Is CO a lewis acid?
Carbon monoxide is in fact a potent Lewis-acid. It is demonstrably an electron pair acceptor.
Carbon monoxide is an excellent ligand towards low valent transition metals. Why?
The usual Lewis formulation is,
For these reasons, carbon monoxide (and isoelectronic cyanide ion) are known as
The HOMO of carbon monoxide (above) is the donor orbital; it is conceived to be occupied, and shunts electron density towards the metal centre. Further ligation of carbon monoxide is unlikely, UNLESS the metal can back donate some electron density; and a mechanism for this process exists inasmuch as the anti-bonding orbitals on carbon monoxide (below) are UNOCCUPIED, and are of appropriate energy and symmetry to receive electron density from the filled non-axial
Both images were from this site.
This back-donation is ANTIBONDING with respect to the carbon monoxide ligand, hence its IR stretching frequency drops from the
Because bound carbon monoxide has demonstrably accepted electron density, it is commonly known as a
anor has a good answer, but I wanted to provide a visual approach. Also,
Although it may seem counterintuitive at first (seeing how there are lone pairs of electrons on both carbon and oxygen), carbon monoxide can accept electrons in its antibonding,
The MO diagram for CO is:
The high-lying pair of MOs (by the carbon
We can see, as an example, in the reaction to form carbonic acid, that
Similarly, carbon monoxide can do that too on the carbon (just focus on the first step).
Since carbon monoxide has a method for accepting electrons, it by definition can be a Lewis acid. But as implied earlier, it can also be a Lewis base.