The cyanide ion, when acting as a nucleophile, typically attacks via carbon as its HOMO has a greater contribution from carbon. But how can we deduce this from the MO diagram?

To me it appears that the nitrogen $\mathrm{2p}$ orbitals are energetically closest to the $3\sigma$ HOMO, so its coefficient should be larger in the HOMO. What is wrong with this line of reasoning?

MO diagram.

  • $\begingroup$ related chemistry.stackexchange.com/q/18449/16683 $\endgroup$
    – orthocresol
    Oct 17, 2016 at 16:58
  • $\begingroup$ can u please @martin-マーチン answer the question ..as in the linked question it is mentioned that We can clearly see that the highest coefficient of the HOMO is at the carbon end, this is consistent with the fact, that it will most likely bond with this end to a more electropositive element. i dont know how to tag , can someone pls tag him. $\endgroup$
    – aks0854
    Oct 17, 2016 at 18:01
  • $\begingroup$ @akso854 There are 4 MOs with the same $\mathrm{\Sigma}$ symmetry. So the exact energies are going to matter here. I suspect that you're getting a much larger contribution from the carbon $\mathrm{2s}$ orbital, and that seems consistent with the shape of the boundary surface in the diagrams. $\endgroup$
    – Zhe
    Oct 17, 2016 at 18:30
  • $\begingroup$ thanks @Zhe ..i also think the same. 2s orbital is behind greater lobe on carbon in HOMO. $\endgroup$
    – aks0854
    Oct 17, 2016 at 18:57
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    $\begingroup$ Whilst I don't think this post in unclear in it's meaning (and so should not be closed), it could definitely do with some tidying up to use better grammar and formatting. You are much more likely to get an answer if you present your posts in a nice way. $\endgroup$
    – bon
    Oct 18, 2016 at 10:40

1 Answer 1


The 2s and 2pz atomic orbitals are of the same symmetry in the molecule. That means that the 2 sigma* and 3 sigma are actually hybrids. The former is a hybrid were most of the electron density points to the bond (making it partially bonding), while in the latter most of the electron density points outside, i.e. a lone pair in the carbon.


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