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Question:

Which of these is least likely to act as a Lewis base?

CO

BF3

F−

PF3

Solution:

CO, F− and PF3 can either donate a lone pair of electrons or pi electrons. Hence, they act as Lewis bases. On the other hand, BF3 has a vacant orbital in which it can accept a lone pair of electrons or pi electrons. Hence, it acts as a Lewis acid. Thus, it has the least tendency to act as a Lewis base.