4
$\begingroup$

In molecular orbital theory, please explain why cyanide ion combines with metal from the carbon end and not from the nitrogen end?

$\endgroup$
  • 1
    $\begingroup$ It can be other way round, depending on cation. Why this theory not another? You need sth more advanced then HSAB? $\endgroup$ – Mithoron Aug 21 '16 at 20:50
  • $\begingroup$ agree with Mith, HSAB is the best explanation imo since cyanide is ambidentate, but I guess if you just want an explanation of why it can coordinate via C then I suppose the shape of the HOMO would explain it. That doesn't mean it always coordinates via C. $\endgroup$ – orthocresol Aug 22 '16 at 13:38
4
$\begingroup$

In this answer of Martin’s, you can find a molecular orbital diagram of $\ce{CO}$. The corresponding diagram for cyanide, $\ce{CN-}$ is essentially identical, there will only be different orbital energies and very slightly different extends of the lobes.

When forming a coordinate bond to a metal centre, cyanide will primarily attack with its highest occupied molecular orbital, the HOMO, since it is a nucleophile (attacking a positively polarised metal centre). You can also see the HOMO depicted as the bottom image of the following set as taken from Wikipedia:

different depictions of cyanide

Note the large lobe extending outwards on carbon’s side. An attack with this lobe is greatly preferred, hence why isocyanides are comparatively rare.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.