What is Back Bonding ? ~ NEET chemistry

Most of the time we confused about dπ-pπ bonding and Back Bonding. Both terms are same or different? If they are are same, then Why we called them with different name? If they are different then what are the difference between them? Many questions arises in our mind. So, today we are discussing about on these topics.

Content

1.What is back bonding

2. Condition of back bonding

3. Types of backbonding

4. Effects of backbonding

4.1. Bond length

4.2 Hybridisation, shape, bond angle

4.3 Lewis acid and Lewis base

4.4 Adduct formation

4.5 Backbonding in Borazine

5. Questions related to backbonding

Quest : What is back bonding?

Ans : It is also known as π backdonation or π backbonding. When an electron is move from an atomic orbital of an to a π*-antibonding orbital of a π-acceptor ligand this known as back bonding. It is found in the transition metals with multi atomic ligands such as ethylene(CH2CH2), carbonmonoxide(CO) and nitrosonium(NO+).In this process the metal having excess of negative charge is used to bind the ligand. Complexes such as (NiCO4) and Zeises salt exhibit backbonding. It involves a synergic process in which the electrons from the filled π-orbitals or lome pair of the ligand are donated into an empty orbital of the metal and at the same time back donations of electrons take place from the nd orbital of the metal to the empty π*-antibonding orbital of the ligand.

The sigma bomd formed by the partial donation of a pair of electrons from an orbital largely localised on the carbon atom to a d orbital on the metal atom.

Back π bonding from filled d orbital on the metal atom into an empty π* orbital on the CO ligand. In most cases, the net back bonding predominates, and electrons density is transferred from the metal to the CO ligand.

Or

1. The sigma bomd formed by the partial donation of a pair of electrons from an orbital largely localised on the carbon atom to a d orbital on the metal atom.

2. Back π bonding from filled d orbital on the metal atom into an empty π* orbital on the CO ligand.

HOMO(bonding)

LUMO (antibonding)

Sigma bond

π - backbonding

HOMO and LUMO of CO and example of a sigma bonding orbital in which CO donates electrons to a metals centre from its HOMO. At the last picture is an example where the metal centre donates electrons through a d orbital to CO LUMO.

Back bonding Can also be visualised as type of resonance.
This type of bonding is possible between atoms in a compound in which one atom has lone pair of electrons and the other has vacamt orbital placed adjacent to each other.
The bonding acquires partial double bond.
Generally back bonding increases stability. It does affect properties of molecules, like hybridisation and dipole moment.

Condition of back bonding:-

The following conditions should be fulfilled for effective overlapping of atomic orbital and formation of back bonds:

Metal atom should have vacant orbital.
Ligand should have lome pair of electrons or filled π- Orbital.
At least one of the atoms should be from 2nd period. As if both are from 3rd period then effective overlapping will not take place.

Types of backbonding :-

Backbonding is basically of two types

Central atom to side atom
Side atom to central atom

As if central atom has lone pair of electrons or filled π orbitals and side atoms have vacant orbitals then central to side backbonding takes place.

For example N(SiH3)3, where lone pair on nitrogen atom are involve in back bonding with empty orbitals of silicon. Opposite is the case with side to central atom backbonding.

For example BF3 where, electrons from filled π orbitals of fluorine are involve in backbonding with empty orbitals of boron atom.

Backbonding can take place in three ways :

Overlapping of 2pπ-2pπ
2pπ-3pπ
3pπ-2pπ

Since in 2pπ-2pπ overlapping,the energy and symmetry of orbitals of both metal and ligands are same therefore Effective overlapping will take place hence the 2pπ-2pπ backbonding is highly stable. Therefore, with respect to effective overlapping of the orbitals, the order of stability of backbonding is

2pπ-2pπ>2pπ-3pπ>3pπ-3pπ

NOTE: 3pπ-3pπ backbonding is not possible as if metal and ligand both are ftom 3rd period the size of both the atoms will be very large and effective overlapping will not take place.

Effect of backbonding :-

(1) BOND LENGTH : Bond length of the molecule always decreases whether the backbonding is from central atom to side atom or from side atomto central atom. As due to backbonding partial double bond character is induced in the molecule as a result of which bond order increases and hence bond length decreases. This is reason for short B-F bond length in BF3 as compared to[ BF4]- ion.

(2) Hybridisation, shape and bond angle:

Taking an example of BF3 molecule, here lone pair on fluorine atom is donayed to vacant π-orbital of boron and results in side to central atom backbonding. In this the hybridisation of molecule will not change and it remains sp² hybridised. Also the shape of the molecule remains trigonal planar. But in N(SiH3)3 as lone pair on nitrogen atom is donated to empty orbital of the silicon and central to side atom backbonding take place, the lone pair on nitrogen atom is no more in hybridisation and hence the hybridisation of the molecule is changes from sp³ to sp²(percentage s character increases) and hence the geometry of the molecule also changes from pyramidal to trigonal planar. Now as the geometry of the molecule changes by change in hybridisation. Bond angle of the molecule also changes.

(3)Lewis acid and Lewis basic character :-

Back bonding is nothing intramolecular Lewis acid base interaction. For example on comparing the Lewis acidic character of BF3 and BH3 molecule, the Lewis acidic character of BF3 is less than that of BH3 because it undergoes backbonding due to which the π- orbitals of the boron atom is no longer vacant and hence BF3 cannot act as a Lewis acid.

Similarly, in the case of N(SiH3)3 the lone pair of electron on nitrogen atom is involved in backbonding with vacant π-orbital of Si atom. They are no longer available for further donation and hence N(SiH3)3 is less basic than N(CH3)3 which do not involve in backbonding as carbon atom do not have vacant π-orbitals.

This is the reason of better Lewis basic character of ethers than that of (SiH3) 2O

(4) Adduct formation :- Either the lone pair or the vacant π-orbital of the molecules are involved in backbonding. In both the cases the efficiency of bond formation decreases. This is the reason for poor adduct of (SiH3) 2O and BF3 as both are involved in backbonding.

Similarly NH3 and BBr3 forms good adducts beacuse none of them are involved in backbonding. Therefore, we cansay that the molecules which are better Lewis base can form better adducts.

(5) Backbonding in Borazine(Inorganic benzene) :-

Borazine is also known as Inorganic benzene because its structure is similar to that of benzene. In borazine nitrrogen and boron are bonded as a 6 membered ring or has 6π electron system similar to that of benezen. But as compared to benzene, borazine is kinetically less stable and thermodyanmically more stable. This is due to backbonding between boron and nirogen atoms which induced polarity within the molecule and the bond between nitrogen and boron become ionic.

Now in borazine although nitrogen is more electronegative than boron but due to backbonding there will be a partial negative charge on boron and partial positive charge

On nitrogen (nitrogen has lone pair of electron which are backbonded with vacant pi orbital of boron). For example if Borazine is going to react with HCl then following reaction will take place.

Question related to backbonding :

Question: Does backbonding affect the. bond angle?

Ans : Yes, back bonding affects the bond angle. In case of molecule having lone pair upon the central atom available for back bonding, bond angle increases. Bond angle in NCl3 is larger than NF3 due to back bonding.

Question : Does backbonding affect bond. length?

Ans : This type of bonding is possible between atoms in a compound in which one atom has lone pair of electron and the other has vacant orbital placed adjacent to each other. Consequently, the backbonding acquires partial double bond character. Backbonding results in a decrese in bond length and increase in bond order.

Question: Does back bonding affect the hybridisation?

Ans : NH2-BH2 , here 2pπ-2pπ back bonding takes place between lone pair orbital of nitrogen (2p) and vacant orbital of boron (2p). So, nitrogen gets sp² character.

(SiH3) 3O , here "2pπ-3dπ" backbonding takes place between lone pair orbital of oxygen (2p) amd vacant orbital of silicon (3d). So, again oxygen gets sp² character.

Question : Which molecule has strong back bonding?

Ans: Electronegative substituents exhibit greater π-backbonding. Thus, strong π- backbonding ligands are tetrafluroethylene (CF2-CF2), tetracyanoethylene and hexafluro-2-butyne.

Question : Is there backbonding in BF4?

Ans : In [BF4-] ion, Boron is sp³ hybridised. It doesnot empty 2p-orbital . So, there is no backbonding in BF4 ion. All the four B-F bonds are purelh single bond.

Question : Does back bondinv affect planarity?

Ans : No, Backbonding does not affect the planarity.

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What is Back Bonding ?

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