The compounds [Co(SO₄)(NH₃)₅]Br and [Co(SO₄)(NH₃)₅]Cl represent (i) linkage isomerism (ii) ionisation isomeris

NCERT Exemplar · Q10

[Co(SO_4)(NH_3)_5]Br

(i) linkage isomerism

(ii) ionisation isomerism

(iii) coordination isomerism

(iv) no isomerism

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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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These two compounds have the SAME coordination sphere, $[Co(SO_4)(NH_3)_5]^{+}$ , but DIFFERENT counter-ions ( $Br^-$ vs. $Cl^-$ ). Because the counter-ion is different, the two compounds don't even share the same overall molecular formula — so they cannot be isomers of each other at all. The correct answer is (iv) no isomerism.

Isomers, by definition, must have the same molecular formula but a different arrangement of atoms or ions. Before deciding which kind of isomerism relates two compounds, you must first check whether they are isomers at all.

Step-by-step reasoning

Identify the coordination sphere in each compound.

In $[Co(SO_4)(NH_3)_5]Br$ , the square brackets enclose $Co^{3+}$ bonded to one sulfate ion ( $SO_4^{2-}$ , acting as a ligand) and five ammonia molecules. Bromide sits outside the sphere as the counter-ion.

In $[Co(SO_4)(NH_3)_5]Cl$ , the coordination sphere is identical — same metal, same ligands (one sulfate, five ammines), same charge on the complex ion. Only the counter-ion is chloride instead of bromide.
Compare the overall molecular formulas.

The first compound's formula contains one bromine atom; the second contains one chlorine atom instead. These are simply two different chemical formulas — a bromide salt and a chloride salt of the same complex cation.
Check each isomerism type against this pair:
- Linkage isomerism requires an ambidentate ligand (like $NO_2^-/ONO^-$ or $SCN^-/NCS^-$ ) bonding through a different atom in the two compounds. No ambidentate ligand is present here.
- Ionisation isomerism requires the same overall molecular formula, with an anion exchanging roles — a ligand in one compound, a free counter-ion in the other — e.g. $[Co(NH_3)_5Br]SO_4$ vs. $[Co(NH_3)_5SO_4]Br$ (both compounds share the formula $CoBrSO_4(NH_3)_5$ ). In our pair, sulfate stays a ligand in both compounds — nothing ever moves in or out of the coordination sphere. Only the identity of the free halide itself changes ( $Br^-$ vs. $Cl^-$ ), which makes the overall formula different between the two compounds — this fails the basic requirement for ionisation isomerism.
- Coordination isomerism requires both the cation and the anion to be complex ions that exchange ligands between them (e.g. $[Co(NH_3)_6][Cr(CN)_6]$ vs. $[Cr(NH_3)_6][Co(CN)_6]$ ). Here the counter-ions ( $Br^-$ , $Cl^-$ ) are simple monatomic anions, not complex ions — this is impossible. …

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Here are the common mistakes students make on this exact question, along with the conceptual fixes to avoid them.

Mistake 1: Confusing the Counter Ion with the Ligand

The Error: Students see $SO_4$ inside the coordination sphere and immediately think "sulfate is a ligand." They then compare the two compounds and think the sulfate is different in each, leading them to choose linkage isomerism (option i).

Why it’s wrong: In these complexes, sulfate ( $SO_4^{2-}$ ) is acting as a monodentate ligand (bonded through one oxygen). It is inside the square brackets, so it is part of the coordination sphere. The difference between the two compounds is the ion outside the brackets: $Br^-$ vs $Cl^-$ .

How to avoid: Always draw a vertical line mentally at the square brackets. Everything inside is the complex ion; everything outside is the counter ion. Ask: "What is different?" Here, the complex ion $[Co(SO_4)(NH_3)_5]^+$ is identical in both. Only the counter ion changes.

Mistake 2: Forgetting the Definition of Ionisation Isomerism

The Error: Students know the counter ion is different, but they think "ionisation isomerism" means the ligand changes its bonding mode (like $SO_4$ switching from inside to outside). They don't realize that ionisation isomerism specifically refers to the exchange of an ion between the coordination sphere and the outside.

Why it’s wrong: Ionisation isomerism occurs when a compound can be written in two ways: one where an anion is a ligand, and another where that same anion is the counter ion. For example:

$[Co(NH_3)_5Br]SO_4$ (bromide is ligand, sulfate is counter ion)
$[Co(NH_3)_5SO_4]Br$ (sulfate is ligand, bromide is counter ion)

In the given question, both compounds have sulfate as a ligand and halide as counter ion. No exchange has occurred. The complex ion is the same; only the counter ion is different.

How to avoid: Memorise the definition: Ionisation isomerism = exchange of an ion between the coordination sphere and the outside. If the complex ion is identical, it cannot be ionisation isomerism.

Mistake 3: Assuming "Different Counter Ions" Means Isomerism

The Error: Students see $Br$ and $Cl$ and think "different atoms = isomerism." They then pick coordination isomerism (option iii) because they vaguely remember that coordination compounds with different ligands can be isomers.

Why it’s wrong: Coordination isomerism occurs when the ligands are exchanged between the cation and anion in a coordination compound. For example:

$[Co(NH_3)_6][Cr(CN)_6]$ vs $[Cr(NH_3)_6][Co(CN)_6]$

Here, the cation and anion are both complex ions, and ligands swap between them. In the given question, the counter ions ( $Br^-$ and $Cl^-$ ) are simple ions, not complex ions. So coordination isomerism is impossible.

How to avoid: Check if both the cation and anion are complex ions (containing a metal). If one is a simple ion (like $Br^-$ , $Cl^-$ , $NO_3^-$ ), coordination isomerism is ruled out. …

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

KEAM 2026Set eng-2026-04184 marksMCQ
$[Ag(NH_3)_2][Ag(CN)_2]$
›Reveal solutionSolution

Both silver centres are +1; use current IUPAC ligand names (ammine, cyanido) and the -ate suffix for the anion.

Cation $[Ag(NH_3)_2]^+$ : two $NH_3$ = diammine, Ag oxidation state $+1$ $\Rightarrow$ diamminesilver(I).

Anion $[Ag(CN)_2]^-$ : two $CN^-$ = dicyanido (modern IUPAC), Ag $+1$ , anionic complex takes -ate on the Latin stem argent $\Rightarrow$ dicyanidoargentate(I). …
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KEAM 2026Set eng-2026-04204 marksMCQ
$[CoCl_2(en)_2]Cl$
›Reveal solutionSolution
Correct IUPAC name = Dichloridobis(ethane-1,2-diamine)cobalt(III)chloride.

Oxidation state of Co: complex $[CoCl_2(en)_2]^{+}$ (charge $+1$ balancing the outer $Cl^{-}$ ); with $2(-1)$ from chlorido and neutral en, Co $=+3$ .

Naming rules:

Ligands cited alphabetically: chlorido (c) before ethane-1,2-diamine (e), so "dichlorido" then "bis(ethane-1,2-diamine)". …
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KEAM 2026Set eng-2026-04214 marksMCQ
$[Co(NH_3)_5(CO)]Cl_2$
›Reveal solutionSolution

Charge balance gives $[Co(NH_3)_5(CO_3)]Cl$ .

Co is $+3$ , five neutral $NH_3$ , carbonato $CO_3^{2-}$ :

$+3 + 5(0) + (-2) = +1.$ …
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KEAM 2026Set pha-2026-0420F4 marksMCQ
$Q. The IUPAC name of the complex [Co(NH3)5ONO]Cl2 is (A) Pentaamminenitritocobalt (III) chloride (B) Pentaamminenitritocobalt (II) chloride (C) Pentaamminenitrocobalt (III) chloride (D) O-Nitritopentaamminecobalt (III) chloride (E) Pentaamminemononitritocobalt (III) chloride$
›Reveal solutionSolution

Co is +3; O-bonded ONO is 'nitrito'; alphabetical ligand order gives Pentaamminenitritocobalt(III) chloride.

Determine the metal oxidation state: NH $_3$ is neutral, the ONO ligand (bonded through oxygen) is nitrito with charge $-1$ , and the two chlorides are counterions ( $-1$ each). For overall neutrality the complex cation is $+2$ , so

$\text{Co} + 5(0) + (-1) = +2 \quad\Rightarrow\quad \text{Co} = +3.$ …
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KEAM 2025Set eng-2025-04254 marksMCQ
$[Co(en)_3]_2(SO_4)_3$
›Reveal solutionSolution
With three neutral $en$ ligands and a sulphate counter-ion, cobalt is in the $+3$ state; the IUPAC name is tris(ethane-1,2-diamine)cobalt(III) sulphate.

In $[Co(en)_3]_2(SO_4)_3$ :

$en$ (ethane-1,2-diamine) is a neutral bidentate ligand; there are three of them $\rightarrow$ prefix tris (as the ligand name contains 'di').

Charge: three $SO_4^{2-}$ give $-6$ shared over two complex cations, so each $[Co(en)_3]^{3+}$ , making cobalt $+3$ . …
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KEAM 2025Set pha-2025-0424F4 marksMCQ
$[Co(NH_3)_3(H_2O)_3]Cl_3$
›Reveal solutionSolution
Name ligands alphabetically (ammine < aqua) and assign Co the +3 oxidation state, so the complex is triamminetriaquacobalt(III) chloride.

For $[Co(NH_3)_3(H_2O)_3]Cl_3$ :

Ligands are named in alphabetical order using the ligand name (ignoring the multiplying prefix): ammine (a) comes before aqua (a-q) → "triammine" is cited before "triaqua".

Oxidation state of cobalt: overall charge zero; three $Cl^-$ counter-ions give the complex ion a +3 charge; $NH_3$ and $H_2O$ are neutral, so Co is +3.

Anion: chloride. …
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KEAM 2025Set pha-2025-0424A4 marksMCQ
$[Cr(H_2O)_3(NH_3)_3]Cl_3$
›Reveal solutionSolution
The IUPAC name is triamminetriaquachromium(III) chloride.

Naming $[Cr(H_2O)_3(NH_3)_3]Cl_3$ :

Ligands are cited in alphabetical order of ligand name: ammine (a) comes before aqua (a-q), so 'triammine' precedes 'triaqua'.

Oxidation state of Cr: the complex cation carries $+3$ (balanced by three $Cl^-$ ), so chromium is $Cr(III)$ . …
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KEAM 2024Set eng-2024-06054 marksMCQ
$[Co(NH_{3})_{4}(H_{2}O)Cl]Cl_{2}$
›Reveal solutionSolution

Alphabetical ligand order ammine < aqua < chlorido, Co oxidation state $+3$ , giving tetraammineaquachloridocobalt(III) chloride.

In $[Co(NH_3)_4(H_2O)Cl]Cl_2$ the two chloride counter-ions make the complex ion $+2$ . Charge balance: $x + 4(0) + 0 + (-1) = +2 \Rightarrow x = +3$ . …
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KEAM 2024Set eng-2024-06064 marksMCQ
$[Cr(NH_{3})_{3}(H_{2}O)_{3}]Cl_{3}$
›Reveal solutionSolution
In IUPAC nomenclature ligands are cited alphabetically (ammine < aqua), and the metal oxidation state is set by charge balance. For $[Cr(NH_3)_3(H_2O)_3]Cl_3$ , three chlorides outside give Cr = $+3$ .

Working it out:

Ligands: 3 $NH_3$ (ammine) and 3 $H_2O$ (aqua). Alphabetical order → triammine then triaqua. …
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KEAM 2024Set eng-2024-06084 marksMCQ
$Hg[Co(SCN)_4]$
›Reveal solutionSolution

$Hg[Co(SCN)_4]$ : Hg is $+2$ , so $[Co(SCN)_4]^{2-}$ makes Co $+2$ ; thiocyanate binds via sulphur, giving Mercury(II) tetrathiocyanato-S-cobaltate(II).

Assign oxidation states. Mercury is $Hg^{2+}$ , so the complex anion carries a $-2$ charge:
$[Co(SCN)_4]^{2-}$
Each thiocyanate ligand is $SCN^{-}$ , so four contribute $-4$ . Letting the cobalt state be $x$ :

$x + 4(-1) = -2 \;\Rightarrow\; x = +2$ …
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Q.The compounds $[Co(SO_4)(NH_3)_5]Br$ and $[Co(SO_4)(NH_3)_5]Cl$ represent

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 …

Step-by-step reasoning

Method: Check Molecular Formula Equality BEFORE Naming the Isomerism Type

Steps

Mistake 1: Confusing the Counter Ion with the Ligand

Mistake 2: Forgetting the Definition of Ionisation Isomerism

Mistake 3: Assuming "Different Counter Ions" Means Isomerism

Q.The compounds [Co(SO4)(NH3)5]Br[Co(SO_4)(NH_3)_5]Br[Co(SO4​)(NH3​)5​]Br and [Co(SO4)(NH3)5]Cl[Co(SO_4)(NH_3)_5]Cl[Co(SO4​)(NH3​)5​]Cl represent

Step-by-step reasoning

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Q.The compounds $[Co(SO_4)(NH_3)_5]Br$ and $[Co(SO_4)(NH_3)_5]Cl$ represent

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