# Explaining why CN- is a soft base

So, I am having difficulty with the following explanation/question.

(Based on your MO diagram and pi bonding theory) explain why $\ce{CN-}$ is a soft base and prefers to bind to soft acids.

Now, I know that the HOMO is the 5 σ MO and that the electron pair in the 5 σ MO is, to a large extent, more concentrated around the carbon. So, is $\ce{CN-}$ a soft base because it has a low energy HOMO but large magnitude HOMO coefficient? Does polarizability have anything to do with it?

When $\ce{CN-}$ reacts with a soft acid what physical changes in the cyanide molecule would such a reaction lead to?

Not really sure about this one. Would it have to do with the donation of the electron pair in the HOMO and therefore change the HOMO?

• But since N is more electronegative than C (i.e. it has lower energy AOs than C), wouldn't more electron density be concentrated on N as N contributes more to the bonding MOs of the cyanide ion? – Tan Yong Boon Nov 8 '17 at 2:39
• If you look at the electron distribution it surrounds the carbon atom instead of the nitrogen as one would expect. This is caused by the interaction of the atomic orbitals of carbon among each other. They contribute more significantly to this 5σ molecular orbital compared to the influence of the 2p atomic orbital. You should not forget that the energy difference between 2s and 2p in Nitrogen is greater than that in Carbon, so the overlap between them is less significant, and thus the total contribution to the 5σ molecular orbital. – MatthewSpire Nov 8 '17 at 3:05