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

Feb 24, 2016

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

CYANIDE COMPARES WELL WITH CARBON MONOXIDE

${\text{CN}}^{-}$ is isoelectronic with $\text{CO}$, and it can act as both a $\setminus m a t h b f \left(\sigma\right)$ donor and $\setminus m a t h b f \left(\pi\right)$ acceptor.

Its MO diagram looks somewhat like that of $\text{CO}$:

We can see the two electrons in the orbital labeled $3 \sigma$, which is its HOMO. Also, its two $1 {\pi}^{\text{*}}$ antibonding orbitals are empty, which are its LUMOs.

Thus, it can donate electrons from its $\sigma$ bonding HOMO and/or accept electrons into its ${\pi}^{\text{*}}$ 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 ${\text{CN}}^{-}$ 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, ${\text{CN}}^{-}$ uses its $3 \sigma$ 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 ${e}_{g}^{\text{*}}$ orbitals (next to them is the label "${z}^{2} , {x}^{2} - {y}^{2}$").

${\text{CN}}^{-}$ ends up donating electrons to the metal in a $\setminus m a t h b f \left(\sigma\right)$ destabilizing interaction. This is Lewis base behavior because it donates electrons.

Then, the $1 {\pi}^{\text{*}}$ antibonding LUMOs of ${\text{CN}}^{-}$ also happen to be compatible with the ${d}_{x y}$, ${d}_{x z}$, and ${d}_{y z}$ atomic orbitals of the transition metal and lowers their energy when generating the three ${t}_{2 g}$ orbitals (next to them is the label "$x y , x z , y z$").

This is done by accepting electrons from the metal in what's called a $\setminus m a t h b f \left(\pi\right)$-backbonding stabilization. This is Lewis acid behavior because it accepts electrons.

Here is the $\pi$ backbonding stabilization happening with $\text{CO}$ and a transition metal's ${d}_{x y}$ and ${d}_{x z}$ 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 ${t}_{2 g}$ orbitals decreased, and the energy of the two now-higher ${e}_{g}^{\text{*}}$ orbitals increased, relative to the original, uncoordinated $d$ atomic orbitals.

Because of the $\pi$-acceptor, i.e. Lewis acid behavior of ${\text{CN}}^{-}$, it is a very strong field ligand, and it often gives rise to "low spin" complexes where electrons are paired in the ${t}_{2 g}$ orbitals first before going into the higher-energy ${e}_{g}^{\text{*}}$ orbitals.