Is it correct to say pyridine’s non-bonding electron pair does not participate in electron resonance while pyrrole’s non-bond electron participates in electron resonance? Would therefore pyridine’s resonance not be disturbed so act the molecule can act as a monodentate ligand?
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1$\begingroup$ I don't understand how pyrrole can be a tridentate ligand, can you give an example where it acts as a tridentate ligand? $\endgroup$– DavePhDCommented Jan 12, 2015 at 11:52
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2$\begingroup$ I think he says about coordination using whole ring, like with cyclopentadienyl $\endgroup$– MithoronCommented Jan 12, 2015 at 13:12
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1$\begingroup$ Still, I've never seen pyrrole coordinated like that - examples welcome! $\endgroup$– MithoronCommented Jan 12, 2015 at 21:38
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1 Answer
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From what I gathered in the comments you confused a few terms. Both compounds are monodentate, i.e. they can only coordinate one metal at a time. However, pyridine usually donates one electron pair, while pyrrole can donate three (to one metal) — I haven’t seen a structure where pyrrole does that, though.
The reason is pretty simple: Pyrrole is much like the cyclopentadienyl anion except that one carbon atom is replaced by nitrogen. If it wants to coordinate in any way, it will have to do that like a slice of bread from its side, forming a $\eta ^5$ complex. Much like ferrocene in the picture below.
Pyridine, on the other hand, has no hydrogen on the nitrogen atom; instead it has a lone pair pointing outwards, but still in the plane of the ring. This allows pyridine to donate two electrons in a $\sigma$-like fashion to a metal centre, $\eta ^1$-style. Of course, pyridine could also perform a sandwich-like coordination much like benzene sometimes does. However, the direct two-electron $\sigma$ donation will be much stronger and therefore largely outweigh a sandwich-type donation in most compounds.
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1$\begingroup$ I have seen $\eta^5$ coordination of pyrrole, e.g. here. Presumably, that means a 6-electron interaction. $\endgroup$ Commented Jun 10, 2015 at 0:00
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1$\begingroup$ There are also examples of $\eta^6$ coordination of pyridine. $\endgroup$ Commented Jun 10, 2015 at 0:01
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$\begingroup$ @GeoffHutchison I didn’t say ‘no-one has seen it’, I said ‘I haven’t seen it’ which held true until now. (But I wouldn’t have gone looking for it, I would have assumed it to exist but being less common.) Thanks for the nice examples! $\endgroup$– JanCommented Jun 10, 2015 at 0:03
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3$\begingroup$ Fair enough. Organometallic chemists seem to relish the idea of making "less common" compounds. $\endgroup$ Commented Jun 10, 2015 at 1:14