Question

Is CO a lewis acid?

Answer 1

Carbon monoxide is in fact a potent Lewis-acid. It is demonstrably an electron pair acceptor.

Explanation:

Carbon monoxide is an excellent ligand towards low valent transition metals. Why?

The usual Lewis formulation is, `:C-=O:`. Given the triple bond, there is a formal negative charge on the carbon, and a formal positive charge on the oxygen. When carbon monoxide binds to a transition metal centre, usually the `C` is the donor atom. Now if the metal is already in a low oxidation state, further coordination to other donors is unlikely, unless the metal can redistribute (i.e. back donate) electron density to the ligand. The vacant anti-bonding orbitals on carbon monoxide are the right symmetry to ACCEPT electron density from filled metal d -orbitals.

For these reasons, carbon monoxide (and isoelectronic cyanide ion) are known as `pi`-acids.

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 `d` orbitals (`d_(xy)`, `d_(xz)`, `d_(yz)`) of the metal centre:

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 `2100` `cm^-1` of the free gas to below `1900` `cm^-1` or lower. Now while the interaction is antibonding with respect to the carbon monoxide ligand, the interaction is bonding with respect to the `M-CO` bond inasmuch as electron density has been shared between metal and ligand.

Because bound carbon monoxide has demonstrably accepted electron density, it is commonly known as a `pi`-acid ligand. There are many zerovalent transition metal complexes with the formula `M(CO)_6` or `M(CO)_4`.

Answer 2

anor has a good answer, but I wanted to provide a visual approach. Also, `"CO"` can be BOTH a Lewis acid and base, and is typically both in Ligand Field Theory. I go more into that here.

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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, `pi^"*"` orbitals, in addition to behaving like a Lewis base.

The MO diagram for CO is:

The high-lying pair of MOs (by the carbon `2p` AOs) that are close in energy are the `pi_x^"*"` and `pi_y^"*"` (if we take the horizontal coordinate axis to be the `z` axis). Those LUMOs would accept the electrons.

We can see, as an example, in the reaction to form carbonic acid, that `"CO"_2` accepts electrons (also in an antibonding, `pi^"*"` orbital on the carbon):

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.