Equivalent Hybridization and Non-equivalent Hybridization
Equivalent and non-equivalent hybridization describe the mixing of atomic orbitals to form new hybrid orbitals, which play an important role in determining molecular geometry and bonding characteristics.
Equivalent Hybridization
Equivalent hybridization is a type of hybridization in which all hybrid orbitals are of the same shape, size, and energy. All bond angles are the same. Thus, all resulting hybrid orbitals are equivalent and such hybrid orbitals are called equivalent hybrid orbitals. It is generally observed when same atoms are attached to the central atom.
Examples: CH4, CCl4 etc.
The value of the mixing coefficient (λ) is the same for all hybrid orbitals. This hybridization occurs whenever there are s and p electrons with a slight difference in energy or almost equal energy.
Examples: Sp3 hybridization (CH4).
One s and three p orbitals (px + py + pz) mix to form four sp3 hybrid orbitals having the same shape (tetrahedral), size, energy, and bond angle, which is equal to 109o 28'.
Non-equivalent Hybridization
Non-equivalent hybridization is a type of hybridization in which all hybrid orbitals don't have the same shape, size, and energy. Bond angles are also different. Thus, all resulting hybrid orbitals are not equivalent and such hybrid orbitals are called non-equivalent hybrid orbitals. It is observed when atoms attached to the central atom are not same (atoms or lone pair).
Examples: CHCl3, NH3 etc.
In CHCl3, the values of the mixing coefficient (λ) for hybrid orbitals are not the same.
If three of the hybrid orbitals have one value of mixing coefficient and the fourth hybrid orbital will have a different value of λ. This type of hybridization is not found in carbon only. They will occur whenever there are s and p electrons with approximately the same energy.
For example: Molecules of PCl5. Hybridization in this molecule is sp3d. Two types of orbitals geometry (trigonal bipyramidal and square pyramidal) are possible in this case.
Trigonal bipyramidal: In trigonal bipyramidal, two types of hybrid orbitals are formed. Three equivalent orbitals formed from the mixing of s + px + py and the bond angle in this case is 120°. The three equatorial bonds formed from the hybridization are shorter due to their spatial arrangement and the effective overlap of orbitals.
Similarly, two equivalent orbitals are formed from Pz + dz2, and the bond angle in this case is 90°. The two axial bonds are longer because the axial bonds experience greater electron repulsion and less effective overlap compared to the equatorial bonds.
Square pyramidal: In square pyramidal, two types of hybrid orbitals are formed. Four equivalent orbitals are developed from combination of these orbitals s + px + py + dx2 - y2. Similarly, One pure pz orbital remained the same.
Similarities and Differences in Bonding
Both types of hybrid orbitals overlap with other orbitals to form sigma bonds. Whether equivalent or non-equivalent, hybrid orbitals make strong covalent bonds.
Equivalent hybrid orbitals are limited to single (sigma) bonds only. That’s a disadvantage, but it gives stability and rigidity to some molecular structures.
Non-equivalent hybrid orbitals are not limited to single bonds only. They can form multiple bonds (double or triple bonds) due to their different shapes by overlapping with orbitals on other atoms. That’s an advantage of non-equivalent hybrid orbitals.
Thus, both equivalent and non-equivalent hybrid orbitals contribute to bond formation in molecules, but non-equivalent hybrid orbitals have an extra ability to form multiple bonds in molecules.
