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:
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?