Why is CO a lewis acid?

1 Answer
Jan 16, 2016

#"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.


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.