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What is the correct way to draw a carbene? On most/all material I have seen, both singlet and triplet carbenes are depicted as so:

enter image description here

With 2 "dots" representing the non-bonding electrons on either a singlet or a triplet carbene.

However in a class today my tutor was adamant that a singlet carbene should be drawn with a positive and a negative sign on the carbon. His argument as such was that "you have an empty and a full orbital and so the empty orbital corresponds to the positive sign, the full orbital corresponds to the negative sign".

So my question is - How is a carbene correctly drawn? As above or with a carbon bearing both a positive and a negative sign?

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    $\begingroup$ You may wish to see Graphical representation standards for chemical structure diagrams (IUPAC Recommendations 2008) especially GR-5.3 where examples of singlet and triplet carbenes are given. This is probably worth an answer but I'm not free now, sorry. $\endgroup$ – orthocresol♦ Jan 17 '18 at 20:11
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    $\begingroup$ There are two types of carbenes: singlet and triplet. Triplet ones should be drawn with two clearly separate dots, while singlet ones should be drawn with an electron pair and often with a vacant orbital (unless it participates in a strong conjugated system). $\endgroup$ – permeakra Jan 17 '18 at 21:17
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There is simply no place for an argument. It is about as insane as if someone forgot the the shape of the Cyrillic letter "Ы" and tried to deduce it from the corresponding sound, or from the shapes of other letters. The information is simply not there. The shape of a letter is a matter of convention, not deduction. Ditto for the way of drawing a carbene.

As for the convention, the one accepted by your tutor seems to differ from the one I'm familiar with. This is not much of a problem, as long as you remember when to use which. So when you are in your class, use the plus/minus form; when you are elsewhere, use two dots, and you'll be understood just fine, Google be my witness.

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    $\begingroup$ "Ы" is just a Russian way of typing "lol". $\endgroup$ – andselisk♦ Jan 17 '18 at 20:47
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    $\begingroup$ In Germany, I would argue that the most common convention is to write a line to symbolise two electrons — which would only be correct for singlet carbenes. (Rather than two dots, an electron pair is drawn as a straight line in all cases.) $\endgroup$ – Jan Jan 19 '18 at 9:20
  • $\begingroup$ BTW, the triplet/singlet question is another thing entirely. When you discuss it, you are typically going to draw phallomorphic appendages pierced by two arrows. $\endgroup$ – Ivan Neretin Jan 19 '18 at 9:32
  • $\begingroup$ While I agree with your response, I added a slightly more .. er, non-judgemental alternative. :-) $\endgroup$ – Geoff Hutchison Jan 23 '18 at 20:10
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Put simply, there's no precedent for this unusual depiction of a singlet carbene.

Your comment about most carbene depictions, for both singlet and triplet carbenes is true, but a bit lazy. Yes, many chemists, frequently just draw two electrons as a lone pair on the carbon for both. In principle, with a moment's thought, you can often deduce the likely spin state.

As described, by the comment above, the standard for 2D chemical structure depiction is the IUPAC guide Graphical representation standards for chemical structure diagrams

enter image description here

In short, there's a difference in representing a triplet (two unpaired electrons), which should show preferably two separate electron "dots."

For a singlet, the lone pair is indicated, emphasizing the connection between the electron spins.

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