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