There are many different types of hybridization depending upon the type of orbitals involved in mixing such as sp3, sp2 , sp. sp3d. sp3d2 , etc. Let us discuss various types of hybridization along with some examples.
The type of hybridization involves the mixing of one orbital of s-sub-level and three orbitals of p-sub-level of the .valence shell to form four sp3 hybrid orbitals of equivalent energies and shape. Each sp3 hybrid orbital bas 25% s-character and 75% p-character. These hybridized orbitals tend to lie as far apart in space as possible so that the repulsive interactions between them are minimum. The four sp3 hybrid orbitals are directed towards the four comers of a tetrahedron. The angle between the sp3 hybrid orbitals is 109.5° (Fig. 9.4).
Fig. 9.4. Tetrahedral orientations of sp3 hybrid orbitals.
sp3 hybridization is also known as tetrahedral hybridization. The molecules in which central atom is sp3 hybridized and is linked to four other atoms directly, have tetrahedral shape.
Some Examples of Molecules where Central Atom Assume Sp3 Hybridization
1. Formation of methane (CH4) . In methane carbon atom acquires sp3 hybrid states as described below: Here, one orbital of 2s-sub-shell and three orbitals of 2p-sub-shell of excited carbon atom undergo hybridization to form four sp3 hybrid orbitals. The process involving promotion of 2s-electron followed by hybridization is shown in Fig. 9.5.
Fig. 9.5. sp3 hybridization of carbon.
As pointed out earlier the sp3 hybrid orbitals of carbon atom are directed towards the comers of regular tetrahedron. Each of the sp3 hybrid orbitals overlaps axially with half filled Is-orbital of hydrogen atom constituting a sigma bond (Fig. 9.6).
Fig. 9.6. Orbital picture of methane.
Because of sp3 hybridization of carbon atom, CH4 molecule has tetrahedral shape.
2. Formation of ethane (CH3-C3). In ethane both the carbon atoms assume sp3 hybrid state. One of the hybrid orbitals of carbon atom overlaps axially with similar orbital of the other carbon atoms to form sp3-sp3 sigma bond. The other three hybrid orbitals of each carbon atom are used informing sp3-s sigma bonds with hydrogen atoms as describedin Fig. 9.7.
Fig. 9.7. Orbital picture of ethane.
Each C-H bond in ethane is sp3-s sigma bond with bond length 109 pm. The C-C bond is sp3-sr sigma bond with bond length 154 pm.
3. Formation of ammonia (NH)3 molecule; In NH3 molecule the nitrogen atom adopts sp3-hybrid state. Three of sp3-hybrid orbitals of the N atom are used for forming sp3-scr (sigma) bonds with H atoms. The fourth sp3-hybrid orbital carry lone pair of electrons. The relatively larger lone pair bond pair interactions cause HNH angle to decrease from 109°.281 to 107°. The ground state, hybrid state of N atom and orbital overlap in are shown in Fig. 9.8.
Fig. 9.8. Formation of NH3 molecule.
4. Formation of water (H2O) molecule. In H2O molecule, oxygen atom adopts sp3 hybrid state. Two of the sp3-hybrid-orbitals of oxygen contains lone pairs of electrons whereas the other two hybrid orbitals constitutesp3-scr (sigma) bonds with H atoms. The lone pair orbitals exert relatively greater repulsive interactions on bond pair-orbitals causing HOH angle to decrease from 109°.28′ to 104.5°. The hybridization Of O atom along with orbital overlap in molecule are shown in Fig 9.9.
Fig. 9.9. Formation of H2O molecule.
This type of hybridization involves the mixing of one orbital of s-sub-level and two orbitals of p-sub-level of the valence shell to form three sp2 hybrid orbitals. These sp2 hybrid orbitals lie in a plane and are directed towards the corners of equilateral triangle (Fig. 9.10).
Fig. 9.10. Trigonal planar orientation of sp2 hybrid orbitals.
Each sp2 hybrid orbital has one-thirds-character and two third p-character. sp2 hybridization is also called trigonal hybridization. The molecules in which central atom is sp2 hybridized and is linked to three other atoms directly have triangular planar shape.
Let us study some examples of the molecules which involve sp2 hybridization.
1. Formation of boron trichloride (BCl2). Boron (5B) atom has ground state configuration as ls2, 2s2, 2p1 But in the excited state its configuration is ls2, 2s1, 2px1, 2p/. One 2s-orbital of boron intermixes with two 2p-orbitals of excited boron atom to form three sp2 hybrid orbitals as shown in Fig. 9.11.
Fig. 9.11. sp2 hybrid state of boron.
The sp2 hybrid orbitals of boron are directed towards the comers of equilateral triangle and lie in a plane. Each of the sp2 hybrid orbitals of boron overlaps axially with 3p-halffilled orbital of chlorine atom to form three B-Cl sigma bonds as shown in Fig. 9.12.
Fig. 9.12. Orbital diagram of BCI3 .
Because of sp2 hybridization of boron, BC~ molecule has triangular planar shape.
2. Formation of ethylene (C2H4). Both the carbon atoms in ethylene assume sp2 hybrid state. In acquiring sp2 hybrid state, one 2s-orbital and two 2p-orbitals of excited carbon atom get hybridized to form three sp2 hybridized orbitals. However, one orbital of 2p-sub-shell of the excited carbon atom does not take part in hybridization. The promotion of electron and hybridization in carbon atom is shown in Fig. 9.13.
Fig. 9.13. sp2 hybrid state of carbon
As already indicated, the three sp2 hybrid orbitals lie in one plane and are oriented in space at an angle of 120° to one another. The unhybridized 2p-orbital is perpendicular to the plane of sp2 hybrid orbitals as shown in Fig. 9.14.
In the formation of ethylene (C2H4) one of the sp2 hybrid orbital of carbon atom overlaps axially with similar orbital of the other carbon atom to form C-C sigma bond. The other two sp2 hybrid orbitals of each carbon atom are utilised for forming sp2-s sigma bond with two hydrogen atoms.
The unhybridised 2p-orbitals of the two carbon atoms overlap sidewise each other to form two 1t clouds distributed above and below the plane of carbon and hydrogen atoms (Fig. 9.15).
Fig. 9.15. Orbital diagram of ethylene
Thus, in ethylene, the six atoms (bonded by sigma bonds) lie in one plane while the 1t bond is projected perpendicular to the plane of six atoms (two C atoms and four H atoms) as shown in Fig. 9.15 (c).
In ethylene molecule, the C = C bond consists of one sp2-sp2 sigma bond and one 1t bond. Its bond length is 134 pm. C-H bond is sp2-s sigma bond with bond length 108 pm.
The H-C-H angle is 117.5° while H-C-C angle is 121°.
This type of hybridization involves the mixing of one orbital of s-sub-level and one orbital of p-sub-level of the valence shell of the atom to form two sp-hybridized orbitals of equivalent shapes and energies. These sp-hybridized orbitals are oriented in space at an angle of 180° (Fig. 9.16). This hybridization is also called diagonal hybridization. Each sp hybrid orbital has equal s and p character, i.e., 50% s-character and 50% p-character. The molecules in which the central atom is sp-hybridized and is linked to two other atoms directly have linear shape.
Fig. 9.16. Formation of sp hybrid orbitals.
Let us study some examples of molecules involving sp-hybridization.
1. Formation of beryllium fluoride (BeCl2). Beryllium· ~Be) atom has a ground state configuration as ls2, 2s2. In the excited state one of the 2s-electron is promoted to 2p-orbitals. One 2s-orbital and one 2p-orbital of excited beryllium atom undergo sp-hybridization to form two sp-hybridized orbitals as described in Fig. 9.17.
Fig. 9.17. sp-hybrid state of Be
The two sp-hybrid orbitals are linear and oriented in opposite directions at an angle of 180°. Each of the sp-hybridized orbital overlaps axially with 3p-half-filled orbital of chlorine atom to form two Be-Cl sigma bonds (fig. 9.18).
Due to the sp-hybridised state of beryllium, BeCl2 molecule has linear shape.
2. Formation of ethyne (CH = CH). Both the carbon atoms in ethyne assume sp-hybrid state. In acquiring sp-hybrid state, one 2s orbital and one 2p-orbital of excited carbon atom (1s2 2s12p1 2p1 2p1) get hybridized to form two spbybridized orbitals (Fig. 9.19).
Fig. 9.19. sp-hybridization of carbon.
The two sp-hybrid orbitals of carbon atom are linear and are directed at an angle of 180° whereas the unhybridized p-orbitals are perpendicular to sp-hybrid orbitals and also perpendicular to each other as shown in Fig. 9.20.
Fig. 9.20 sp-hybrid state of carbon.
In the formation of ethyne, carbon atom uses its one of the sp-hybrid orbital for overlapping with similar orbital of the other carbon to form C–C sigma bond. The other sp-hybrid orbital of each C atom overlaps axially with Is-orbital of H atom to form C-H sigma bond. Each of the two unhybridized orbitals of both the carbon atoms overlap sidewise to form two n-bonds. The electron clouds of one n-bond lie above and below the inter-nuclear axis whereas those of the other 1t bond lie in front and back of the inter-nuclear axis. The overlapping of orbitals has been shown in Fig. 9.21
Fig. 9.21. Orbital diagram of ethyne.
The four 1t clouds so formed further merge into one another to form a single cylindrical electron cloud around the inter-nuclear axis representing C–C sigma bond. It has been shown in Fig. 9.22.
Fig. 9.22. Cylindrical n-electron cloud in ethyne.
Thus, in ethyne molecule,
C = C bond consists of one’ sp-sp σ bond along with two n-bonds. The C = C bond length is 120 pm. C-H bond is sp-s sigma bond. The H-C~C angle is 180°, i.e., the molecule is linear.
HYBRIDIZATION INVOLVING d-ORBITALS
The participation of d-orbitals in hybridization scheme can take place when d-orbitals are lying vacant or partly filled. s and p-orbitals of the outer most shell can use d-orbitals of the outer shell as well as d-orbitals of the lower shell for hybridization depending upon the nature of molecule. For example, 3d-orbitals can be involved in hybridization 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 4th shell. The important hybridization schemes of s, p, d-orbitals are summarised below in tabular form:
It may be noted that here, we shall restrict our discussion to the hybrid schemes involving outer shell d-orbitals. We shall discuss the examples of sp3-d and sp3cf- hybridization.
This type of hybridization involves mixing of three p and one d-orbitals to form five sp3-d -hybridized orbitals which adopt trigonal bipyramidal geometry as shown in Fig. 9.23. Three of the hybrid orbitals lie in horizontal plane at angle of 120° to one another. These are called equatorial orbitals (marked as e). The other two hybrid orbitals, lie in vertical plane at right angle to the plane of equatorial orbitals and are called axial orbitals (marked as a).
Fig. 9.23. Formation of five sp3d hybrid orbitals.
1. Formation of PC15 The ground state electronic configuration of phosphorus is ls2 2s2 2p6 3s2 3p3 Under the conditions of bond formation the 3s-electrons get unpaired and one of the electron .is promoted to vacant 3d 2 orbital. Z The ground state and excited state configurations of phosphorus are shown in Fig. 9.24.
Fig. 9.24. Ground state and excited state configuration of phosphorus.
Now the five orbitals, one (3s), three (3p) and one (3dz2) which are half-filled, hybridized to yield a set of five sp3 d hybrid orbitals, which point towards the five corners of a trigonal bipyramid (Fig. 9.25).
Fig. 9.25 Trigonal bipyramidal geometry of PCI5 molecule.
In the formation of PC15, each of the five sp3d hybrid orbitals overlap axially w1th bipyramidal geometry of half-filled 3p-orbital of Cl atom to form five P-Cl sigma bonds. Three of the five P-Cl bonds lie in one plane at an angle of 120° to one another. These are called equatorial bonds. The other two P-Cl bonds are at right angle to the plane of equatorial bonds, 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 bond pairs, therefore, axial P-CI bonds become slightly longer (219 pm) than the equatorial bonds (204 pm). The unequal length of axial and equatorial P-Cl bonds in PC15 has been confirmed by X-ray diffraction technique. This causes axial bonds to become slightly weaker than the equatorial bonds; which makes PC15 molecule more reactive.
In this type of hybridization one s, three p and two d-orbitals undergo intermixing to form six identical sp3 d2 hybrid orbitals. These six orbitals are directed towards the corners of an octahedron and lie in space at an angle of 90° to one another.
Fig. 9.26. Formation of six sp3d2-hybrid orbitals.
1. Formation of SF 6 The ground state outer configuration of 16S is 3s23p4 In the excited state the electron pairs in 3s and 3px orbitals get unpaired, and one out of each pair is promoted to vacant 3dz2 and 3dx2-y2 orbitals. The ground state and excited state configurations of 16S are given as follows:
Now, six orbitals; one (3s), three (3p) and two (3d) orbitals which are half-filled, hybridized to form six new sp3d2 hybrid orbitals which are projected towards the six corners of a regular octahedron as shown in Fig. 9.27.
Fig. 1.27. Octahedral geometry of SF 8.
In the formation of SF6, these six sp3 d2 hybrid orbitals overlap with half-filled orbitals of fluorine atoms to form six S-F sigma bonds.
Because of sp3d2 hybridization of sulphur, SF 6 has regular octahedral geometry as shown in Fig. 9.27. All the six S-F bonds in SF 6 have same bond length.