Answer
(a) Lewis acid: $Fe$; Lewis base: $CO$
(b) Lewis acid: $BCl_3$; Lewis base: $NH_3$
(c) Lewis acid: $Hg^{2+}$; Lewis base: $I^-$
Work Step by Step
(a)
Transition metals like $Fe$ are capable of acting like Lewis acids because of their not completely filled "d" orbital, which can receive pairs of electrons.
Drawing the Lewis dot structure for CO, we can see that it has 2 free electron pairs, so, it can act as a Lewis base.
(b)
In $BCl_3$, the boron atom is making 3 covalent bonds, but, since it is from the third group, it can make 5 covalent bonds. Therefore, it has a free space to receive one pair of electrons in its orbitals.
In $NH_3$, the nitrogen atom had 5 free electrons in its valence shell, and it made 3 covalent bonds, so, there are 2 free electrons, making $NH_3$ capable of acting as a Lewis acid.
(c)
As the positive charge indicates, $Hg^{2+}$ is capable of receiving a pair of electrons.
Normally, since it is an halogen with a single negative charge, it has 8 electrons in its electron shell, which it can donate, making this ion a Lewis base.
