75.Covalent Bonding(21)- Hybridization in solid-state.

How do compounds exist in solid-state?

Compounds exist as ions in the solid-state. Consider the following example –  

PCl₅ molecule – This molecule has trigonal bipyramidal (TBP) geometry, which is a little less stable as compared to octahedral or tetrahedral geometry.  As there are 5 substituents, this molecule is sp³d hybridized. Thus, in the solid-state, it splits into two ions as follows – 

[PCl₆] ^– ion has six substituents/ligands and so it has an octahedral geometry.

The central atom phosphorous is pentavalent i.e it has 5 valence electrons. However, in this ion, it is attached to six chlorine atoms. This is possible because a phosphorous atom can have an expanded octet i.e it has vacant valence d-orbitals, where it can accommodate an extra electron, from the sixth chlorine atom.  This one extra electron, which is accommodated in the P atom’s d-orbital (which comes from another PCl₅ molecule)  gives this ion a negative charge.

[PCl₄] ⁺ ion has four substituents/ligands and so it has a tetrahedral geometry. It is sp³ hybridized.

Although phosphorous is pentavalent, in this ion, it is attached to only 4 substituents. The fifth valence electron is transferred to another PCl₅ molecule to form a [PCl₆] ^– ion. As one electron is transferred to another species, the net charge on this ion is +1. 

The transfer can be shown as follows – 

Many compounds show this phenomenon. Some examples are –

1. PBr₅   →PBr₄⁺+ Br ^–…. Note that this molecule forms ions differently than PCl₅ as the size of the bromine atom is bigger than that of chlorine. The six bigger Br atoms do not sterically fit around the P atom. Thus, we see some distortion and trigonal bipyramidal geometry. The two bromine atoms are at the axial positions, and the other three are at the equatorial positions.PI₅ does not exist as iodine atoms are too large to fit into an octahedral or square pyramidal geometry.
2. Cl₂0₆ → Cl₂0₂⁺  + Cl₂0₄^–

In our next post, we shall study another rule which is important in the hybridization series. Till then, 

Be a perpetual student of life and keep learning… 

Good day!

References and Further Reading –

1. https://www.youtube.com/watch?v=6xBRMuqPoss