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

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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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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) …

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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

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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” …

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

AP EAPCET 2026Set eng-2026-05-13-AN1 markMCQ
$[Co(en)_3][Fe(CN)_6]$
›Reveal solutionSolution
This tests naming-to-formula conversion for coordination compounds — figuring out ligand charges, metal oxidation states, and then balancing overall charge between cation and anion. The answer is $[Co(en)_3]_4[Fe(CN)_6]_3$ .

Concept and Intuition

When a coordination compound's name specifies both a cationic complex and an anionic complex (as here — cobalt complex is the cation, ferrate complex is the anion), the overall salt must be electrically neutral. You find each complex ion's own charge first, then combine cation and anion in whatever whole-number ratio makes the total charge zero — exactly like balancing a simple ionic salt such as $Al_2O_3$ (from $Al^{3+}$ and $O^{2-}$ ).

Step-by-Step Solution

Cation — tris(ethane-1,2-diamine)cobalt(III): "ethane-1,2-diamine" (en) is a neutral bidentate ligand ( $NH_2CH_2CH_2NH_2$ ), and "tris" means three of them are attached: $[Co(en)_3]$ . The oxidation state of cobalt is given as (III), i.e. $+3$ . Since en is neutral, the whole complex ion carries the metal's charge: $[Co(en)_3]^{3+}$ .

Anion — hexacyanidoferrate(II): "hexacyanido" means six $CN^-$ ligands, each carrying $-1$ charge, i.e. $-6$ total from ligands. "ferrate(II)" tells us iron is in the $+2$ state. Net charge of the complex ion $= +2 + (-6) = -4$ : $[Fe(CN)_6]^{4-}$ . …
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AP EAPCET 2026Set ap-2026-05-19-AN1 markMCQ
$[PtCl_2(H_2N{-}CH_2{-}CH_2{-}NH_2)_2](NO_3)_2$
›Reveal solutionSolution
This tests IUPAC coordination nomenclature: computing the metal's oxidation state and applying the alphabetical-ligand-order + no-prefix-on-a-Stock-named-counter-ion rules. Answer: Dichloridobis(ethane-1,2-diamine)platinum(IV) nitrate.

Concept and Intuition

A coordination compound name is built as [cationic complex or ligands][metal(oxidation state)] [anion]. Two separate rules govern it here: (1) ligands inside the complex are cited in alphabetical order by the ligand's own name, ignoring any multiplying prefix (mono/di/bis, tris...); (2) once the metal's oxidation state is stated in Roman numerals (Stock notation), the charge of the whole complex ion is fixed, so the number of counter-ions needed to balance it is implicit and is NOT restated with a multiplying prefix on the counter-ion's name.

Step-by-Step Solution

Ligands: $Cl^-$ (chlorido, charge $-1$ each, two of them) and $en$ = ethane-1,2-diamine (neutral, a bidentate ligand named with "bis" since its own name contains a numerical locant "1,2").

Overall salt is $[PtCl_2(en)_2](NO_3)_2$ , so the complex cation carries $+2$ charge (two nitrate anions balance it).

Let Pt oxidation state be $x$ : $x + 2(-1) + 2(0) = +2 \Rightarrow x = +4$ . So it's platinum(IV).

Ligand naming: "chlorido" for $Cl^-$ ×2 → "dichlorido"; "ethane-1,2-diamine" ×2 → "bis(ethane-1,2-diamine)" (bis is used, not "di", because the ligand name itself already has numbers in it — using "di" would be ambiguous).

Alphabetical citation order compares "chlorido" (c) vs "ethane-1,2-diamine" (e): c precedes e, so chlorido-containing part is written first: "Dichloridobis(ethane-1,2-diamine)platinum(IV)". …
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AP EAPCET 2025Set eng-2025-05-26-AN1 markMCQ
$K_3[\mathrm{Co(ox)_3}]$
›Reveal solutionSolution
IUPAC naming of coordination compounds: ligand 'oxalato' × 3 = 'trioxalato', anionic complex metal gets the '-ate' suffix ('cobaltate'), oxidation state by charge balance is +3, and the counter-cation 'potassium' is stated without a multiplying prefix — giving Potassium trioxalatocobaltate(III).

Concept and Intuition

Coordination-compound IUPAC names follow a fixed template: cation first, then ligands alphabetically with multiplying prefixes (bis/tris for complex ligand names, di/tri for simple ones like oxalato), then the metal name — with the special rule that if the complex ion is an anion, the metal name is modified to end in '-ate' (e.g. cobalt → cobaltate, iron → ferrate), followed by the oxidation state in Roman numerals in brackets.

Step-by-Step Solution

Formula: $K_3[\mathrm{Co(ox)_3}]$ , where 'ox' = oxalate ion ( $\mathrm{C_2O_4^{2-}}$ ), a bidentate ligand named 'oxalato' in coordination nomenclature.

Charge balance: complex ion is $[\mathrm{Co(ox)_3}]^{3-}$ (balanced by 3 $K^+$ ). Let Co oxidation state be $x$ : $x + 3(-2) = -3 \Rightarrow x = +3$ .

Ligand prefix: three oxalato ligands → 'trioxalato' (using tri- since oxalato is a simple, unsubstituted ligand name).

Since the complex ion is an anion, the metal name takes the anionic suffix: cobalt → 'cobaltate'.

Combine: 'trioxalatocobaltate', with oxidation state (III) in brackets: 'trioxalatocobaltate(III)'. …
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AP EAPCET 2023Set eng-2023-05-16-FN1 markMCQ
$\mathrm{[Co(H_2NCH_2CH_2NH_2)_3]SO_4}$
›Reveal solutionSolution
The complex cation [Co(en)₃]³⁺ needs 2 cations for every 3 sulphate ions to balance charge, giving [Co(en)₃]₂(SO₄)₃.

Concept and Intuition

"Ethane-1,2-diamine" (en) is the neutral bidentate ligand $H_2NCH_2CH_2NH_2$ . Cobalt(III) with three such neutral ligands gives a complex cation charge equal to the metal's oxidation state alone: $[Co(en)_3]^{3+}$ . To form a neutral salt with sulphate ( $SO_4^{2-}$ ), the overall charges must balance using the smallest whole-number ratio — LCM of 3 and 2 is 6, so 2 cations (total +6) balance 3 sulphates (total −6).

Step-by-Step Solution

Write the complex cation: $[Co(H_2NCH_2CH_2NH_2)_3]^{3+}$ (Co³⁺ + 3 neutral en ligands).

Sulphate anion charge: $SO_4^{2-}$ .

Balance charges: need ratio such that $2\times3=3\times2=6$ → 2 cations : 3 sulphates. …
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AP EAPCET 2023Set eng-2023-05-17-AN1 markMCQ
$[\mathrm{Pt(NH_3)_2Cl(NH_2CH_3)}]\mathrm{Cl}$
›Reveal solutionSolution
Naming ligands alphabetically (ammine, chloro, methanamine) and computing Pt's oxidation state as +2 gives Diamminechloro(methanamine)platinum(II) chloride.

Concept and Intuition

IUPAC coordination nomenclature: (1) name ligands alphabetically using IUPAC ligand names (ignoring di/tri prefixes for alphabetization); (2) anionic ligands get '-o' suffix (chloro for Cl-); (3) metal oxidation state deduced from charge balance; (4) counter-ion named last.

Step-by-Step Solution

Ligands in $[\mathrm{Pt(NH_3)_2Cl(NH_2CH_3)}]\mathrm{Cl}$ : two $\mathrm{NH_3}$ (ammine, neutral), one $\mathrm{Cl^-}$ (chloro, -1), one $\mathrm{NH_2CH_3}$ = methanamine (neutral, the IUPAC ligand name for methylamine).

Alphabetize ignoring 'di': ammine (a) < chloro (c) < methanamine (m) -> "diamminechloro(methanamine)". …
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AP EAPCET 2022Set eng-2022-07-05-FN1 markMCQ
$[Co(NH_3)_4Br_2]^{\oplus}$
›Reveal solutionSolution
A homoleptic complex has all identical ligands; only $[\mathrm{Co(NH_3)_6}]^{3+}$ qualifies among the given options.

Concept and Intuition

Coordination complexes are classified as homoleptic (all ligands of one single type bound to the metal) or heteroleptic (two or more different kinds of ligands). This is purely a matter of reading off the ligands present in each formula and checking whether they are all the same species.

Step-by-Step Solution

$[\mathrm{Co(NH_3)_4Br_2}]^{+}$ — ligands present: $\mathrm{NH_3}$ and $\mathrm{Br^-}$ (two types) → heteroleptic.

$[\mathrm{Co(C_2O_4)(NH_3)_4}]^{+}$ — ligands present: oxalate ( $\mathrm{C_2O_4^{2-}}$ ) and $\mathrm{NH_3}$ (two types) → heteroleptic.

$[\mathrm{Co(NH_3)_6}]^{3+}$ — ligand present: only $\mathrm{NH_3}$ , six of them → homoleptic. …
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AP EAPCET 2022Set eng-2022-07-07-FN1 markMCQ
$\mathrm{[Co(NH_2CH_2CH_2NH_2)_2Br_2]Br}$
›Reveal solutionSolution
The oxidation state of Co is $+3$ (from a $+1$ complex cation with two neutral en and two $\mathrm{Br^-}$ ligands), and IUPAC alphabetical ligand ordering puts "bromido" before "(ethylenediamine)," giving dibromidobis(ethylenediamine)cobalt(III) bromide.

Concept and Intuition

Naming a coordination compound requires: (1) determining the metal's oxidation state from overall charge balance, (2) naming ligands with multiplying prefixes (di-, tri- for simple ligands; bis-, tris- for more complex/substituted ligand names like ethylenediamine, to avoid ambiguity), and (3) listing ligands in strict alphabetical order by ligand name (ignoring the multiplying prefix), followed by the metal name+oxidation state, then the counter-ion name.

Step-by-Step Solution

The compound is $[\mathrm{Co(en)_2Br_2}]\mathrm{Br}$ — one $\mathrm{Br^-}$ is outside the coordination sphere (counter-ion), so the complex cation is $[\mathrm{Co(en)_2Br_2}]^{+}$ .

Charge balance inside the bracket: en is neutral (×2 = 0), $\mathrm{Br^-}$ ligands contribute $-2$ (×2), so $\mathrm{Co} + 0 - 2 = +1 \Rightarrow \mathrm{Co} = +3$ .

Ligand names: "ethylenediamine" (a bidentate amine, needs "bis" not "di" since its name itself could be ambiguous with a simple prefix) and "bromido" for $\mathrm{Br^-}$ .

Alphabetical order of ligand names (ignoring multiplying prefixes): "bromido" (b) before "ethylenediamine" (e). …
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AP EAPCET 2022Set eng-2022-07-08-AN1 markMCQ
$[Cr(NH_3)_3(H_2O)_2Cl]Cl_2$
›Reveal solutionSolution
Naming $[\mathrm{Cr(NH_3)_3(H_2O)_2Cl}]\mathrm{Cl_2}$ requires listing ligands alphabetically (ammine, aqua, chlorido) with multiplying prefixes, then the oxidation state of Cr (found by charge balance) in Roman numerals, followed by the anion name — giving "triamminediaquachloridochromium(III) chloride".

Concept and Intuition

IUPAC naming of coordination compounds lists ligands in alphabetical order (by the first letter of the ligand name itself, ignoring multiplying prefixes like di-/tri-), followed by the name of the central metal with its oxidation state in Roman numerals in parentheses, and finally the counter-ion name (as a separate word) if the complex is a cation. The oxidation state of the metal is found by balancing charges: neutral ligands (NH₃, H₂O) contribute 0, anionic ligands contribute their charge, and the sum plus the metal's oxidation state must equal the overall charge of the complex ion.

Step-by-Step Solution

Identify ligands inside the bracket: 3 NH₃ ("ammine" ×3 → "triammine"), 2 H₂O ("aqua" ×2 → "diaqua"), 1 Cl⁻ ("chlorido").

Alphabetize by ligand name ignoring prefixes: compare "ammine", "aqua", "chlorido" — a-m-m... vs a-q-u-a: 'm' precedes 'q', so "ammine" comes before "aqua"; "chlorido" (starting with 'c') comes after both 'a' words. Order: ammine, aqua, chlorido.

Combine with multiplying prefixes in that order: "triamminediaquachlorido".

Determine oxidation state of Cr: the compound is neutral with 2 Cl⁻ as counter ions outside the bracket, so the complex ion charge is +2. Inside the bracket: 3(NH₃, 0) + 2(H₂O, 0) + 1(Cl⁻, −1) = −1. So Cr + (−1) = +2 → Cr = +3.

Complete metal name: "chromium(III)". …
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Q.Which of the following species is not expected to be a ligand?

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