# Why is CO a lewis acid?

$\text{CO}$ 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.
An example is nickel tetracarbonyl, ${\text{Ni(CO)}}_{4}$, 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 $\text{d}$ orbital of the metal in a process called back-bonding.