an ambidentate ligand uses one of two.

Nickel dimethylglyoximate and copper sulphate pentahydrate8 rows

Feature	Value	Reason
Colour of $\mathrm{[Ni(dmgH)_2]}$	Red (rosy red precipitate)	Used to detect Ni²⁺ in ammoniacal solution
Geometry and magnetism of $\mathrm{[Ni(dmgH)_2]}$	Square planar, diamagnetic	d⁸ Ni²⁺ with dsp² hybridisation; N–Ni–N angles close to 90°
H atoms in $\mathrm{[Ni(dmgH)_2]}$	14	Two ligands of $\mathrm{C_4H_7N_2O_2^-}$
H atoms in hydrogen bonds	2	One O–H···O bridge on each side; the other 12 H are in methyl groups
Five-membered rings in $\mathrm{[Ni(dmgH)_2]}$	2	One chelate ring per dmgH⁻; the other two rings are six-membered
Charge of the ligand as bound	−1 (dmgH⁻)	One oxime proton is lost; dmgH₂ itself is neutral
Waters bonded to Cu in $\mathrm{CuSO_4\cdot 5H_2O}$	4	Secondary valency 4 in the textbook formula $\mathrm{[Cu(H_2O)_4]SO_4\cdot H_2O}$
Hydrogen-bonded water in $\mathrm{CuSO_4\cdot 5H_2O}$	1	Held between sulphate and coordinated water, not bonded to Cu

Nickel dimethylglyoximate and blue vitriol are asked as facts, so the numbers in this table are worth memorising.

Watch out for (7)

A chelating ligand is not ambidentate
→ Denticity and types of ligands
Nitrogen and phosphorus ligands bond differently
→ Denticity and types of ligands
Wilkinson's catalyst carries triphenylphosphine
→ Denticity and types of ligands
dmgH⁻ is an anion when it binds
→ Nickel dimethylglyoximate and copper sulphate pentahydrate
Blue vitriol has a longer story than four waters
→ Nickel dimethylglyoximate and copper sulphate pentahydrate
Oxido, not oxo; manganate, not permanganate
→ IUPAC names, oxidation state and d-electron count of complexes
Bis and tris for ligands that already carry a number
→ IUPAC names, oxidation state and d-electron count of complexes

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