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I was just reading about linkage isomerism, that usually arise due to the fact that some ligands are ambidentate (i.e. $\ce{SCN}$ and $\ce{NCS}$).

I then think to myself, considering only the ligand structure, $\ce{CN-}$ and $\ce{CO}$ ligand also should also be ambidentate ligands (I'm not including bridging). However, I haven't found any reference to that. Here's now, my questions:

  • Are $\ce{CN-}$ and $\ce{CO}$ really ambidentate? why or why not?
  • How different are the complexes? (i.e. $\ce{Cr(OC)6}$ vs $\ce{Cr(CO)6}$)

Also, if anyone found a complex with $\ce{NC-}$ or $\ce{OC}$ ligand, please do put the reference to that.

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    $\begingroup$ And why not include bridging? That's the main situation you get O-coordinated carbonyls. $\endgroup$
    – Mithoron
    Jul 2, 2023 at 16:30
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    $\begingroup$ Mostly for personal preference. However, I felt like bridging force the ligand to use more than one electrons pair at the same time, instead of having 2 (or more) electrons pair that can be used in multiple ways. $\endgroup$
    – Tensor
    Jul 2, 2023 at 16:41
  • $\begingroup$ Well, in the answer, there sure is bridging... $\endgroup$
    – Mithoron
    Jul 3, 2023 at 13:15

1 Answer 1

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In the case of cyanide we actually do have iron coordinated to the nitrogen ends of the cyanide ligands in Prussian blue. In fact iron atoms are coordinated to both ends of the cyanide ion ligands; the iron forms a simple cubic lattice with the cyanide ions laid out along the edges of the cube like an expanded perovskite structure:

enter image description here

From Wikimedia Commons user Ben Mills

enter image description here

From Wikipedia user Smokefoot

In general, the pi-acceptor ligand that cyanide ion is will bond first through the less electronegative carbon atom, where the $\pi^*$ orbitals that accept electrons from the metal have higher amplitudes (and thus potentially better overlap). But when that bond is already taken up, as in the Prussian blue structure or with nitrile ligands where the cyanide carbon is bonded to another carbon atom, then the nitrogen end of the cyanide can from a second bond.

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