Question

Which of the following species (molecule or ion) can behave both as a Lewis Base as well as a Bronsted base?

• A. $B{F}_3$
• B. $C{l}^{-}$
  
• C. $CO$
  
• D. None of the above

Answer 1

The correct option is B $C{l}^{-}$


According to Bronsted-Lowry and Lewis Concept of acid and bases:
Species (molecule or ion) which has a tendency to accept a proton can act as a Bronsted Lowry base.

Species which has a lone pair of electrons available for donation can act as a Lewis base.

$B{F}_3$ has a empty p orbital, and can accept a lone pair of elecron to act as a lewis acid, not a lewis base.
$CO$ cannot accept a proton and hence cannot act as a Bronsted base. It can act as a Lewis base, as the lone pair on carbon can donates it's electrons, (for example forming transition metal coordination complexes).

While $C{l}^{-}$ accepts a proton to form $HCl$. It also has a lone pair of electrons available for donation. So, it can act both as a Bronsted Lowry base as well as Lewis base.