Question

Why is CO a lewis acid?

Answer 1

`"CO"` can act as a Lewis acid because it has a low-lying, vacant, antibonding π orbital that can accept electrons from an electron-pair donor.

Explanation:

The antibonding orbital looks like the image below.

An example is nickel tetracarbonyl, `"Ni(CO)"_4`, in which the vacant `pi` orbital on the carbon monoxide has the right symmetry and the right energy to accept electron density from a filled `"d"` orbital of the metal in a process called back-bonding*.

Because the carbon monoxide is accepting electron density, it is behaving as a Lewis acid.