Back Bonding

Back Bonding occurs between atoms where electrons jump from the atomic orbital to the anti-bonding orbital of the acceptor ligand. Back Bonding is also known as π-Back Bonding or Back Donation. It is a type of covalent bonding where electrons from a filled atomic orbital jump to an empty or partially filled atomic orbital.

In this article, we will learn what back Bonding is, its definition, characteristics, and Back Bonding in BF3 and Metal Carbonyls.

What is Back Bonding?

Back Bonding is a concept in chemistry where electrons move from an atomic orbital on one atom to an appropriate symmetry anti-bonding orbital on a π-acceptor ligand. It is also called π backdonation. This electron transfer strengthens the metal-ligand bond and weakens the ligand-ligand bond. It is especially common in the organometallic chemistry of transition metals.

In simple terms, it refers to the interaction where an atomic orbital on one atom shares its electrons with an anti-bonding orbital on another atom.

Learn, Chemical Bonding

Back bonding Definition

Back Bonding, also known as π back bonding or π back donation, is where electrons move from an atomic orbital on one atom to an appropriate symmetry anti-bonding orbital on a π-acceptor ligand. This electron transfer strengthens the metal-ligand bond and weakens the ligand-ligand bond.

Characteristics of Back bonding

The characteristics of Back bonding are:

• Back bonding results in a partial double-bond character, reducing the bond length while increasing the bond order.
• Back bonding influences molecule properties such as hybridization and dipole moment and can enhance chemical compounds’ stability.
• Back bonding is a form of resonance that can be visualized.
• Back bonding can improve the strength of metal-carbon bonds and weaken the carbon-oxygen bonds in metal carbonyls, nitrosyls, and isocyanides.
• Back bonding can result in longer M-P bonds when R3P-M complexes are oxidized.

Pi Back bonding

Pi Back Bonding, or pi back donation, is a chemical phenomenon in which electrons move from an atomic orbital on one atom to an appropriate symmetry anti-bonding orbital on another atom or ligand.

This type of bonding occurs when one atom in a compound has a lone pair of electrons, and the other atom has vacant orbitals adjacent to it.

Characteristics of Pi-back Bonding

• It occurs between atoms in a compound when one atom has a lone pair of electrons and the other has a vacant orbital next to it.
• As a result, the bonding takes on a partial double-bond character.
• It is especially common in the organometallic chemistry of transition metals with multi-atomic ethylene or the nitrosonium cation.
• Sometimes, pi back bonding can also occur between a phosphorus atom and a transition-metal atom, with the phosphine ligand acting as a σ-donor and a π-acceptor.

Back Bonding in BF₃

In BF₃, boron has an empty p-orbital, while each of the three fluorine atoms possesses a lone pair of electrons in its p-orbital. Consequently, boron acts as a Lewis acid, while fluorine serves as a Lewis base. The p-electron from fluorine drifts into the empty p-orbital of boron, resulting in a pi-bonding interaction known as back bonding.

This type of bond formation is known as back bonding or back donation. Thus, the B-F bond has some double bond characteristics. Back bonding in BF₃ does not alter the molecule’s bond angle, planarity, or geometry. The BF₃ molecule exhibits a ‘Trigonal Planar’ geometry.

Back Bonding in Metal Carbonyls

Back Bonding in metal carbonyls is a type of bonding that occurs between a metal center and a carbonyl ligand. In this type of bonding, electrons are partially transferred from a d-orbital of the metal to anti-bonding molecular orbitals of the carbonyl ligand. This interaction strengthens the metal-carbonyl bond and weakens the C-O bond.

• Carbon monoxide is a strong π-acceptor and a good σ-donor, making it a suitable ligand for back bonding.
• The back bonding in metal carbonyls is a synergistic bonding that involves both sigma donation by the carbonyl and pi-back bonding from the metal.
• The more sigma donation by the carbonyl, the stronger the pi-back bonding interaction.
• The presence of extra electron density in the anti-bonding orbital on carbon monoxide weakens the C-O bond.
• The back bonding in metal carbonyls is reflected in the M-CO bond’s vibrational frequencies and bond lengths.
• IR spectroscopy is an essential diagnostic technique in metal-carbonyl chemistry.

Back Bonding vs Coordinate Bonding

The difference between Back bonding and Coordinate bonding is as follows:

Features	Coordinate Bonding	Back Bonding
Nature of Bonding	Coordinate bonding involves the sharing of electrons between two atoms, where the electrons are donated by one atom (ligand) to another (metal ion or central atom)	Back bonding involves the transfer of electrons from an atomic orbital of one atom to an antibonding orbital of another atom or ligand
Occurrence	Coordinate bonds commonly occur in coordination complexes, where a central metal atom is bonded to the ligands through coordinate bonds. Here, the ligands share their lone electron pairs with the metal atom	It is common in organometallic chemistry and occurs between atoms in a compound when one atom has a lone pair of electrons and the other has empty orbitals next to it
Bonding Character	It results in a covalent bond, and the shared pair of electrons comes from the same atom (the ligand)	It results in a partial double-bond character, and the electrons flow from the metal ion to the ligand
Influence on Molecule Properties	Coordinate bonding does not significantly impact molecule properties such as hybridization and dipole moment	Back bonding can affect molecule properties such as hybridization and dipole moment, and it can enhance the stability of chemical compounds
Stability	Coordinate bonding contributes to the stability of coordination complexes	Back bonding generally increases the stability of chemical compounds, as it can enhance the strength of bonds and reduce their length

Back Bonding JEE Questions

Q1: The bond dissociation energy of B–F in BF₃ is 646 kJ mol^-1, whereas that of C–F in CF₄ is 515 kJ mol^–1. The correct reason for higher B–F bond dissociation energy as compared to that of C–F is

1. Significant pπ–pπ interaction between B and F in BF₃ whereas there is no possibility of such interaction between C and F in CF₄.
2. Lower degree of pπ–pπ interaction between B and F in BF₃ than that between C and F in CF₄
3. Smaller size of B-atom as compared to that of C-atom
4. Stronger bond between B and F in BF₃ as compared to that between C and F in CF₄.

Solution:

Because of pπ – pπ back bonding in BF₃ molecule, all B-F bonds have partial double bond character.

Hence option (1) is the answer.

Q2: The formation of molecular complex BF₃–NH₃ results in a change in the hybridisation of boron

1. From sp³ to sp³d
2. From sp² to dsp²
3. From sp³ to sp²
4. From sp² to sp³

Solution:

In BF₃, Boron atom has 3 bond pairs of electrons and 0 lone pairs of electrons. It is sp² hybridized. In F₃B-NH₃, Boron atom has 4 bond pairs of electrons and 0 lone pairs of electrons. It is sp³ hybridized. So the formation of molecular complex results in a change in the hybridization of boron from sp² to sp³.

Hence option (4) is the answer.

Back Bonding Frequently Asked Questions

What is Meant by Back Bonding?

Back bonding refers to the sharing of electron pairs from a filled orbital of one atom to an empty orbital on another. It often involves a metal donating electrons back to a pi-acceptor ligand.

What is Difference between Pi Bonding and Back Bonding?

Pi Bonding is formed by overlapping of atomic orbitals while Back Bonding is formed by donation of elec tron from filled orbital to partial or empty anti-bonding orbital

Which Elements show Back bonding?

Elements with available d orbitals, commonly transition metals, are more likely to exhibit back bonding. For example, metals like iron, platinum, and palladium.

Does Back Bonding increase Stability?

Yes, back bonding can increase the stability of a complex by forming stronger bonds and reducing electron repulsion between adjacent electron-rich regions.

What does Back Bonding depend on?

Back bonding depends on the availability of filled orbitals on the metal and empty orbitals on the ligand. It is influenced by the nature of the ligands and the metal’s electronic structure.

Does Back Bonding affect Bond Length?

Yes, back bonding can affect bond lengths. Due to the shared electron density, it often leads to a shorter bond between the metal and the ligand involved in back bonding.

Is Anti-Bonding and Back Bonding Same?

No Anti-Bonding and Back Bonding are not same.