Arrange the following complexes in the increasing order of conductivity of their solution: [Co(NH₃)₃Cl₃], [Co(

NCERT Exemplar · Q15

[Co(NH_3)_3Cl_3]

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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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Conductivity depends on the number of ions produced per formula unit in solution. Counting the ions from each complex gives the order: $[Co(NH_3)_3Cl_3] < [Co(NH_3)_4Cl_2]Cl < [Cr(NH_3)_5Cl]Cl_2 < [Co(NH_3)_6]Cl_3$ .

Why conductivity tells us about the complex

When a coordination compound dissolves in water, the complex ion stays intact (usually), but the counter ions — the ones outside the coordination sphere — break off and become free ions in solution. The more free ions a compound releases, the higher its electrical conductivity.

So the trick is simple: look at the formula, identify which ligands are inside the coordination sphere (written inside the square brackets) and which are outside. The ones outside are the ones that will dissociate.

Watch out

A common mistake is to count all chlorine atoms as dissociating. But if chlorine is inside the coordination sphere (like in $[Co(NH_3)_3Cl_3]$ ), it is covalently bonded to the metal and does not dissociate. Only the chlorines written outside the brackets become chloride ions.

Step-by-step breakdown

1. $[Co(NH_3)_3Cl_3]$ — This complex has no ions outside the brackets. Everything is inside the coordination sphere. When dissolved, it remains as a neutral molecule. It produces 0 ions in solution. Conductivity: negligible.

2. $[Co(NH_3)_4Cl_2]Cl$ — Here, one chlorine is outside the brackets. That chlorine dissociates as $\text{Cl}^-$ . The complex ion $[Co(NH_3)_4Cl_2]^+$ stays intact. So total ions per formula unit = 2 (one cation, one anion).

3. $[Cr(NH_3)_5Cl]Cl_2$ — Two chlorines are outside the brackets. They dissociate as $2\text{Cl}^-$ . The complex ion $[Cr(NH_3)_5Cl]^{2+}$ remains. Total ions = 3 (one cation, two anions). …

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Here are the most common mistakes students make when solving this type of conductivity ordering problem, along with how to avoid each.

Mistake 1: Confusing "Conductivity" with "Concentration of Complex"

The Mistake: Students think the larger or more complex the coordination sphere, the higher the conductivity. They might order by molecular weight or number of ligands.
Why it’s wrong: Conductivity depends on the number of ions the complex dissociates into in solution, not the size of the complex ion itself.
How to avoid: Always ask: "How many total ions does this formula produce when dissolved?" Conductivity ∝ (number of ions).

Mistake 2: Forgetting to Count the Counter Ions Outside the Bracket

The Mistake: Only counting the ions inside the coordination sphere. For example, seeing $[Co(NH_3)_4Cl_2]Cl$ and thinking it gives only 1 ion.
Why it’s wrong: The $Cl$ outside the bracket is a free chloride ion. The complex dissociates into $[Co(NH_3)_4Cl_2]^+$ and $Cl^-$ → 2 ions.
How to avoid: Circle every atom/group outside the square brackets. Those are the ions that will dissociate. Count them first.

Mistake 3: Misidentifying the Charge on the Complex Ion

The Mistake: Guessing the charge without using oxidation states. For example, assuming $[Co(NH_3)_3Cl_3]$ is neutral because it "looks balanced."
Why it’s wrong: You must calculate the charge on the complex ion using the oxidation state of the metal and the charges of the ligands.
How to avoid: Use the formula:

Charge on complex = (Oxidation state of metal) + (Sum of charges of all ligands inside bracket)

For $[Co(NH_3)_3Cl_3]$ :
- Co is +3 (common for Co in ammine complexes)
- $NH_3$ is neutral (0)
- $Cl^-$ is -1 each, three of them = -3
- Total = +3 + (-3) = 0 → neutral complex → 0 ions in solution.

Mistake 4: Forgetting That Neutral Complexes Give Zero Conductivity

The Mistake: Placing $[Co(NH_3)_3Cl_3]$ somewhere in the middle of the order, thinking it gives some conductivity.
Why it’s wrong: A neutral complex does not dissociate into ions. It contributes zero to conductivity.
How to avoid: Always check if the overall charge on the complex is zero. If yes, it is a non-electrolyte → lowest conductivity.

Mistake 5: Reversing the Order (Increasing vs Decreasing) …

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

AHSEC Higher Secondary (HS) Final Examination 2024Set ANNUAL1 mark
$Q. Write the chemical formula for the following coordination compound: Mercury (I) tetrathiocyanato-s-cobaltate (III)$
›Reveal solutionSolution
The complex anion [Co(NCS)4]⁻ (Co³⁺ + 4 SCN⁻, S-bonded) carries charge −1, balanced by one Hg(I) unit, giving Hg[Co(NCS)4].

Breaking the name down:

"Cobaltate(III)" → the central metal is cobalt in the +3 oxidation state, named as an anion (suffix -ate) because the whole complex ion is negatively charged.

"Tetrathiocyanato-S-" → four thiocyanate ligands (SCN⁻), each attached to cobalt through the sulfur atom (hence written as NCS⁻ or specified "-S-"), each ligand carrying a charge of −1.

Charge on the complex ion = (charge on Co) + 4 × (charge on each SCN⁻) = (+3) + 4(−1) = −1, so the anion is [Co(NCS)4]⁻. …
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AHSEC Higher Secondary (HS) Final Examination 2022Set ANNUAL1 mark
$Q. Write down the formula of Tetraamineaquachloridocobalt(III) chloride.$
›Reveal solutionSolution
Build the complex ion from the named ligands, balance its charge with Co(III), then add the counter chloride ions.

Read the name piece by piece:

Tetraammine → 4 NH3 ligands (neutral)

Aqua → 1 H2O ligand (neutral)

Chlorido → 1 Cl⁻ ligand (charge −1)

Cobalt(III) → central Co with oxidation state +3

This gives a hexacoordinate (octahedral, 4+1+1 = 6 ligands) complex ion: [Co(NH3)4(H2O)Cl].

Charge on the complex ion = (charge on Co) + (sum of ligand charges) = (+3) + (0×4 + 0 + (−1)) = +2.

…
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AHSEC Higher Secondary (HS) Final Examination 2022Set ANNUAL1 mark
$Q. Write the IUPAC name of the following compound: [Pt(NO2)(NH3)BrCl]$
›Reveal solutionSolution
List the 4 ligands alphabetically, find Pt's oxidation state by charge balance (ligand charges sum to −3, complex is neutral, so Pt = +3), and combine.

Ligands present: ammine (NH3, neutral), bromido (Br⁻, charge −1), chlorido (Cl⁻, charge −1), nitro (NO2⁻ bonded through N, charge −1). This gives a 4-coordinate complex.

Oxidation state of Pt: the complex [Pt(NO2)(NH3)BrCl] is written with no charge and no counter-ion, so the whole species is neutral. Sum of ligand charges = 0 (NH3) + (−1)(Br) + (−1)(Cl) + (−1)(NO2) = −3.

Oxidation state of Pt + (−3) = 0 (overall charge) ⇒ Oxidation state of Pt = +3.

Naming (IUPAC rules for coordination compounds):

Name ligands in alphabetical order (ignoring multiplying prefixes): ammine, bromido, chlorido, nitro. …
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AHSEC Higher Secondary (HS) Final Examination 2019Set ANNUAL1 mark
$Q. Write the IUPAC names of [Co(NH3)4Cl(NO2)]Cl and K2[NiCl4].$
›Reveal solutionSolution
Both complexes are named by (1) naming ligands alphabetically with multiplying prefixes, (2) naming the metal with its oxidation state in brackets, (3) cation before anion — the metal's oxidation state is found by balancing the overall charge.

[Co(NH3)4Cl(NO2)]Cl This is a complex cation [Co(NH3)4Cl(NO2)]⁺ paired with one chloride counter-ion (shown by the Cl outside the bracket). Ligands inside the coordination sphere: 4 NH3 (ammine, neutral), 1 Cl⁻ (chloro, −1), 1 NO2⁻ bonded through N (nitrito-N, −1). Oxidation state of Co: overall complex-ion charge = +1 (since it balances one Cl⁻ outside). Let Co = x: x + 4(0) + (−1) + (−1) = +1 ⟹ x = +3. Ligands are cited alphabetically by first letter of ligand name (ignoring multiplying prefixes): ammine (a) < chloro (c) < nitrito-N (n). Name: tetraamminechloronitrito-N-cobalt(III) chloride

K2[NiCl4] The complex anion is [NiCl4]²⁻ (charge −2, balancing the 2 K⁺ outside). …
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←Previous questionQ14 — IUPAC name of [Pt(NH₃)₂Cl(NO₂)] is: (i) Platinum diaminechloronitrite …→Next questionQ16 — A coordination compound CrCl₃ · 4H₂O precipitates silver chloride when…

Q.Arrange the following complexes in the increasing order of conductivity of their solution: $[Co(NH_3)_3Cl_3]$ , $[Co(NH_3)_4Cl_2]Cl$ , $[Co(NH_3)_6]Cl_3$ , $[Cr(NH_3)_5Cl]Cl_2$

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 …

Why conductivity tells us about the complex

Step-by-step breakdown

Method: Conductivity Based on Number of Ions in Solution

Steps

Mistake 1: Confusing "Conductivity" with "Concentration of Complex"

Mistake 2: Forgetting to Count the Counter Ions Outside the Bracket

Mistake 3: Misidentifying the Charge on the Complex Ion

Mistake 4: Forgetting That Neutral Complexes Give Zero Conductivity

Mistake 5: Reversing the Order (Increasing vs Decreasing) …

Why conductivity tells us about the complex

Step-by-step breakdown

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