Which of the following species is not expected to be a ligand? (i) NO (ii) NH₄⁺ (iii) NH₂CH₂CH₂NH₂ (iv) CO

NCERT Exemplar · Q12

Q. Which of the following species is not expected to be a ligand?

(i)

NO

(ii)

NH_4^{+}

(iii)

NH_2CH_2CH_2NH_2

(iv)

CO

Rajasthan RbseMCQ· 1mImportance★★★★★

59% · 60/101 Questions

🔒 Locked · start free trial →

You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.

Start your 14-day free trial to unlock the full solution →

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 …

Start your 14-day free trial to unlock the full solution →

A ligand must have at least one lone pair of electrons to donate to a metal centre. $NH_4^{+}$ has no lone pair on nitrogen (all four bonds are used), so it cannot act as a ligand. The correct answer is (ii).

To decide whether a species can be a ligand, you need to recall the fundamental definition: a ligand is an ion or molecule that donates a pair of electrons to a central metal atom to form a coordinate bond. That means the species must possess at least one lone pair of electrons that is available for donation.

Let’s examine each option with this idea in mind.

Option (i): $NO$ (Nitric oxide)

Nitric oxide has an unpaired electron and a lone pair on nitrogen. It is a well-known ligand (often called nitrosyl) that can donate its lone pair to a metal. In fact, it forms stable complexes like $[Fe(NO)(H_2O)_5]^{2+}$ . So it is expected to be a ligand.
Option (ii): $NH_4^{+}$ (Ammonium ion)

In $NH_4^{+}$ , nitrogen is bonded to four hydrogen atoms via four sigma bonds. All four valence electrons of nitrogen are used in these bonds — there is no lone pair left. Without a lone pair, it cannot donate electrons to a metal.

Watch out

A common mistake is to confuse $NH_4^{+}$ with $NH_3$ . Ammonia ( $NH_3$ ) has a lone pair and is a classic ligand, but the ammonium ion has lost that lone pair by forming a fourth bond to $H^{+}$ . They are chemically different species.

Therefore, $NH_4^{+}$ cannot act as a ligand.
Option (iii): $NH_2CH_2CH_2NH_2$ (Ethylenediamine) …

Start your 14-day free trial to unlock the full solution →

Concept: Ligands and Donor Atoms

A ligand is a molecule or ion that donates a lone pair of electrons to a central metal atom/ion to form a coordinate bond. For a species to act as a ligand, it must have at least one lone pair of electrons available for donation.

Method: Lone Pair Availability Check

Step 1: Identify the donor atom(s) in each species.

Step 2: Check if the donor atom has a lone pair that is free to donate.

Step 3: If no lone pair exists, the species cannot act as a ligand.

Applying the method:

Species	Donor atom(s)	Lone pair available?	Ligand?
NO	N	Yes (N has a lone pair)	✓ Yes
NH₄⁺	N	No — all 4 bonds to H, no lone pair left	✗ No

Start your 14-day free trial to unlock the full solution →

Common Mistakes: Identifying Ligands in Coordination Chemistry

The Concept in Brief

A ligand is a molecule or ion that donates a lone pair of electrons to a central metal atom/ion to form a coordinate bond. For a species to act as a ligand, it must have at least one lone pair available for donation.

Mistake #1: Assuming All Neutral Molecules Are Ligands

The error: Students think that because $NO$ and $CO$ are neutral, they automatically qualify as ligands. They forget to check if the lone pair is actually available for donation.

Why it’s wrong:

$CO$ has a lone pair on carbon and is a classic ligand (carbonyl).
$NO$ also has a lone pair on nitrogen and acts as a ligand (nitrosyl). Both are valid ligands — the mistake is ruling them out without checking.

How to avoid:

Always check for lone pairs on the atom that would donate.
Neutral molecules can be ligands, but only if they have a free lone pair.

Mistake #2: Confusing $NH_4^+$ with $NH_3$

The error: Students see nitrogen and hydrogen and assume $NH_4^+$ can donate like $NH_3$ .

Why it’s wrong:

$NH_3$ has a lone pair on nitrogen → acts as a ligand.
$NH_4^+$ has no lone pair — all four bonds are used, and the positive charge means no extra electrons to donate.

How to avoid:

Count total valence electrons and bond pairs.
If every valence electron is used in bonding (and there’s a positive charge), there is no lone pair left.
Key check: $NH_4^+$ is not a ligand; $NH_3$ is.

Mistake #3: Overlooking the Role of Charge

The error: Students ignore the charge on a species when deciding if it can donate electrons.

Why it’s wrong:

A positive charge often means the species has lost electrons — making lone pairs less likely.
$NH_4^+$ is a classic example: it’s electron-deficient, not electron-rich.

How to avoid:

For cations, ask: Does this ion still have a lone pair?
For anions (like $Cl^-$ , $CN^-$ ), they usually have extra lone pairs → good ligands.
For neutral species, check the Lewis structure carefully.

Mistake #4: Thinking “Bigger Molecule = Always a Ligand” …

Start your 14-day free trial to unlock the full solution →

←Previous questionQ11 — A chelating agent has two or more than two donor atoms to bind to a si…→Next questionQ13 — What kind of isomerism exists between [Cr(H₂O)₆]Cl₃ (violet) and [Cr(H…

Q.Which of the following species is not expected to be a ligand?

🎓Unlock everything free for 14 days

Unlock everything free for 14 days