Back Bonding

The back bonding definition involves a type of chemical bonding in which electron density is donated from a filled orbital of one atom to an empty orbital of another atom in a compound. 

1.0What is Back Bonding?

Back bonding meaning involves pi back bonding (π-back bonding) or back-donation, it is a concept in chemistry where electron density from filled p or d orbitals of one atom is donated to an empty orbital on another atom, typically with lower energy. This process occurs simultaneously with the formation of a covalent bond between the two atoms.

In simpler terms, back bonding involves the transfer of electron density from one atom's filled orbital to another atom's empty orbital, resulting in a partial sharing of electrons between the two atoms.

Back bonding is commonly observed in transition metal complexes and certain organic molecules. It can influence the electronic structure, stability, and reactivity of compounds, often leading to unique properties.

2.0Types of Back Bonding

pπ-pπ back bonding 

pπ-pπ back bonding is a form of covalent bonding where an electron pair is shared between a p orbital of one atom that has electron density to donate (filled orbital) and a p orbital of another atom that can accept electron density (empty orbital).

pi back bonding formed by the sidewise overlapping of two p orbitals.      

Order of Strength: 

For example- Back Bonding in BF₃: 

Due to boron's electron deficiency and vacant p orbitals, back bonding from fluorine to boron strengthens the B-F bonds by adding a partial double bond character. This leads to shorter bond lengths and higher bond energies.

pπ-dπ Back Bonding

In pπ-dπ back bonding, an electron pair from a filled pπ orbital of a non-metal (the donor) is partially shared with an empty dπ orbital of a transition metal (the acceptor). This interaction not only helps to fill the empty d orbitals of the metal but also stabilizes the electron pair in the non-metal's p orbital.

For example- Trisilyl amine, (N(SiH₃)₃)  (pπ-dπ overlapping)

Note- In Trimethylamine,  (CH₃)₃N Back bonding is not possible due to the absence of vacant orbital while in Trisilyl amine (N(SiH₃)₃)  pπ-dπ back bonding is possible because lone-pair on the Nitrogen atom is donated to the vacant d orbital of Silicon. That’s why Trimethylamine will be a stronger lewis base than Trisilyl amine.

3.0Back Bonding Examples and Observations 

Examples of back bonding include Transition metal carbonyls like nickel tetracarbonyl [Ni(CO)₄], alkenes and alkynes. Let’s learn in detail-

(a) Lewis acidity of Boron and Beryllium halides: 

Back bonding decreases with increasing size of the halogen, leading to stronger Lewis acidity. Boron and Beryllium halides follow an order of increasing Lewis acidity as back bonding decreases.

Lewis acidic strength of boron trihalide:

BF₃ < BCl₃ < BBr₃ < BI₃

(b) Back bonding in metal carbonyls: 

Back bonding in metal carbonyls involves the overlap of partially filled d orbitals of a transition metal with the π* orbital of a carbonyl ligand. This interaction stabilizes the complex by redistributing electron density, strengthening the metal-ligand bond. 

The electronic configuration of CO molecule shows that it has lone pair of electrons on carbon and oxygen atoms each. Carbon atom can donate its electron pair of a transition metal atom (M), forming a coordinate bond (OC → M).

(c) Hybridization in molecules with back bonding: 

If lone pairs participate in back bonding, they are not considered in hybridization. For example, in B₃N₃H₆ (inorganic benzene), both B and N exhibit sp² hybridization.

(d) Back bonding effect on dimer or polymer formation: 

The presence of back bonding reduces the tendency to form dimers or polymers. For instance, BF₃ and BeF₂, where back bonding occurs, exhibit a decreased tendency for dimer or polymer formation.

Basically, the effect of back bonding enhances molecular stability, alters reactivity, modifies electronic structure, affects bond strength, and plays a crucial role in coordination chemistry.

4.0Conditions for Back Bonding

• 1. Presence of π-acceptor Ligands: Back bonding commonly occurs in transition metal complexes where ligands can act as π-acceptors. These ligands have available π* orbitals that can accept electron density from a filled d orbital of the metal center.
• 2. Empty d Orbitals on Central Atom: The central atom should have empty d orbitals available for back donation. This is often observed in transition metals, where the partially filled d orbitals can interact with the ligands' π* orbitals.
• 3. Both bonded atoms must belong to 2nd period or one bonded atom must belong to 2nd period and other belong to 3rd period.

As a result of back bonding between the bonded atoms, bond length decreases and bond energy increases.