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I am wondering why nitrogen lone pairs are delocalized in NHC metal complexes. What I can think of is, when complex is formed, plus charge is formed on the carbon atom and therefore delocalization from the nitrogen atoms. But there should be then "plus" sign like in the picture below.

enter image description here

But these kind of carbene complexes are always depicted without the plus sign. Can anyone please tell me what I am missing out?

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  • $\begingroup$ They're just depicting the merged resonance structures. Just draw out one of the resonance structures, and it should be perfectly clear. This is basicially a N,N-disubstituted imidazoline. $\endgroup$ – Zhe Mar 8 at 16:08
  • $\begingroup$ Imidazolium ion is depicted normally (with the plus sign) but NHC metal complexes are not. As I said I think NHC metal complexes should be written with plus sign, but normally they are written without it and I was wondering why. $\endgroup$ – happysad Mar 8 at 16:19
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    $\begingroup$ To my knowledge that is correct. You only care about the overall charge of the organometallic complex anyway, and you're just polluting the structure with charges. Should there be a charge on the chloride? They're X-type ligands but the phosphine is L-type. Nah, just draw brackets around the whole thing as needed and write the total charge. $\endgroup$ – Zhe Mar 8 at 16:28
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    $\begingroup$ Note this exact issue in this example. With the formal positive charge in that imidazoline, what is the overall charge on the complex? If one answers correctly with neutral, then why, in the name of all that is holy, is there a positive charge with no corresponding negative charge? The answer is that it's a carbene, and there's a hidden negative from that, but again, we've chosen not to show the negatives so why bother with the positive? $\endgroup$ – Zhe Mar 8 at 16:29
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Some parts of this answer probably border a bit on personal opinion, but I will at least try to justify having such an opinion.

The first thing to do is to actually examine the heterocycle. What kind of ligand is it?

carbene

This is a carbene. It donates two electrons via the lone pair on the carbon in plane with the ring. Notice that there is a smear of positive charge from the delocalization of the $\pi$ system between the N-C-N system. That is balanced by the negative chage on the carbon, however. Overall, this ligand is neutral, so it is an L-type ligand.

What's the overall charge on this metal complex? This is a ruthenium(II) ion (2 chlorides, everything else neutral), so this is a $d^{8}$ metal ion. You have 2 electrons from the NHC, 2 from each chloride (4 total), 2 from the phosphine ligand, and 2 from the carbene (the double bond is back-bonding). That gives 10 electrons from ligands, so this is a fine-looking neutral 18 electron complex.

For a complex that's overall charge zero, why is there a plus charge hanging out by itself? Well, it turns out that we're extremely sloppy with charges. For example, the chlorides are X-type ligands but the phosphine is not. Do we stipulate this difference in the structure? No we do not, though we use it during electron counting. The authors here felt that the smear of positive charge of the carbene, which is how you would draw the structure in isolation, should be shown but chose not to show the negative charge of the carbene, which is the case by convention. That seems somehow inconsistent to me because the overall charge of this ligand is neutral.

Look at these other representations:

NHC example 1

Mendeleev Communications, 2019, 29 (1), pp 38-40. (10.1021/om3009298)

NHC example 2

Organometallics, 2012, 31 (23), pp 8355–8359. (doi: 10.1016/j.mencom.2019.01.011)

Both representations of the NHC are used: without explicit $\pi$ system and with.

Note that neither of them say anything about the positive charge. This seems more consistent to me, and these authors, at least, agree with me.

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