Which of the following complexes are homoleptic? (i) [Co(NH₃)₆]³⁺ (ii) [Co(NH₃)₄Cl₂]⁺ (iii) [Ni(CN)₄]²⁻ (iv) [

NCERT Exemplar · Q36

Q. Which of the following complexes are homoleptic?

(i)

[Co(NH_3)_6]^{3+}

(ii)

[Co(NH_3)_4Cl_2]^{+}

(iii)

[Ni(CN)_4]^{2-}

(iv)

[Ni(NH_3)_4Cl_2]

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Concept understanding — Coordination Compound Nomenclature

Coordination Compound Nomenclature: From Intuition to Precision

Imagine you're naming a person. You'd say "Ravi Sharma" — family name first, then given name. Coordination compounds have a similar logic, but the "family name" is the metal, and the "given names" are the groups attached to it. The rules are just a systematic way of writing that name so any chemist anywhere can draw the exact structure from it.

The Core Idea

A coordination compound has a central metal ion surrounded by molecules or ions called ligands. Think of the metal as the nucleus and ligands as planets orbiting it. The entire assembly (metal + ligands) is called the coordination sphere, and it's written inside square brackets: [Co(NH₃)₆]Cl₃.

The nomenclature rules tell you:

What order to list things
How to name each ligand
How to indicate the metal's oxidation state
How to handle the counter-ions outside the brackets

The Rules, Step by Step

1. Cation before anion (just like NaCl is sodium chloride)

If the complex ion is positive, it's named first. If it's negative, it's named last. Simple.

2. Within the coordination sphere: ligands first, then metal

This is the big rule. Ligands are named before the metal, in alphabetical order (ignoring prefixes like di-, tri-).

Note

Alphabetical order is based on the ligand's name, not its formula. So NH₃ (ammine) comes before H₂O (aqua), even though N comes after H in the alphabet.

3. Naming ligands

Ligand type	Name	Example
Neutral molecule (NH₃)	ammine	`[Co(NH₃)₆]³⁺` → hexaamminecobalt(III)
Neutral molecule (H₂O)	aqua	`[Cu(H₂O)₄]²⁺` → tetraaquacopper(II)
Neutral molecule (CO)	carbonyl	`[Ni(CO)₄]` → tetracarbonylnickel(0)
Negative ion (Cl⁻)	chloro	`[PtCl₆]²⁻` → hexachloroplatinate(IV)
Negative ion (CN⁻)	cyano	`[Fe(CN)₆]⁴⁻` → hexacyanoferrate(II)
Negative ion (OH⁻)	hydroxo	`[Al(OH)₄]⁻` → tetrahydroxoaluminate(III)

Watch out

ammine (with two m's) is for NH₃ as a ligand. amine (one m) is for organic compounds like ethylamine. Don't mix them up — exam setters love this trap.

4. Prefixes for multiple ligands

Use Greek prefixes: di-, tri-, tetra-, penta-, hexa-, hepta-, octa-.

If the ligand name already contains a number (like ethylenediamine), use bis-, tris-, tetrakis- instead.

Tip

[Co(en)₃]³⁺ is tris(ethylenediamine)cobalt(III), not triethylenediaminecobalt(III). The parentheses around the ligand name are mandatory when using bis/tris/tetrakis.

5. Oxidation state of the metal

Write it in Roman numerals in parentheses right after the metal name. No space.

[Fe(CN)₆]³⁻ → hexacyanoferrate(III) (iron is in +3 state)

6. If the complex is an anion, change the metal's ending

Metal	Anionic form
Cobalt	cobaltate
Copper	cuprate
Iron	ferrate
Nickel	nickelate
Platinum	platinate
Zinc	zincate

General pattern: [M(L)ₙ]Xₘ \to cation name = [prefix-ligands]metal(oxidation state) anion name = [prefix-ligands]metal ate (oxidation state)

Worked Examples

Example 1: K₃[Fe(CN)₆]

Cation: potassium (K⁺)
Complex anion: [Fe(CN)₆]³⁻
Ligands: 6 cyano → hexacyano
Metal: iron → ferrate (because it's an anion)
Oxidation state: Fe is +3 (since 6 CN⁻ = -6, total charge -3, so Fe must be +3)
Answer: Potassium hexacyanoferrate(III)

Example 2: [Co(NH₃)₅Cl]Cl₂

Cation: [Co(NH₃)₅Cl]²⁺
Ligands: 5 ammine + 1 chloro → alphabetical: ammine before chloro → pentaamminechloro
Metal: cobalt
Oxidation state: Co is +3 (5 NH₃ neutral, 1 Cl⁻ = -1, total +2, so Co = +3) …

Why this formula?

Coordination Compound Nomenclature: Why the Rules Work

Coordination compound nomenclature isn't about a single formula — it's a system of rules built on a few core principles. Let's understand the why behind each major rule, so you never have to memorise blindly.

1. The Central Idea: Ligands as "Guests" Around a Metal "Host"

A coordination compound has a central metal atom/ion surrounded by ligands (molecules or ions that donate electron pairs). The naming reflects this relationship:

Cation first, then anion (like normal ionic compounds)
Ligands named before the metal (because they modify the metal's identity)

Why?

In chemistry, we name the more electropositive part first (cation). The metal-ligand complex is treated as a single unit — the ligands are "attached" to the metal, so they come first in the complex name.

2. Key Rule: Ligand Order — Alphabetical, Not by Charge

Rule: Ligands are named in alphabetical order (ignoring prefixes like di-, tri-).

Why?

If we ordered by charge or size, the name would change every time a ligand is replaced.
Alphabetical order is universal and unambiguous — it doesn't depend on the metal or oxidation state.
Example: [Co(NH₃)₄Cl₂]⁺ is tetraamminedichlorocobalt(III) — "ammine" (a) before "chloro" (c).

3. Oxidation State: Why Roman Numerals?

Rule: The metal's oxidation state is written in Roman numerals in parentheses after the metal name.

Why?

The oxidation state tells you the charge on the metal after accounting for ligand charges.
Roman numerals avoid confusion with Arabic numbers (which are used for ligand counts).
Example: [Fe(CN)₆]³⁻ → hexacyanoferrate(III) — the iron is Fe³⁺, not Fe²⁺.

Derivation of oxidation state:

Let the complex charge = $Q$ , ligand charges = sum of ligand charges $L$ , number of ligands = $n$ .

Then:

\text{Metal oxidation state} = Q - L

For [Fe(CN)₆]³⁻: CN⁻ has charge -1, so $L = 6 \times (-1) = -6$ , $Q = -3$ .

\text{Fe oxidation state} = -3 - (-6) = +3

4. Anionic Ligands: The "-o" Ending

Rule: Anionic ligands (negative ions) end in -o (e.g., Cl⁻ → chloro, CN⁻ → cyano, OH⁻ → hydroxo).

Why?

This distinguishes them from neutral ligands (e.g., NH₃ → ammine, H₂O → aqua).
The suffix -o signals "this ligand came from an anion" — crucial for charge balance.

Common examples:

Anion	Ligand name
Cl⁻	chloro
CN⁻	cyano
OH⁻	hydroxo
SO₄²⁻	sulfato

5. Neutral Ligands: Special Names

Rule: Neutral ligands keep their molecular name, except for a few with special names:

NH₃ → ammine (not "ammonia")
H₂O → aqua
CO → carbonyl
NO → nitrosyl

Why?

"Ammine" avoids confusion with ammonia (NH₃) as a free molecule.
These special names are historical but standardised — you must memorise them for exams.

6. Prefixes: di-, tri-, tetra-, etc.

Rule: Use Greek prefixes to indicate the number of each ligand:

2 → di, 3 → tri, 4 → tetra, 5 → penta, 6 → hexa

Why?

Without prefixes, [Co(NH₃)₆]³⁺ would be "hexaamminecobalt(III)" — the "hexa" tells you there are six ammines.
For ligands with complex names (e.g., ethylenediamine), use bis-, tris-, tetrakis- to avoid confusion.

Example:

[Co(en)₃]³⁺ → tris(ethylenediamine)cobalt(III) — "tris" because "triethylenediamine" would sound like three ethylenediamine molecules (which is correct, but "tris" is clearer).

7. Anionic Complexes: The "-ate" Suffix …

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Homoleptic complexes contain only one type of ligand. Among the given options, (i) $[Co(NH_3)_6]^{3+}$ and (iii) $[Ni(CN)_4]^{2-}$ are homoleptic; (ii) and (iv) have two different ligands and are heteroleptic.

The term homoleptic comes from Greek roots: homo (same) and leptos (taken/held). In coordination chemistry, a homoleptic complex is one where the metal ion is bound to only one kind of ligand. If even two different types of ligands are present, the complex is heteroleptic.

This is a classification based purely on ligand identity — not on charge, geometry, or oxidation state. So the task is simply to check each complex: does it contain only one type of ligand, or more than one?

Let’s examine each option.

Option (i): $[Co(NH_3)_6]^{3+}$

The only ligand here is ammonia ( $NH_3$ ). All six coordination sites are occupied by the same ligand.

→ Homoleptic.
Option (ii): $[Co(NH_3)_4Cl_2]^{+}$

Two different ligands are present: ammonia ( $NH_3$ ) and chloride ( $Cl^-$ ). Four sites have $NH_3$ , two have $Cl^-$ .

→ Heteroleptic.
Option (iii): $[Ni(CN)_4]^{2-}$

The only ligand is cyanide ( $CN^-$ ). All four coordination sites are identical in ligand type.

→ Homoleptic.
Option (iv): $[Ni(NH_3)_4Cl_2]$

Again, two different ligands: ammonia and chloride. Even though the complex is neutral, the ligand set is mixed. …

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Common Mistakes on Homoleptic vs. Heteroleptic Complexes

The Concept in One Line

A homoleptic complex has only one type of ligand attached to the central metal atom/ion. A heteroleptic complex has two or more different types of ligands.

Mistake #1: Confusing "homoleptic" with "only one ligand molecule"

The error:

Students think a complex is homoleptic if it contains only one molecule of a ligand (e.g., $[Co(NH_3)_6]^{3+}$ has 6 NH₃ molecules, but some think "one type" means "one molecule").

Why it's wrong:

"One type" refers to the chemical identity of the ligand, not the number of ligand molecules. $[Co(NH_3)_6]^{3+}$ has six identical NH₃ ligands — it is homoleptic.

How to avoid:

Ask: Are all ligands chemically the same?
Count the kinds of ligands, not the total number.

Mistake #2: Thinking charge or coordination number determines homoleptic nature

The error:

Students assume that if a complex has a charge (like $[Ni(CN)_4]^{2-}$ ) or a specific coordination number, it must be homoleptic.

Why it's wrong:

Charge and coordination number are unrelated to ligand diversity. $[Ni(CN)_4]^{2-}$ is homoleptic because all four ligands are CN⁻, not because it's charged.

How to avoid:

Ignore charge, geometry, and coordination number.
Focus only on whether the ligands are all the same.

Mistake #3: Misidentifying neutral complexes as heteroleptic

The error:

A neutral complex like $[Ni(NH_3)_4Cl_2]$ is often mistakenly called homoleptic because it is neutral.

Why it's wrong:

Neutrality does not imply ligand uniformity. Here, NH₃ and Cl⁻ are two different ligands — so it is heteroleptic.

How to avoid:

List the ligands explicitly: NH₃ and Cl⁻ are different.
If the list has more than one entry → heteroleptic.

Mistake #4: Overlooking the ligand identity in coordination compounds

The error: …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

GSEB Higher Secondary Certificate (HSC) Examination 2025Set ANNUAL1 markMCQ
$Q. How many total number of ions will be obtained by ionisation of Iron (III) - Hexacyanido Ferrate (II) in aqueous medium.$
(a) 2
(b) 7
(c) 5
(d) 3
›Reveal solutionSolution

Iron(III) hexacyanidoferrate(II) is Fe4[Fe(CN)6]3 (Prussian blue), which ionises into 4 Fe3+ ions and 3 [Fe(CN)6]4- complex ions - 7 ions total.

Iron(III) hexacyanidoferrate(II) = Fe4[Fe(CN)6]3.

On dissolving in water, it ionises as: Fe4[Fe(CN)6]3 -> 4Fe3+(aq) + 3[Fe(CN)6]4-(aq)

…

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GUJCET 2024Set 131 markMCQ
$[Co(en)_3]_3(SO_4)_2$
›Reveal solutionSolution

The complex cation is $[Co(en)_3]^{3+}$ and the anion is $SO_4^{2-}$ ; balance $+3$ and $-2$ to get a 2:3 ratio.

Concept: Ethane-1,2-diamine (en) is a neutral bidentate ligand, so with cobalt(III) the cation is $[Co(en)_3]^{3+}$ . To neutralise with $SO_4^{2-}$ : …
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GUJCET 2022Set 171 markMCQ
$NO_3^-$
›Reveal solutionSolution
[!TLDR] The ambidentate ligands here are $NO_2^-$ (Q) and $SCN^-$ (S), option (C).

Concept

An ambidentate ligand has two different potential donor atoms but coordinates through only one at a time, giving linkage isomers.

Solution

$NO_2^-$ (Q): binds through N (nitro, –NO $_2$ ) or through O (nitrito, –ONO). Ambidentate.

$SCN^-$ (S): binds through S (thiocyanato) or through N (isothiocyanato). Ambidentate. …
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GUJCET 2021Set 151 markMCQ
$[Co(NH_3)_5(CO_3)]Cl$
›Reveal solutionSolution

The complex ion charge is $+3+(-2)=+1$ , balanced by one chloride: [Co(NH₃)₅(CO₃)]Cl.

Concept: Pentaamine = 5 NH₃ (neutral), carbonato = CO₃²⁻ (charge −2), cobalt(III) = Co³⁺. …
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GSEB Higher Secondary Certificate (HSC) Examination 2020Set ANNUAL1 markMCQ
$Q. Molecular formula of Tetraamineaquachlorido cobalt (III) chloride is ______.$
(a) [Co(NH3)4(H2O)]Cl3
(b) [Co(NH3)4(H2O)Cl]Cl2
(c) [Co(NH3)4(H2O)Cl]Cl3
(d) [Co(NH3)4(H2O)Cl]3Cl2
›Reveal solutionSolution

Naming from the pieces: 'tetraammine' = 4 NH3, 'aqua' = 1 H2O, 'chlorido' = 1 Cl- as a ligand, cobalt(III) means Co3+; balancing charge fixes how many Cl- counter-ions are needed outside the bracket.

Ligands in the coordination sphere: 4 x NH3 (neutral) + 1 x H2O (neutral) + 1 x Cl- (as a ligand, charge -1) = coordination number 6. Overall charge of the complex ion = (charge of Co) + (sum of ligand charges) = (+3) + (-1) = +2. So …

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Q.Which of the following complexes are homoleptic?

Concept understanding — Coordination Compound Nomenclature

Coordination Compound Nomenclature: From Intuition to Precision

The Core Idea

The Rules, Step by Step

1. Cation before anion (just like NaCl is sodium chloride)

2. Within the coordination sphere: ligands first, then metal

3. Naming ligands

4. Prefixes for multiple ligands

5. Oxidation state of the metal

6. If the complex is an anion, change the metal's ending

Worked Examples

Why this formula?

Coordination Compound Nomenclature: Why the Rules Work

1. The Central Idea: Ligands as "Guests" Around a Metal "Host"

2. Key Rule: Ligand Order — Alphabetical, Not by Charge

3. Oxidation State: Why Roman Numerals?

4. Anionic Ligands: The "-o" Ending

5. Neutral Ligands: Special Names

6. Prefixes: di-, tri-, tetra-, etc.

7. Anionic Complexes: The "-ate" Suffix …

Concept: Homoleptic vs Heteroleptic Complexes

Method: Ligand-Type Inspection

Steps:

Applying to each option:

Common Mistakes on Homoleptic vs. Heteroleptic Complexes

The Concept in One Line

Mistake #1: Confusing "homoleptic" with "only one ligand molecule"

Mistake #2: Thinking charge or coordination number determines homoleptic nature

Mistake #3: Misidentifying neutral complexes as heteroleptic

Mistake #4: Overlooking the ligand identity in coordination compounds

Concept

Solution

Q.Which of the following complexes are homoleptic?

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