Hybridisation

Hybridisation is the process of intermixing of the orbitals of slightly different energies so as to redistributed their energies, resulting in the formation of new set of orbitals of equivalent energies and shape.

Salient features of hybridisation

1. The number of hybrid orbitals is always equal to the number of atomic orbitals that get hybridised.

2. These hybridised orbitals are always having similar energy and shape.

3. These hybridised orbitals are more effective in forming stable bonds in comparison of the pure atomic orbitals.

4. These hybridised orbitals are directed in space in some preferred direction to have minimum repulsion between electron pairs and thus a stable arrangement. Hybridisation always indicates the geometry of the molecules.

Important conditions for hybridisation

1. Orbitals of the valence shell of the atom are hybridised.

2. The orbitals taking part is hybridisation must have only a small difference of energies.

3. Promotion of electron is not mandatory condition prior to hybridisation.

4. It is not required that only half filled orbitals participate in hybridisation. In some cases, even filled orbitals of valence shell or vacant orbital can take part in hybridisation.

Types of hybridisation

Let us discuss various types of hybridisation along with some examples:

(i) sp-hybridisation:

In this hybridisation one s and one p orbitals hybridise (or intermix) to produce two equivalent hybrid orbitals, known as sp hybrid orbitals. The orbitals used for sp hybridisation are s and p_z, if the hybrid orbitals are to lie along the z-axis. The two sp-hybrid orbitals are oriented in a straight line making an angle of 180⁰ and therefore the molecule possesses linear geometry.

Each of hybrid orbitals has 50% s-character and 50% p-character. This hybridisation is also known as diagonal hybridisation. The two sp hybrids point in the opposite direction along the z-axis with projecting positive lobes are very small negative lobes, which provides more effective overlapping resulting in the formation of stronger bonds.

Example of molecules having sp-hybridisation are BeF₂, BeCl₂, BeH₂, HC CH, CO₂, HgCl₂ etc.

(ii) sp²-hybridisation :

In sp² hybridisation one s and two (p_x and p_y) orbitals of one atom hybridize to give three equivalent sp² hybrid orbitals. These three sp² hybrid orbitals are directed towards the three corners of an equilateral triangle with an angle of 120⁰ and give a triangular geometry to the molecule. Each sp² hybrid orbitals has 1/3 (or 33.33%) s-character and 2/3 (or 66.7%) p-character.

Example, of sp² hybridisation are compounds of boron (e.g., BF₃, BCl₃, BH₃ etc.), Aluminium (e.g., AlCl₃), carbon containing double bond (e.g., CH₂=CH₂) and SO₂, SO₃, etc.

In BF₃, boron is the central atom. Its electronic configuration in the ground state and excited state are one half filled 2s orbital and two half filled 2p orbital undergo hybridisation and produce three equivalent half filled sp²­ hybrid orbitals. These hybrid orbitals are trigonal planar and are oriented at an angle of 120⁰ to each other. These three sp² hybrid orbitals overlap with half filled 2p orbital of three fluorine atoms to form three B-F sigma bonds. The resultant geometry of BF₃ molecule is trigonal planar.

(iii) sp³ hybridisation :

In this hybridisation one s and three p-orbitals intermix to form sp³ hybrid orbitals of equivalent energy and identical shape. These four sp³ hybrid orbitals are directed towards the four corners of a tetrahedron separated by an angle of 109⁰, 28’. sp³ hybrid orbitals have ¼ (or 25%) s-character and 3/4 (or 75%) p-character.

sp³ hybridisation is observed in alkaline (CH₄, C₂H₆, C₃H₈, C₄H₁₀ etc.), CCl₄, SiCl₄, NH₃, , H₂O, H₃O⁺, .

Hybridisation of elements involving d-orbitals

The participation of d-orbitals in hybridisation scheme can take place when d-orbitals of the lower shell used for hybridisation depending upon the nature of molecules. For example, 3d-orbitals can be involved in hybridisation with 3s, 3p-orbitals and also with 4s and 4p-orbital. It is because of the fact that the energy of 3d orbitals is comparable to 3s and 3p orbitals and also to s and p-orbitals of 4^th shell.

1. sp³d-Hybridisation

This type of hybridisation involves mixing of one s, three p and one d-orbitals to form five sp³d hybridised orbitals which adopt trigonal bipyramidal . Three of the hybrid orbitals lie in horizontal plane and lie at angle of 120⁰ to one another. These are called equatorial orbitals (marked as e). The other two hybrid orbital, lie in vertical plane at right angle to the plane of equatorial orbitals and are called axial orbitals (marked as a).

Formation of PCl₅ :

The ground state electronic configuration of phosphorus is 1s² 2s² 2p⁶ 3s² 3p³. Under the conditions of bond formation the 3s-electrons get unpaired and one of the electron is promoted to vacant  orbital.

Now the five orbitals, one (3s) three (3p) and one  ( )which are half-filled, hybridise to yield a set of five sp³d hybrid orbitals, which point towards the five corners of a trigonal bipyramidal.

In the formation PCl₅ each of the five sp³d hybrid orbitals overlap axially with half filled 3p-orbitals of Cl atom to form five P-Cl sigma bonds. Three of the five P-CI bonds lie in one plane at an angle of 120⁰ to one another. These are called equatorial bonds. The other two P-Cl bonds are at right angle of 90⁰ to the plane of equatorial bond, i.e., one above and other below the plane. These are called axial bonds.

As the axial bond pairs suffer more repulsive interaction from the equatorial pairs, therefore axial P-Cl bonds become slightly longer (219 pm) than the equatorial bonds (204 pm). The unequal length of axial and equatorial P-Cl bonds in PCl₅ has been confirmed by X-ray diffraction technique. This causes axial bonds to become slightly weaker than the equatorial bonds; which makes PCl₅ molecule more reactive.

2. sp³d² – Hybridisation

In this type of hybridisation one s , three p and two d-orbitals undergo intermixing to form six identical sp³d² hybrid orbitals. These six orbitals are directed towards the corners of an octahedron and lie in space at an angle of 90⁰ to one another.

For example the ground state outer configuration of ₁₆S is 3s²3p⁴. In the excited state the electron pairs in 3s and 3p_x orbitals get unpaired and one out of each pair is promoted to vacant   and  orbitals. Now, six orbitals; one (3s), three (3p) and two (3d) orbitals which are half-filled hybridise to form six new sp³d² hybrid orbtials which are projected towards the six corners of a regular octahedron.

These six sp³d² hybrid orbitals overlap with half-filled orbitals of fluorine atoms to form six S-F sigma bonds.

Because of sp³d² hybridisation of sulphur, SF₆ has regular octahedral geometry. All the bonds in SF₆ have same bond length.

3. sp³d³ – Hybridisation

This type of hybridisation involves the mixing of one s three p and three d orbitals to form seven sp³d³ hybrid orbitals which adopt pentagonal bipyramidal arrangement.

Formation of IF₇ : The ground state configuration of ₅₃l is 5s²5p⁵. In the excited state one electron from 5s and two electron from 5p-orbital are promoted to 5d-orbtials. The seven orbitals now hybridise to give seven sp³d³ hybrid orbitals each of these hybrid orbitals overlap with half filled 2p-orbital of fluorine atoms to form IF₇ molecule which adopts the pentagonal bipyramidal geometry.

In this geometry, the five I-F bonds lie in one plane at an angle of 72⁰, these are equatorial bonds. The two I-F bonds lie perpendicular to the plane of equatorial bonds. These are axial bonds (marked as a). The axial bonds are relatively longer than equatorial bonds.