Chemistry Stack Exchange is a question and answer site for scientists, academics, teachers, and students in the field of chemistry. It only takes a minute to sign up.
Sign up to join this community
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?
.
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.
