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Chemistry · Coordination Compounds

Important Terms — Ligand, Coordination Number, Charge

Every problem in coordination chemistry rests on a small, exact vocabulary. NCERT §5.2 and NIOS §22.2 define a handful of terms — central atom, ligand, denticity, coordination number, coordination sphere, oxidation number, charge — that decide the formula, the name, the geometry and the magnetic behaviour of a complex. In NEET this vocabulary is tested directly, in counting questions and statement-matching, and indirectly in almost every other question of the chapter. This deep dive fixes each definition with worked examples.

The coordination entity

A coordination entity is a central metal atom or ion bonded to a fixed number of ions or molecules. The entity is written inside square brackets and behaves as a single unit that does not dissociate in solution. For example, $\ce{[CoCl3(NH3)3]}$ is a coordination entity in which the cobalt ion is surrounded by three ammonia molecules and three chloride ions. Other examples that recur throughout NEET are $\ce{[Ni(CO)4]}$, $\ce{[PtCl2(NH3)2]}$, $\ce{[Fe(CN)6]^{4-}}$ and $\ce{[Co(NH3)6]^{3+}}$.

This single idea — that the bracketed species is one robust unit — is what separates a complex from a double salt. A double salt such as carnallite, $\ce{KCl.MgCl2.6H2O}$, dissociates completely into simple ions in water, whereas the complex ion $\ce{[Fe(CN)6]^{4-}}$ of $\ce{K4[Fe(CN)6]}$ does not break down into $\ce{Fe^{2+}}$ and $\ce{CN^-}$. Every term defined below is a property of this entity or of the species attached to it.

Central atom or ion (Lewis acid)

In a coordination entity, the atom or ion to which a fixed number of ions or groups are bound in a definite geometrical arrangement is the central atom or ion. In $\ce{[NiCl2(H2O)4]}$, $\ce{[CoCl(NH3)5]^{2+}}$ and $\ce{[Fe(CN)6]^{3-}}$ the central ions are $\ce{Ni^{2+}}$, $\ce{Co^{3+}}$ and $\ce{Fe^{3+}}$ respectively. Because the central atom accepts electron pairs from the ligands, it acts as a Lewis acid; each ligand, donating at least one lone pair, is a Lewis base. This Lewis acid–base picture is the bonding foundation that Werner's theory first systematised.

Ligands and denticity

The ions or molecules bound to the central atom are the ligands. They range from simple ions such as $\ce{Cl^-}$, through small molecules such as $\ce{H2O}$ and $\ce{NH3}$, to larger molecules such as ethane-1,2-diamine $\ce{H2NCH2CH2NH2}$, and even to macromolecules such as proteins. The atom in the ligand that is bound directly to the metal is the donor atom; in $\ce{[Cu(NH3)4]^{2+}}$ the donor atom is nitrogen.

Ligands are classified by their denticity — the number of donor atoms a single ligand uses to grip the metal. This number, not the count of ligands, is what most NEET counting questions hinge on.

ClassDonor atoms usedExamplesDonor atoms
Unidentate (monodentate)1$\ce{Cl^-}$, $\ce{H2O}$, $\ce{NH3}$, $\ce{CN^-}$, $\ce{CO}$Cl, O, N, C, C
Bidentate (didentate)2en ($\ce{H2NCH2CH2NH2}$), oxalate ($\ce{C2O4^{2-}}$)2 N; 2 O
Polydentateseveral$\ce{N(CH2CH2NH2)3}$ (tren)4 N
Hexadentate6EDTA4−2 N + 4 O

A ligand that binds through a single donor atom — as $\ce{Cl^-}$, $\ce{H2O}$ or $\ce{NH3}$ do — is unidentate. When a ligand binds through two donor atoms, as in en or oxalate $\ce{C2O4^{2-}}$, it is bidentate. When several donor atoms are present in one ligand it is polydentate; the ethylenediaminetetraacetate ion, $\ce{EDTA^{4-}}$, is an important hexadentate ligand that binds through two nitrogen and four oxygen atoms.

Figure 1 — Denticity Bidentate — en (2 N donors) M N N C-C one 5-membered chelate ring Hexadentate — EDTA (2 N + 4 O) M N N O O O O six donor atoms wrap one metal

A bidentate ligand grips through two donor atoms forming one chelate ring; EDTA's six donor atoms enclose the metal in several rings, giving exceptional stability.

Chelate ligands and the chelate effect

When a di- or polydentate ligand uses two or more donor atoms simultaneously to bind a single metal ion, it is a chelate ligand (from the Greek chele, "claw"), and the resulting complex is a chelate complex. NCERT states the key consequence plainly: chelate complexes tend to be more stable than similar complexes containing unidentate ligands. This extra stability is the chelate effect. It is the reason $\ce{[Co(en)3]^{3+}}$ outranks $\ce{[Co(NH3)6]^{3+}}$ in stability even though both bind cobalt through six nitrogen donors, and the reason $\ce{[CoCl2(en)2]NO3}$ was the answer to a 2023 "most stable complex" question.

NEET Trap

Denticity is the ligand's property; coordination number is the metal's

Students routinely report the coordination number of $\ce{[Co(en)3]^{3+}}$ as 3 because there are three en ligands. But coordination number counts donor atoms, and each en supplies two. The correct value is $3 \times 2 = 6$.

Coordination number = (number of ligands) × (denticity of each), summed over all ligands.

Ambidentate ligands

A ligand that has two different donor atoms but binds through only one of them at a time is an ambidentate ligand. The classic NEET examples are the nitrite and thiocyanate ions. The nitrite ion $\ce{NO2^-}$ can coordinate through nitrogen (nitro, written $\ce{-NO2}$) or through oxygen (nitrito, written $\ce{-ONO}$). The thiocyanate ion $\ce{SCN^-}$ can coordinate through sulphur ($\ce{-SCN}$) or through nitrogen ($\ce{-NCS}$). Because the donor atom can switch, ambidentate ligands give rise to linkage isomerism — the red and yellow forms of $\ce{[Co(NH3)5(NO2)]Cl2}$ being the textbook case.

Build on this

Once the vocabulary is fixed, the next step is converting these terms into systematic names. See Nomenclature of Coordination Compounds.

Coordination number

The coordination number (CN) of a metal ion in a complex is the number of ligand donor atoms to which the metal is directly bonded. In $\ce{[PtCl6]^{2-}}$ and $\ce{[Ni(NH3)4]^{2+}}$ the coordination numbers of Pt and Ni are 6 and 4. In $\ce{[Fe(C2O4)3]^{3-}}$ and $\ce{[Co(en)3]^{3+}}$ the coordination number of both Fe and Co is 6, because oxalate and en are bidentate.

One refinement that NCERT stresses: the coordination number is determined only by the number of sigma bonds formed by the ligand with the central atom. Pi bonds, if formed, are not counted. The most common coordination numbers in NEET problems are 2, 4 and 6.

ComplexLigand(s)DenticityCoordination number
$\ce{[Ag(NH3)2]^+}$$\ce{NH3}$12
$\ce{[Ni(NH3)4]^{2+}}$$\ce{NH3}$14
$\ce{[PtCl6]^{2-}}$$\ce{Cl^-}$16
$\ce{[Co(en)3]^{3+}}$en26
$\ce{[Fe(C2O4)3]^{3-}}$oxalate26

Coordination sphere and counter ions

The central atom together with its attached ligands, enclosed in the square bracket, is the coordination sphere. The ionisable groups written outside the bracket are the counter ions. In $\ce{K4[Fe(CN)6]}$ the coordination sphere is $\ce{[Fe(CN)6]^{4-}}$ and the counter ion is $\ce{K^+}$. The counter ions balance the charge of the coordination sphere and dissociate freely in water, whereas the sphere stays intact — exactly the behaviour Werner's cobalt-ammine series demonstrated.

Figure 2 — Anatomy of a formula K 4 [ Fe ( CN ) 6 ] counter ion (K⁺) coordination sphere [Fe(CN)₆]⁴⁻ central ion ligand × 6

In $\ce{K4[Fe(CN)6]}$, the bracket separates the intact coordination sphere from the dissociable counter ions. The number of counter ions sets the solution conductance.

Coordination polyhedron and geometry

The spatial arrangement of the ligand donor atoms directly attached to the central atom defines the coordination polyhedron. The three most common polyhedra are octahedral, square planar and tetrahedral. For example, $\ce{[Co(NH3)6]^{3+}}$ is octahedral, $\ce{[Ni(CO)4]}$ is tetrahedral and $\ce{[PtCl4]^{2-}}$ is square planar. Coordination number and polyhedron are linked but distinct: a coordination number of 4 may be tetrahedral or square planar, which is why the geometry of $\ce{[Ni(CO)4]}$ versus $\ce{[Ni(CN)4]^{2-}}$ has been examined repeatedly in NEET.

Coordination numberPolyhedronExample
2Linear$\ce{[Ag(NH3)2]^+}$
4Tetrahedral$\ce{[Ni(CO)4]}$, $\ce{[NiCl4]^{2-}}$
4Square planar$\ce{[Ni(CN)4]^{2-}}$, $\ce{[PtCl4]^{2-}}$
6Octahedral$\ce{[Co(NH3)6]^{3+}}$, $\ce{[Fe(CN)6]^{3-}}$

Oxidation number and charge

The oxidation number of the central atom is the charge it would carry if all the ligands were removed along with the electron pairs they share with the metal. It is written as a Roman numeral in parentheses after the name of the entity — for example, copper in $\ce{[Cu(CN)4]^{3-}}$ is $+1$, written Cu(I). The working rule, stated in NIOS §22.2, is an algebraic balance:

(oxidation number of metal) + (sum of ligand charges) = (overall charge of the coordination entity)

Neutral ligands — $\ce{NH3}$, $\ce{H2O}$, $\ce{CO}$, en — contribute zero, so when only neutral ligands are present the metal's oxidation number equals the charge on the complex ion. The charge of the complex ion itself is simply the sum of the metal's oxidation number and all the ligand charges; this charge then determines how many counter ions appear in the neutral compound.

NEET Trap

Count ligand charges, not ligand atoms

In $\ce{[PtCl6]^{2-}}$ each chloride is $-1$, so six chlorides total $-6$. The error is to forget that the overall $-2$ is the net charge: $x + (-6) = -2$ gives $x = +4$, so platinum is Pt(IV). Mixing up the entity charge with the metal oxidation number is the single most common slip in this section.

Always write the balance explicitly: $x + (\text{ligand charges}) = (\text{entity charge})$, then solve for $x$.

Homoleptic vs heteroleptic

A complex in which the metal is bound to only one kind of donor group is homoleptic, e.g. $\ce{[Co(NH3)6]^{3+}}$ or $\ce{[Ni(CO)4]}$. A complex in which the metal is bound to more than one kind of donor group is heteroleptic, e.g. $\ce{[Co(NH3)4Cl2]^+}$. The distinction depends only on the variety of ligands, never on their number — a point NEET tested verbatim in 2024 with a statement-pair question on $\ce{[Co(NH3)6]^{3+}}$ and $\ce{[Co(NH3)4Cl2]^+}$.

Worked examples

Worked Example 1

Find the oxidation number of the central metal and the coordination number in $\ce{K3[Fe(CN)6]}$.

Charge balance: potassium is $+1$, and three $\ce{K^+}$ balance a sphere of charge $-3$, so $\ce{[Fe(CN)6]^{3-}}$. Each $\ce{CN^-}$ is $-1$; with six of them, $x + 6(-1) = -3$, giving $x = +3$. Iron is Fe(III).

Coordination number: cyanide is unidentate, so six $\ce{CN^-}$ give CN = 6 (octahedral).

Worked Example 2

Determine the oxidation number and coordination number of cobalt in $\ce{[Co(en)2(H2O)(CN)]^{2+}}$.

Charge balance: en and $\ce{H2O}$ are neutral; $\ce{CN^-}$ is $-1$. So $x + 0 + 0 + (-1) = +2$, giving $x = +3$. Cobalt is Co(III).

Coordination number: two bidentate en supply $2 \times 2 = 4$ donor atoms, plus one from $\ce{H2O}$ and one from $\ce{CN^-}$, totalling 6. The complex is octahedral and heteroleptic.

Worked Example 3

For $\ce{[CoBr2(en)2]^+}$, state the oxidation number, coordination number and whether it is homoleptic or heteroleptic.

Charge balance: each $\ce{Br^-}$ is $-1$ (two give $-2$); en is neutral. So $x + (-2) + 0 = +1$, giving $x = +3$, i.e. Co(III).

Coordination number: two $\ce{Br^-}$ (unidentate) plus two en (bidentate, $2\times2$) = $2 + 4 = 6$. Two kinds of ligand are present, so the complex is heteroleptic.

Quick Recap

The seven terms at a glance

  • Central atom/ion — the Lewis-acid metal centre, e.g. $\ce{Fe^{3+}}$ in $\ce{[Fe(CN)6]^{3-}}$.
  • Ligand & denticity — donor species; unidentate (1), bidentate (en, ox = 2), hexadentate (EDTA = 6).
  • Ambidentate — two possible donor atoms, one used at a time: $\ce{NO2^-}$ (N or O), $\ce{SCN^-}$ (S or N).
  • Coordination number — total donor atoms sigma-bonded to the metal; common values 2, 4, 6.
  • Coordination sphere / counter ions — inside vs outside the bracket; counter ions dissociate, the sphere does not.
  • Oxidation number & charge — solve $x + (\text{ligand charges}) = (\text{entity charge})$.
  • Homoleptic vs heteroleptic — one kind of ligand vs more than one.

NEET PYQ Snapshot — Important Terms

Real NEET questions that turn directly on ligand denticity, counter ions and the homoleptic/heteroleptic distinction.

NEET 2021

Ethylenediaminetetraacetate (EDTA) ion is:

  1. Tridentate ligand with three "N" donor atoms
  2. Hexadentate ligand with four "O" and two "N" donor atoms
  3. Unidentate ligand
  4. Bidentate ligand with two "N" donor atoms
Answer: (2)

EDTA4− binds a single metal ion through six donor atoms — four carboxylate oxygens and two amine nitrogens — making it hexadentate.

NEET 2024

Statement I: $\ce{[Co(NH3)6]^{3+}}$ is a homoleptic complex whereas $\ce{[Co(NH3)4Cl2]^+}$ is a heteroleptic complex. Statement II: $\ce{[Co(NH3)6]^{3+}}$ has only one kind of ligand but $\ce{[Co(NH3)4Cl2]^+}$ has more than one kind of ligand.

  1. Both Statement I and Statement II are true
  2. Both Statement I and Statement II are false
  3. Statement I is true but Statement II is false
  4. Statement I is false but Statement II is true
Answer: (1)

A single ligand type ($\ce{NH3}$) makes the first complex homoleptic; two ligand types ($\ce{NH3}$ and $\ce{Cl^-}$) make the second heteroleptic. Statement II correctly explains Statement I.

NEET 2023

The homoleptic complex from the following is:

  1. Triamminetriaquachromium(III) chloride
  2. Potassium trioxalatoaluminate(III)
  3. Diamminechloridonitrito-N-platinum(II)
  4. Pentaamminecarbonatocobalt(III) chloride
Answer: (2)

$\ce{K3[Al(C2O4)3]}$ contains only oxalate as the donor group, so it is homoleptic. The other entities each carry two or more distinct ligand types.

NEET 2017

The correct order of the stoichiometries of AgCl formed when excess $\ce{AgNO3}$ is treated with $\ce{CoCl3.6NH3}$, $\ce{CoCl3.5NH3}$, $\ce{CoCl3.4NH3}$ respectively is:

  1. 2AgCl, 3AgCl, 1AgCl
  2. 1AgCl, 3AgCl, 2AgCl
  3. 3AgCl, 1AgCl, 2AgCl
  4. 3AgCl, 2AgCl, 1AgCl
Answer: (4)

Only chloride present as a counter ion precipitates as AgCl. $\ce{[Co(NH3)6]Cl3}$ has 3 counter $\ce{Cl^-}$ (3 AgCl); $\ce{[Co(NH3)5Cl]Cl2}$ has 2 (2 AgCl); $\ce{[Co(NH3)4Cl2]Cl}$ has 1 (1 AgCl). Chloride inside the sphere does not precipitate.

FAQs — Important Terms

The definitions students most often confuse, settled in one or two lines each.

What is the difference between denticity and coordination number?
Denticity is a property of the ligand — the number of donor atoms a single ligand uses to bind the metal. Coordination number is a property of the metal — the total number of ligand donor atoms directly sigma-bonded to it. They coincide only for unidentate ligands. In [Co(en)3]3+, en is bidentate (denticity 2), three en ligands give coordination number 6.
Why is the coordination number of [Fe(C2O4)3]3- equal to 6 and not 3?
Oxalate (C2O4 2-) is a bidentate ligand: each oxalate uses two oxygen donor atoms to bind iron. Three oxalates therefore supply 3 x 2 = 6 donor atoms, so the coordination number is 6. Counting ligands (3) instead of donor atoms (6) is the trap.
How do you calculate the oxidation number of the central metal?
The oxidation number is the charge the central atom would carry if every ligand were removed with its shared electron pair. Set: (oxidation number of metal) + (sum of ligand charges) = (overall charge of the coordination entity). Neutral ligands like NH3, H2O and CO contribute zero. For [Fe(CN)6]3-, x + 6(-1) = -3, so x = +3.
What is an ambidentate ligand? Give examples.
An ambidentate ligand has two different donor atoms but binds through only one at a time. The nitrite ion NO2- can bind through nitrogen (nitro, -NO2) or oxygen (nitrito, -ONO), and the thiocyanate ion SCN- can bind through sulphur (-SCN) or nitrogen (-NCS). This choice of donor atom gives rise to linkage isomerism.
Why is EDTA a hexadentate ligand?
Ethylenediaminetetraacetate ion (EDTA 4-) can bind a single metal ion through six donor atoms simultaneously — two nitrogen atoms and four carboxylate oxygen atoms. Because it wraps the metal in multiple chelate rings, the resulting complexes are exceptionally stable, which is why EDTA is used in water-hardness titrations and lead-poisoning therapy.
What is the difference between a homoleptic and a heteroleptic complex?
A homoleptic complex has only one kind of donor group, e.g. [Co(NH3)6]3+. A heteroleptic complex has more than one kind of donor group, e.g. [Co(NH3)4Cl2]+. The classification depends only on the variety of ligands, not on their number.
Related Topics
Coordination Compounds Back to the full chapter hub. Werner's Theory Primary and secondary valence — the origin of the coordination sphere. Nomenclature Turn these terms into systematic IUPAC names. Isomerism Linkage isomerism from ambidentate ligands and more. d- and f-Block Elements Why transition metals form complexes so readily.