Some electron deficient compounds, tend to attain octet by back bonding. Back bonding acceptor must have a vacant orbital and back bonding donor must have atleast one lone pair of electrons.
I.
CO
![](https://df0b18phdhzpx.cloudfront.net/ckeditor_assets/pictures/800009/original_25a.png)
It has a complete octet, there is no back bonding.
II.
PCl5
![](https://df0b18phdhzpx.cloudfront.net/ckeditor_assets/pictures/799899/original_25b.png)
It has an expanded octet and hence, there is no back bonding.
III.
BF3
B→ empty 2p orbital and electron deficient.
F→ 3 lone pairs.
∴ shows back bonding.
IV.
BCl3
B→ empty 2p orbital.
Cl→ 3 lone pairs.
∴ shows back bonding.
V.
SiH4
Si has a complete octet and
H has no lone pair. There is no back bonding
VI.
SiF4
![](https://df0b18phdhzpx.cloudfront.net/ckeditor_assets/pictures/799914/original_25f.png)
It has a complete octet. However, since
Si has vacant d-orbital & flourine has lone pair, hence it undergoes back bonding.
VII.
CCl4
C→ no empty orbital.
Cl→ 3 lone pairs.
∴ No back bonding.
VIII.
BeCl2
![](https://df0b18phdhzpx.cloudfront.net/ckeditor_assets/pictures/799930/original_25h.png)
Though electron deficient, it does not undergo back bonding. It undergoes polymerization.
IX.
N(SiH3)3
![](https://df0b18phdhzpx.cloudfront.net/ckeditor_assets/pictures/799956/original_25j.png)
It has a complete octet. However, since
Si has vacant d-orbital &
N has lone pair, hence it undergoes back bonding.
X.
H2O
O→ No empty orbital.
H→ No lone pairs.
∴ No back bonding.