Question

If CN- is a lewis base, can it also act as a lewis acid?

Answer 1

Actually, it could, but not often. Occasionally it acts as a Lewis acid to stabilize interactions with a transition metal, for instance.

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CYANIDE COMPARES WELL WITH CARBON MONOXIDE

`"CN"^(-)` is isoelectronic with `"CO"`, and it can act as both a `\mathbf(sigma)` donor and `\mathbf(pi)` acceptor.

Its MO diagram looks somewhat like that of `"CO"`:

We can see the two electrons in the orbital labeled `3sigma`, which is its HOMO. Also, its two `1pi^"*"` antibonding orbitals are empty, which are its LUMOs.

Thus, it can donate electrons from its `sigma` bonding HOMO and/or accept electrons into its `pi^"*"` antibonding LUMOs. That makes it both a Lewis base and a Lewis acid for the respective reasons.

SOMETIMES, CYANIDE CAN BE A LEWIS ACID

One situation where `"CN"^(-)` acts like a Lewis acid is after it `sigma` bonds via its carbon onto a transition metal to form a metal-ligand complex, such as hexacyanochromate(III), i.e. `["Cr"("CN")_6]^(3-)`.

This behavior can be summarized in the following diagram which is based off of the angular overlap method, which is basically a simplified approach to approximate d-orbital splitting that ignores s and p interactions:

As you can see, it gives a similar d-orbital splitting as one would get from Crystal Field Theory. (However, it gives an inaccurate representation of the ligand `sigma` MO energies!)

At first, `"CN"^(-)` uses its `3sigma` HOMO to interact with the compatible `d_(z^2)` and `d_(x^2-y^2)` atomic orbitals of the transition metal and raises their energy when generating the two `e_g^"*"` orbitals (next to them is the label "`z^2, x^2 - y^2`").

`"CN"^(-)` ends up donating electrons to the metal in a `\mathbf(sigma)` destabilizing interaction. This is Lewis base behavior because it donates electrons.

Then, the `1pi^"*"` antibonding LUMOs of `"CN"^(-)` also happen to be compatible with the `d_(xy)`, `d_(xz)`, and `d_(yz)` atomic orbitals of the transition metal and lowers their energy when generating the three `t_(2g)` orbitals (next to them is the label "`xy,xz,yz`").

This is done by accepting electrons from the metal in what's called a `\mathbf(pi)`-backbonding stabilization. This is Lewis acid behavior because it accepts electrons.

Here is the `pi` backbonding stabilization happening with `"CO"` and a transition metal's `d_(xy)` and `d_(xz)` orbitals.

Overall, this increases the ligand field splitting energy, which one might call `Delta_o` for octahedral complexes, because the energy of the three now-lower `t_(2g)` orbitals decreased, and the energy of the two now-higher `e_g^"*"` orbitals increased, relative to the original, uncoordinated `d` atomic orbitals.

Because of the `pi`-acceptor, i.e. Lewis acid behavior of `"CN"^(-)`, it is a very strong field ligand, and it often gives rise to "low spin" complexes where electrons are paired in the `t_(2g)` orbitals first before going into the higher-energy `e_g^"*"` orbitals.