Haloalkanes: Compare the mechanisms of SN1 and SN2 reactions regarding stereochemical inversion

Chapter: Haloalkanes and Haloarenes

Topic: SN1 and SN2 Reactions – Stereochemical Inversion

SN1 vs SN2 – Stereochemical Behaviour

SN1 and SN2 are two important nucleophilic substitution mechanisms. Their stereochemical behaviour is different because the nucleophile approaches the substrate differently in the two mechanisms.

SN1 → Carbocation intermediate → Planar → Racemisation

SN2 → Back-side attack → Inversion of configuration

SN1: Usually racemisation
SN2: Walden inversion

SN2 = Substitution Nucleophilic Bimolecular

SN2 reaction occurs in a single concerted step. The nucleophile attacks the carbon atom from the side opposite to the leaving group.

Nu⁻ + R–X
↓
Back-side attack
↓
R–Nu + X⁻

Back-side Attack

The nucleophile cannot effectively attack from the same side as the leaving group. Therefore, it approaches from the opposite side.

Nu⁻ ↓ C / \ R X

Back-side attack

Walden Inversion

Because of the back-side attack, the three-dimensional arrangement around the carbon atom is inverted. This is called Walden inversion.

SN2 → Back-side attack → Inversion of configuration

SN1 = Substitution Nucleophilic Unimolecular

SN1 reaction occurs in two main steps and involves formation of a carbocation intermediate.

Step 1 – Formation of Carbocation

R–X → R⁺ + X⁻

This is generally the slow, rate-determining step.

Step 2 – Nucleophilic Attack

The carbocation is approximately trigonal planar. Therefore, the nucleophile can attack from either side.

Nu⁻ ↙ ↘ R⁺ ↖ ↗ Two possible sides

Stereochemical Result

Attack from both sides can produce both configurations. Therefore, an optically active substrate generally gives a mixture of products, often described as racemisation.

SN1 → Planar carbocation → Attack from both sides → Racemisation

Feature	SN1	SN2
Number of steps	Two or more steps	Single step
Intermediate	Carbocation	No intermediate
Attack	Can occur from either face	Back-side attack
Intermediate geometry	Planar	No carbocation
Stereochemical result	Racemisation / mixture	Inversion of configuration
Special term	Racemisation	Walden inversion

Walden inversion is the inversion of spatial configuration at a stereogenic carbon atom during an SN2 reaction.

Reactant
⬇
Nucleophile attacks from back side
⬇
Leaving group departs
⬇
Product with inverted configuration

Important: SN2 reaction requires back-side attack because the nucleophile must overlap with the antibonding orbital associated with the C–X bond.

SN2 stereochemical hallmark = Walden inversion

In SN1, the leaving group first leaves and produces a carbocation. The carbocation is approximately planar.

R–X
↓
R⁺ + X⁻
↓
Planar carbocation

Since the carbocation is planar, the nucleophile can approach from either face.

Attack from front → Product A
Attack from back → Product B

The two configurations can therefore be formed. Hence, an optically active substrate generally undergoes racemisation in an SN1 reaction.

SN1 → Planar carbocation → Two-side attack → Racemisation

SN1 is favoured by:

• Stable carbocation formation
• 3° alkyl halides generally react readily
• Polar protic solvents
• Good leaving groups
• Weak nucleophiles can participate

SN2 is favoured by:

• Less sterically hindered substrates
• Methyl and primary halides generally react readily
• Strong nucleophiles
• Polar aprotic solvents
• Good leaving groups

SN1 → Carbocation stability is important
SN2 → Steric hindrance is very important

Q. What is the stereochemical result of an SN2 reaction?

Answer:

In an SN2 reaction, the nucleophile attacks the substrate from the back side. This causes inversion of configuration at the stereogenic carbon atom.

This inversion is called Walden inversion.

Q. Why does SN1 reaction generally lead to racemisation?

Answer:

1. SN1 reaction forms a carbocation intermediate.
2. The carbocation is approximately planar.
3. The nucleophile can attack from either face.

Therefore, both configurations may be formed, resulting in racemisation or a mixture of stereoisomers.

Q. Compare the stereochemical outcomes of SN1 and SN2 reactions.

SN1	SN2
Forms a planar carbocation	No carbocation intermediate
Attack from either face	Back-side attack
Usually gives racemisation	Gives inversion
Two-step mechanism	One-step mechanism

Q. Explain the mechanisms of SN1 and SN2 reactions and compare their stereochemical outcomes.

SN1 Mechanism

The leaving group first departs, producing a carbocation.

R–X → R⁺ + X⁻

The planar carbocation is then attacked by the nucleophile from either side, producing both possible configurations.

Planar R⁺
↙     ↘
Nu attack   Nu attack

Hence SN1 generally gives racemisation.

SN2 Mechanism

The nucleophile attacks from the side opposite to the leaving group while the leaving group leaves simultaneously.

Nu⁻ + R–X → R–Nu + X⁻
Back-side attack

This causes inversion of configuration, known as Walden inversion.

SN1 → Racemisation
SN2 → Walden inversion

Q. Explain SN1 and SN2 mechanisms in detail and compare their stereochemical inversion.

SN1 Mechanism

SN1 stands for substitution nucleophilic unimolecular. It proceeds through a carbocation intermediate.

Step 1 – Ionisation

R–X → R⁺ + X⁻

The carbocation formed is approximately trigonal planar.

Step 2 – Nucleophilic Attack

The nucleophile can attack either face of the planar carbocation. Therefore, both configurations may be obtained.

SN1
Planar carbocation
↓
Attack from either face
↓
Racemisation / mixture

SN2 Mechanism

SN2 stands for substitution nucleophilic bimolecular. It occurs in one concerted step.

Nu⁻ + R–X
↓
Back-side attack
↓
R–Nu + X⁻

The nucleophile attacks opposite to the leaving group. This produces inversion of configuration.

SN2
Back-side attack
↓
Walden inversion

Final Comparison

Property	SN1	SN2
Intermediate	Carbocation	None
Attack	Either face	Back side
Stereochemistry	Racemisation	Inversion
Mechanism	Stepwise	Concerted

SN1 → Planar carbocation → Attack from both sides → Racemisation

SN2 → Back-side attack → Walden inversion

1. SN1 stands for:

A) Substitution Nucleophilic Unimolecular

B) Substitution Nucleophilic Bimolecular

C) Substitution Neutral Unimolecular

D) Simple Nucleophilic Reaction

✅ Answer

A

---

2. SN2 reaction occurs in:

A) Two steps

B) One concerted step

C) Three steps

D) Four steps

✅ Answer

B

---

3. SN1 reaction involves formation of:

A) Carbanion

B) Carbocation

C) Free radical

D) Carbene

✅ Answer

B

---

4. The SN2 nucleophile attacks from:

A) Front side

B) Back side

C) Any direction equally

D) Above only

✅ Answer

B) Back side

---

5. Stereochemical inversion in SN2 is called:

A) Markovnikov inversion

B) Walden inversion

C) Saytzeff inversion

D) Peroxide inversion

✅ Answer

B

---

6. The carbocation intermediate in SN1 is approximately:

A) Linear

B) Trigonal planar

C) Tetrahedral

D) Octahedral

✅ Answer

B

---

7. SN1 generally results in:

A) Complete inversion only

B) Racemisation

C) No reaction

D) Elimination only

✅ Answer

B

---

8. SN2 stereochemistry results mainly from:

A) Front-side attack

B) Back-side attack

C) Carbocation formation

D) Radical formation

✅ Answer

B

---

9. Which mechanism has no carbocation intermediate?

A) SN1

B) SN2

C) Both

D) Neither

✅ Answer

B

---

10. SN1 is favoured by:

A) Stable carbocation formation

B) Severe steric hindrance against ionisation

C) No leaving group

D) Absence of substrate

✅ Answer

A

---

11. SN2 is generally fastest for:

A) Highly hindered tertiary substrate

B) Methyl substrate

C) Very bulky substrate

D) Neopentyl substrate only

✅ Answer

B

---

12. Which mechanism is bimolecular in its rate-determining event?

A) SN1

B) SN2

C) Both

D) Neither

✅ Answer

B

---

13. The stereochemical product of a pure SN2 substitution at a chiral centre is associated with:

A) Inversion

B) Retention only

C) Racemisation only

D) No stereochemical change

✅ Answer

A

---

14. Why can SN1 give attack from both sides?

A) Because carbocation is planar

B) Because nucleophile is planar

C) Because halide is planar

D) Because substrate is always linear

✅ Answer

A

---

15. The key stereochemical feature of SN2 is:

A) Carbocation rearrangement

B) Back-side attack

C) Free-radical formation

D) Planar intermediate

✅ Answer

B

---

16. Which reaction is more sensitive to steric hindrance?

A) SN1

B) SN2

C) Both equally

D) Neither

✅ Answer

B) SN2

---

17. Which mechanism commonly uses a planar carbocation intermediate?

A) SN1

B) SN2

C) Both

D) Neither

✅ Answer

A) SN1

---

18. In SN2, the leaving group and nucleophile:

A) React in separate stages

B) Participate in the same concerted step

C) Never interact

D) Form a carbocation first

✅ Answer

B

---

19. Which combination is correct?

A) SN1 – inversion only

B) SN2 – racemisation only

C) SN1 – racemisation; SN2 – inversion

D) SN1 – no stereochemical effect; SN2 – retention

✅ Answer

C

---

20. Walden inversion is associated with:

A) SN1

B) SN2

C) E1

D) E2

✅ Answer

B) SN2

🎯 Quick Revision

SN1: Substitution Nucleophilic Unimolecular
SN2: Substitution Nucleophilic Bimolecular
SN1: Carbocation intermediate
SN2: No intermediate
SN1: Planar carbocation → attack from either face
SN2: Back-side attack
SN1: Generally racemisation
SN2: Walden inversion

Bonding in Carbonyls: Explain the synergic bonding mechanism in metal carbonyls

Chapter: Coordination Compounds

Topic: Bonding in Metal Carbonyls – Synergic Bonding

What is Synergic Bonding?

In metal carbonyls, the bonding between the metal and CO ligand is synergic because it involves two simultaneous interactions:

CO → Metal
σ-donation

Metal → CO
π-back donation

These two interactions reinforce each other. Therefore, the bonding is called synergic bonding.

Synergic Bonding = σ-donation + π-back donation

Carbon monoxide (CO) is a strong ligand because it can interact with a transition metal in two ways.

1. The carbon atom of CO donates an electron pair to the metal through a σ bond.
2. The metal donates electron density back into the vacant π* antibonding orbital of CO.

CO → M
σ-donation

M → CO(π*)
π-back donation

σ-Donation

The carbon atom in CO possesses a lone pair of electrons. This electron pair is donated to an empty orbital of the metal.

CO : → M
Lone pair on C → Empty metal orbital

Thus a coordinate covalent σ bond is formed between carbon and the metal.

CO → Metal = σ-donation

π-Back Donation

The metal atom can donate electron density from its filled d-orbitals into the vacant antibonding π* orbital of CO.

Metal d-orbital
↓
CO π* antibonding orbital
π-back donation

This interaction is called π-back bonding or π-back donation.

Metal → CO(π*) = π-back donation

Step 1 – σ Donation

CO donates its lone pair from carbon to the metal.

CO → M
σ bond

Step 2 – π Back Donation

The metal simultaneously donates electron density from filled d-orbitals into the vacant π* orbital of CO.

M → CO(π*)
π-back bonding

Step 3 – Mutual Reinforcement

The σ-donation increases electron density on the metal. The metal can then participate in π-back donation. At the same time, π-back donation strengthens the overall metal–CO interaction.

σ-donation ⇄ π-back donation

Both interactions reinforce each other.
Therefore → Synergic Bonding

Effect on Metal–Carbon Bond

Both σ-donation and π-back donation strengthen the metal–carbon bond.

Effect on C–O Bond

When electrons are transferred into the antibonding π* orbital of CO, the C–O bond becomes weaker.

π-back donation ↑
↓
CO π* population ↑
↓
C–O bond order ↓
↓
C–O bond weakens

Important: π-back donation strengthens the M–C bond but weakens the C–O bond.

Nickel Carbonyl

Ni(CO)₄

Nickel tetracarbonyl is a well-known metal carbonyl.

Iron Carbonyl

Fe(CO)₅

Iron pentacarbonyl contains five CO ligands coordinated to iron.

Dicobalt Octacarbonyl

Co₂(CO)₈

Common examples: Ni(CO)₄, Fe(CO)₅, Co₂(CO)₈

Feature	σ-Donation	π-Back Donation
Direction	CO → Metal	Metal → CO
Orbital involved	CO lone pair	Metal d-orbital → CO π*
Bond type	σ bond	π interaction
Effect	Helps form M–C bond	Strengthens M–C and weakens C–O

Q. What is synergic bonding in metal carbonyls?

Answer:

Synergic bonding is the simultaneous interaction of CO with a metal through:

σ-donation: CO → Metal
π-back donation: Metal → CO

Both interactions reinforce each other, producing strong metal–CO bonding.

Q. Explain the two components of synergic bonding in metal carbonyls.

There are two components:

1. σ-donation: CO donates a lone pair from carbon to an empty orbital of the metal.
2. π-back donation: The metal donates electron density from filled d-orbitals into the vacant π* orbital of CO.

CO → M = σ-donation
M → CO(π*) = π-back donation

Q. Explain synergic bonding in metal carbonyls with a suitable representation.

Metal carbonyls exhibit synergic bonding due to two simultaneous interactions.

1. σ-donation

CO donates its lone pair on carbon to an empty orbital of the metal.

CO → M

2. π-back donation

The metal donates electron density from filled d-orbitals into the vacant π* orbital of CO.

M → CO(π*)

These two interactions reinforce each other and produce a strong metal–carbon bond.

Synergic bonding = σ-donation + π-back donation

Q. Explain the synergic bonding mechanism in metal carbonyls and discuss its effect on M–C and C–O bonds.

σ-Donation

The carbon atom of CO donates a lone pair into an empty orbital of the metal, forming a σ bond.

CO → M
σ-donation

π-Back Donation

Filled metal d-orbitals donate electron density into the vacant π* antibonding orbital of CO.

M(d) → CO(π*)
π-back donation

Synergic Effect

The σ and π interactions occur simultaneously and reinforce each other. This makes the metal–carbon bond stronger.

Effect on C–O Bond

Back donation into the π* antibonding orbital reduces the C–O bond order and therefore weakens the C–O bond.

M–C bond → strengthened
C–O bond → weakened

Q. Explain the synergic bonding mechanism in metal carbonyls in detail.

1. Nature of CO Ligand

CO is a strong ligand that coordinates with transition metals mainly through the carbon atom.

2. σ-Donation

The carbon atom of CO contains a lone pair which is donated to an empty orbital of the metal.

CO → M
σ-donation

3. π-Back Donation

The metal has filled d-orbitals. These orbitals can overlap with the vacant π* antibonding orbital of CO and donate electron density into it.

M(d) → CO(π*)
π-back donation

4. Synergic Interaction

The σ-donation and π-back donation occur simultaneously. They reinforce one another and result in strong metal–CO bonding.

CO → M
σ-donation

M → CO(π*)
π-back donation

5. Effect on Bond Strength

The metal–carbon bond becomes stronger due to the combined σ and π interactions.

At the same time, electron density entering the CO π* orbital weakens the C–O bond.

6. Examples

Ni(CO)₄     Fe(CO)₅     Co₂(CO)₈

Final Summary

CO → Metal = σ-donation
Metal → CO(π*) = π-back donation
Both together = Synergic bonding

M–C bond becomes stronger
C–O bond becomes weaker

1. CO acts as a ligand through which atom?

A) Oxygen

B) Carbon

C) Both equally

D) Neither

✅ Answer

B) Carbon

---

2. CO is generally classified as a:

A) Weak ligand

B) Strong ligand

C) Non-ligand

D) Negative ion only

✅ Answer

B) Strong ligand

---

3. σ-donation in metal carbonyls occurs from:

A) Metal to CO

B) CO to metal

C) Metal to metal

D) Oxygen to metal only

✅ Answer

B) CO to metal

---

4. The electron pair involved in σ-donation comes mainly from:

A) Carbon of CO

B) Oxygen of CO

C) Metal nucleus

D) Counter ion

✅ Answer

A) Carbon

---

5. π-back donation occurs from:

A) CO to metal

B) Metal to CO

C) Ligand to ligand

D) Counter ion to ligand

✅ Answer

B) Metal to CO

---

6. In π-back bonding, metal electrons enter the:

A) CO σ orbital

B) CO π* orbital

C) Metal s orbital

D) C–O core orbital

✅ Answer

B) CO π* orbital

---

7. The metal orbitals involved in back donation are mainly:

A) Filled d-orbitals

B) Empty s-orbitals only

C) Nuclear orbitals

D) Core orbitals only

✅ Answer

A) Filled d-orbitals

---

8. Synergic bonding consists of:

A) Only σ-donation

B) Only π-donation

C) σ-donation and π-back donation

D) Ionic bonding only

✅ Answer

C

---

9. π-back donation generally makes the C–O bond:

A) Stronger

B) Weaker

C) Unchanged always

D) Ionic

✅ Answer

B) Weaker

---

10. π-back donation generally strengthens the:

A) C–O bond only

B) M–C bond

C) O–O bond

D) C–C bond

✅ Answer

B) M–C bond

---

11. Which orbital of CO accepts back-donation?

A) π*

B) σ*

C) 1s

D) Core orbital

✅ Answer

A) π*

---

12. Which of the following is a metal carbonyl?

A) Ni(CO)₄

B) NaCl

C) H₂O

D) NH₃

✅ Answer

A) Ni(CO)₄

---

13. Fe(CO)₅ is known as:

A) Iron pentacarbonyl

B) Iron carbonate

C) Iron carbide

D) Iron oxide

✅ Answer

A

---

14. In synergic bonding, the two interactions:

A) Oppose each other

B) Reinforce each other

C) Cancel each other

D) Are unrelated

✅ Answer

B) Reinforce each other

---

15. CO is a:

A) σ donor only

B) π acceptor only

C) σ donor and π acceptor

D) Neither donor nor acceptor

✅ Answer

C

---

16. During σ-donation, CO donates:

A) A proton

B) An electron pair

C) A neutron

D) A metal atom

✅ Answer

B) An electron pair

---

17. Back donation increases electron density in:

A) CO π*

B) CO nucleus

C) Metal nucleus

D) Counter ion

✅ Answer

A) CO π*

---

18. Which statement is correct?

A) Back donation strengthens C–O

B) Back donation weakens C–O

C) CO cannot accept electrons

D) Metal cannot donate electrons

✅ Answer

B

---

19. Synergic bonding is especially important in:

A) Metal carbonyls

B) Alkali metal chlorides only

C) Simple ionic salts only

D) Noble gases

✅ Answer

A) Metal carbonyls

---

20. The correct direction of synergic bonding is:

A) CO → M and M → CO

B) M → M only

C) CO → CO only

D) Ligand → ligand only

✅ Answer

A

🎯 Quick Revision

CO: Strong ligand
σ-donation: CO → Metal
π-back donation: Metal → CO(π*)
Metal orbital: Filled d-orbital
CO orbital: Vacant π* antibonding orbital
Synergic bonding: σ-donation + π-back donation
Effect on M–C: Bond becomes stronger
Effect on C–O: Bond becomes weaker
Examples: Ni(CO)₄, Fe(CO)₅, Co₂(CO)₈

Ligands: Define Ambidentate ligands and provide an example of Linkage Isomerism

Chapter: Coordination Compounds

Topic: Ambidentate Ligands & Linkage Isomerism

What are Ambidentate Ligands?

Ambidentate ligands are ligands that have two different donor atoms through which they can coordinate with the central metal ion, but they coordinate through only one donor atom at a time.

Ambidentate Ligand
↓
Two possible donor atoms
↓
Coordinates through only one at a time

Examples: NO₂⁻, SCN⁻

1. Nitrite Ion (NO₂⁻)

Nitrite ion can coordinate through either the nitrogen atom or an oxygen atom.

M–NO₂
Nitro: M–NO₂ (through N)

Nitrito: M–ONO (through O)

2. Thiocyanate Ion (SCN⁻)

SCN⁻ can coordinate through either sulfur or nitrogen.

M–SCN
Thiocyanato-S: M–SCN

Thiocyanato-N: M–NCS

NO₂⁻ → N or O donor
SCN⁻ → S or N donor

Linkage isomerism is a type of structural isomerism shown by coordination compounds containing an ambidentate ligand.

It occurs when the same ligand coordinates with the central metal ion through different donor atoms.

Same ligand
↓
Different donor atom
↓
Different coordination compound
↓
Linkage Isomerism

Linkage isomerism is possible because of ambidentate ligands.

A common example involves the ambidentate nitrite ion, NO₂⁻.

Nitro Complex

[Co(NH₃)₅(NO₂)]Cl₂
Co–N bond

Here, the nitrite ligand coordinates through its nitrogen atom. This form is called the nitro form.

Nitrito Complex

[Co(NH₃)₅(ONO)]Cl₂
Co–O bond

Here, the same nitrite ligand coordinates through its oxygen atom. This form is called the nitrito form.

[Co(NH₃)₅(NO₂)]Cl₂ ⇄ [Co(NH₃)₅(ONO)]Cl₂

Nitro ⇄ Nitrito
Linkage Isomerism

The thiocyanate ion, SCN⁻, is also an ambidentate ligand. It can coordinate through S or N.

Through Sulphur

M–S–C≡N
Thiocyanato-S

Through Nitrogen

M–N=C=S
Thiocyanato-N

SCN⁻ → S donor or N donor

Point	Ambidentate Ligand	Linkage Isomerism
Meaning	Ligand having two possible donor atoms	Isomerism due to different donor atoms
Cause	Presence of two donor atoms	Different coordination through those donor atoms
Example	NO₂⁻, SCN⁻	Nitro–Nitrito complexes

• Ambidentate ligands have two different donor atoms.
• They coordinate through only one donor atom at a time.
• NO₂⁻ can coordinate through N or O.
• SCN⁻ can coordinate through S or N.
• Linkage isomerism is a type of structural isomerism.
• It occurs because an ambidentate ligand can attach to the metal through different donor atoms.
• NO₂⁻ through N gives the nitro form.
• NO₂⁻ through O gives the nitrito form.
• SCN⁻ through S gives thiocyanato-S.
• SCN⁻ through N gives thiocyanato-N.

Ambidentate Ligand → Different donor atoms → Linkage Isomerism

Q. Define an ambidentate ligand. Give one example.

Answer:

An ambidentate ligand is a ligand having two different donor atoms through which it can coordinate with the central metal ion, but it coordinates through only one donor atom at a time.

Example: NO₂⁻ can coordinate through N or O.

Q. What is linkage isomerism? Explain with an example.

Linkage isomerism occurs in coordination compounds containing ambidentate ligands. It arises when the same ligand coordinates with the central metal through different donor atoms.

For NO₂⁻:

[Co(NH₃)₅(NO₂)]Cl₂
Nitro form – Co–N

[Co(NH₃)₅(ONO)]Cl₂
Nitrito form – Co–O

These are linkage isomers.

Q. Explain ambidentate ligands and linkage isomerism with suitable examples.

Ambidentate Ligands

Ligands having two different donor atoms capable of coordinating with a metal ion through either one of them are called ambidentate ligands.

Examples include NO₂⁻ and SCN⁻.

Linkage Isomerism

When an ambidentate ligand coordinates through different donor atoms, the resulting coordination compounds are called linkage isomers.

[Co(NH₃)₅(NO₂)]Cl₂ ⇄ [Co(NH₃)₅(ONO)]Cl₂

The first is the nitro form and the second is the nitrito form.

Q. Explain linkage isomerism in coordination compounds with suitable examples.

Linkage isomerism is a type of structural isomerism found in coordination compounds containing ambidentate ligands.

An ambidentate ligand contains two possible donor atoms but coordinates through only one of them at a time.

Example 1 – NO₂⁻

[Co(NH₃)₅(NO₂)]Cl₂
Nitro – Co–N

[Co(NH₃)₅(ONO)]Cl₂
Nitrito – Co–O

Example 2 – SCN⁻

M–SCN → Through S

M–NCS → Through N

Different donor atoms of the same ambidentate ligand produce linkage isomers.

Q. Define ambidentate ligands. Explain linkage isomerism in detail with suitable examples.

1. Definition of Ambidentate Ligand

An ambidentate ligand is a ligand which contains two different donor atoms and can coordinate with a central metal ion through either one of them, but not both simultaneously at the same coordination site.

2. Examples

NO₂⁻ → N or O donor
SCN⁻ → S or N donor

3. Linkage Isomerism

Linkage isomerism is a type of structural isomerism in which the same ambidentate ligand is attached to the central metal ion through different donor atoms.

4. Nitro–Nitrito Example

[Co(NH₃)₅(NO₂)]Cl₂
Nitro form
Co–N bond

[Co(NH₃)₅(ONO)]Cl₂
Nitrito form
Co–O bond

Both compounds have the same molecular formula but differ in the atom of NO₂⁻ through which coordination occurs.

5. Thiocyanate Example

M–S–C≡N
Thiocyanato-S

M–N=C=S
Thiocyanato-N

6. Conclusion

Ambidentate ligand → Two possible donor atoms → Different mode of attachment → Linkage Isomerism

1. An ambidentate ligand contains:

A) One donor atom

B) Two different donor atoms

C) Three donor atoms

D) No donor atom

✅ Answer

B

---

2. Which is an ambidentate ligand?

A) NH₃

B) H₂O

C) NO₂⁻

D) CO

✅ Answer

C) NO₂⁻

---

3. NO₂⁻ can coordinate through:

A) Only N

B) Only O

C) N or O

D) N and O simultaneously

✅ Answer

C) N or O

---

4. SCN⁻ can coordinate through:

A) S or N

B) C or S

C) Only C

D) Only S

✅ Answer

A) S or N

---

5. Linkage isomerism is associated with:

A) Monodentate ligands only

B) Ambidentate ligands

C) Metals only

D) Counter ions only

✅ Answer

B

---

6. Linkage isomerism is a type of:

A) Structural isomerism

B) Optical isomerism

C) Geometrical isomerism

D) Conformational isomerism

✅ Answer

A

---

7. Coordination through nitrogen of NO₂⁻ gives:

A) Nitrito

B) Nitro

C) Nitrato

D) Nitride

✅ Answer

B) Nitro

---

8. Coordination through oxygen of NO₂⁻ gives:

A) Nitro

B) Nitrito

C) Nitrate

D) Nitride

✅ Answer

B) Nitrito

---

9. The donor atom in the nitro form is:

A) Oxygen

B) Nitrogen

C) Chlorine

D) Hydrogen

✅ Answer

B) Nitrogen

---

10. The donor atom in the nitrito form is:

A) Nitrogen

B) Oxygen

C) Chlorine

D) Hydrogen

✅ Answer

B) Oxygen

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11. Which pair represents linkage isomers?

A) cis/trans complexes

B) [Co(NH₃)₅(NO₂)]Cl₂ and [Co(NH₃)₅(ONO)]Cl₂

C) optical isomers

D) ionisation isomers

✅ Answer

B

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12. In thiocyanato-S linkage, the metal is bonded through:

A) N

B) S

C) C

D) H

✅ Answer

B) S

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13. In thiocyanato-N linkage, the metal is bonded through:

A) S

B) N

C) C

D) O

✅ Answer

B) N

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14. Which ligand is NOT ambidentate?

A) NO₂⁻

B) SCN⁻

C) NH₃

D) Both A and B are ambidentate

✅ Answer

C) NH₃

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15. In linkage isomerism, the molecular formula is:

A) Different

B) Same

C) Always doubled

D) Always halved

✅ Answer

B) Same

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16. Linkage isomerism arises due to difference in:

A) Oxidation state

B) Donor atom of the same ligand

C) Number of metal atoms

D) Number of counter ions

✅ Answer

B

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17. Which ligand can show nitro-nitrito linkage?

A) NH₃

B) H₂O

C) NO₂⁻

D) CO

✅ Answer

C) NO₂⁻

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18. Which of the following has S and N as possible donor atoms?

A) NO₂⁻

B) SCN⁻

C) NH₃

D) H₂O

✅ Answer

B) SCN⁻

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19. Which statement is correct about ambidentate ligands?

A) They coordinate through both donor atoms simultaneously at one site

B) They have two possible donor atoms

C) They have no donor atoms

D) They cannot form coordination compounds

✅ Answer

B

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20. The correct sequence is:

A) Ambidentate ligand → Different donor atom → Linkage isomerism

B) Monodentate ligand → Optical isomerism only

C) Metal → Linkage ligand → Isomerism

D) Counter ion → Linkage isomerism

✅ Answer

A

🎯 Quick Revision

Ambidentate Ligand: Two different donor atoms
NO₂⁻: N or O donor
SCN⁻: S or N donor
Linkage Isomerism: Same ligand attaches through different donor atoms
Nitro: Co–N–O₂
Nitrito: Co–O–N=O
Important Example: [Co(NH₃)₅(NO₂)]Cl₂ ⇄ [Co(NH₃)₅(ONO)]Cl₂