Hybridization

Key points: Hybrid orbitals are formed when atomic orbitals on the same atom interfere; specific hybridization schemes correspond to each local molecular geometry.

The description of the bonding in AB₄ molecules of Group 14 is still incomplete because it appears to imply the presence of three Ϭ bonds of one type (formed from X_B and X_A2p orbitals) and a fourth bond of a distinctly different character (formed from X_B and XA₂s), whereas all the experimental evidence (bond lengths and strengths) points to the equivalence of all four A B bonds, as in CH₄, for example.

This problem is overcome by realizing that the electron density distribution in the promoted atom is equivalent to the electron density in which each electron occupies a hybrid.

orbital formed by interference, or ‘mixing’, between the A2s and the A2p orbitals. The origin of the hybridization can be appreciated by thinking of the four atomic orbitals, which are waves centred on a nucleus, as being like ripples spreading from a single point on the surface of a lake: the waves interfere destructively and constructively in different regions, and give rise to four new shapes. The specific linear combinations that give rise to four equivalent hybrid orbitals are

Asa result of the interference between the component orbitals, each hybrid orbital con sists of a large lobe pointing in the direction of one corner of a regular tetrahedron and a smaller lobe pointing in the opposite direction (Fig. 2.7). The angle between the axes of the hybrid orbitals is the tetrahedral angle, 109.47°. Because each hybrid is built from ones orbital and three p orbitals, it is called an sp³ hybrid orbital.

It is now easy to see how the VB description of a CH₄ molecule is consistent with a tetra hedral shape with four equivalent C H bonds. Each hybrid orbital of the promoted carbon atom contains a single unpaired electron; an electron in H1s can pair with each one, giving rise to a bond pointing in a tetrahedral direction. Because each sp3 hybrid orbital has the same composition, all four bonds are identical apart from their orientation in space. A further feature of hybridization is that a hybrid orbital has pronounced directional character, in the sense that it has enhanced amplitude in the internuclear region. This directional character arises from the constructive interference between the s orbital and the positive lobes of the p orbitals. As a result of the enhanced amplitude in the internuclear region, the bond strength is greater than for an s or p orbital alone. This increased bond strength is another factor that helps to repay the promotion energy.

Hybrid orbitals of different compositions are used to match different molecular geometries and to provide a basis for their VB description. For example, sp² hybridization is used to reproduce the electron distribution needed for trigonal-planar species, such as on B in BF₃ and N in NO₃^- , and sp hybridization reproduces a linear distribution. Table 2.4 gives the hybrids needed to match the geometries of a variety of electron distributions.

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مواضيع ذات صلة

Polytypism

The close packing of spheres

Lattices and unit cells

The orbitals

Bonding and antibonding orbitals

Hypervalence

Promotion

Polyatomic molecules

Homonuclear diatomic molecules

The hydrogen molecule

The basic shapes

Resonance