Question

Which of the following is not a Lewis acid?
A.$B{F_3}$
B.$AlC{l_3}$
C.$FeC{l_3}$
D.$P{H_3}$

Answer 1

Mostly structures having trigonal planar geometry are known to be Lewis acids because they have empty p-orbitals. Any electron deficient pi system can act as a Lewis acid as it can accept electron pairs.

Complete answer:
First let us see what Lewis acids are. Many acid-bases theories were proposed and one of them was Lewis acid and Lewis base. This concept was based upon the concept of acceptance and donation of electron pairs.
The chemical species which have orbitals and thus which are able to accept electron pairs are known as Lewis acids. The pentahalides of group 15 are good Lewis acids such as Arsenic, Antimony, Phosphorous.
The chemical species which are able to donate electron pairs are known as Lewis bases. They have highly localized HOMO (Highest Occupied Molecular Orbital). They can form an adduct by donating electron pairs to Lewis acids. Ammonia, alkyl amines and other amines are known to be good Lewis bases. They are known as electrophiles. Group 16 elements act as a good Lewis base.
Since we know structures having trigonal planar geometry are known to be Lewis acids so $B{F_3}$ has trigonal planar structure and it can act as good Lewis acid. $P{H_3}$ is a base it does not want electrons so it is not a Lewis acid

So, option D is the correct answer.

Note:
Boron trifluoride is very helpful in organic synthesis because it is a good Lewis acid. It is used in production of sealant chemicals, adhesives and lubricants. It is also used as a catalyst in esterification, alkylation and condensation. Molecular association does not place in $B{F_3}$ because of back donation of electrons from fluorine to boron that is why $B{F_3}$ is a gas.