Hybridization is the mixing of atomic orbitals to form a new orbital. The main three types of hybridization possible involving only s and p orbitals are sp3, sp2, sp. Sp3 results in single bonds, sp2 for double bonds, and sp for triple bonds. The new hybridized orbital has properties and energy which is almost an average of the unhybridized orbitals which took part in hybridization. Hybridization of elements of the third period includes the involvement of d orbitals which otherwise remain vacant in Phosphorus. Phosphorus is an element of the third period and is a p-block element. Phosphorus has five valence electrons which can take part in hybridization with one electron taken by each Chlorine atom and the resultant hybridization for PCl5 is thus sp3d. Chlorine is also a p-block element with vacant d orbitals and has seven valence electrons with a tendency to take one electron to fill its valence shell. The sp3d hybridization results in trigonal bipyramidal shape following the VSEPR theory with 5 chlorine atoms occupying the five corners of the trigonal bipyramid. PCl5 is a useful compound with chlorinating properties.
Let us discuss the essential hybridizations, including the s, p, d orbitals below.
Important Hybridizations
|
Shape of Molecule |
Atomic Orbitals |
Type of Hybridization |
Example |
|
Trigonal Bipyramidal |
s+p(3)+d |
sp3d |
PCl5 |
|
Square Planar |
d+s+p(2) |
dsp2 |
[Ni(CN)4]2– |
|
Square Pyramidal |
s+p(3)+d(2) |
sp3d2 |
BrFl5 |
|
Octahedral |
s+p(3)+d(2), d(2)+s+p(3) |
Sp3d, d2sp3 |
CrF6CrF63–, Co(NH3)6Co(NH3)63+ |
Formation of PCl5
The excited state and ground state of outer electron configurations for Z=15 (phosphorus) are represented below.
The sp3d hybridization can be represented as follows.
The five orbitals, viz 1s, 3p, and 1d orbitals, are free for the hybridization process. Thus, it can obtain a 5sp3d hybrid orbital set, which is directed to the 5 corners of a trigonal bipyramidal (according to the VSEPR theory).
It is prominent that the entire bond angles present in the trigonal bipyramidal geometry are identical. The 5sp3d orbitals present in the PCl5 of phosphorus overlap with the p chlorine atom’s orbitals, where the p orbitals are singly occupied. They form 5 P–Cl sigma bonds together.
Types of Bonds Formed During PCl5 Hybridization
Axial Bonds: 2 P–Cl bonds at which one lies above the equatorial plane and the other bond below the plane to form an angle with the plane. The angle made with the plane is given as 90°.
Equatorial Bonds: 3 P–Cl bond at which lies in one plane to form an angle with each other. The angle made between them is given as 120°.
Because the axial bond pairs agonize massive repulsive interaction from the equatorial bond pairs, the axial bonds tend to be a bit longer. Thus, it makes it a bit weaker than the equatorial bonds, resulting in a more reactive PCl5 molecule.
Hybridization of PCl5 Central Atom
In solid-phase, the PCl5 molecule remains as an ion pair of PCl4+ and PCl6–. Whereas, in PCl4+, the central atom P contains 4 bonding electron pairs and zero lone pair electrons. Thus, it can be given as sp3 hybridized, and PCl4+ is represented in tetrahedral in shape. Moreover, in PCl6–, the P atom contains 6 bonding electron pairs and zero lone pair electrons, and PCl6– becomes octahedral in shape.
In the gaseous phase, this compound remains as PCl5. Here, the central atom P is bonded to 5 Cl atoms via 5 sigma bonds, and there is zero lone electron pair. Thus, in the gaseous phase, P can be given as PCl5 is sp3d hybridized, and the shape of the molecule is trigonal bipyramidal.
Calculation of Hybridization
One of the best and easiest methods to calculate the hybridization is to count the surrounding atom leaving the primary atom. For example, in the NH4+ compound, N is the primary atom, and there are 4 surrounding atoms. So, according to the formula,
H = SA+½ (G-V+E for negative charge and -E for a positive charge )
Where H is the hybridization,
SA is the surrounding atom,
G is the valence electron for the primary atom,
V is the valency of all surrounding atoms,
E is the number of charges.
In NH4+ compound, H is 4+ ½ (5-4-1) = 4+0.
Where 4 is the bond pair, 0 is the lone pair = 4.
Therefore, here, the hybridization is sp3, and the shape is tetrahedral. We have to count the full primary atom, simple valency, and atom valence electron in V for all surrounding atoms in this hybridization. For example, in the iodine compound, G is 7, whereas V is 1. But, in the NO2+ compound, H = 2+ ½ (5-4-1) = 2. It is also sp hybridized and linear. In the same way, we can try for more hybridization.
Polarity of PCl5
PCl< /span>5 is polar. The structure is not covalent, but it is ionic. It means PCl4+/Cl–. Phosphorus trichloride (PCl3) is a significantly less polar liquid, with a boiling point of 73°. If we are likely to form an acid chloride (for suppose, p-chlorobenzene acid with a melting point of 242°), we had better not try heating either with thionyl chloride (SOCl2) or PCl3. It is because they are not polar enough to dissolve the high-melting crystals even at their respective boiling points. To make that acid chloride, we need to melt the PCl5 compound and the chlorobenzene acid together ~170°, and the conversion will become smooth on continued heating.