Ambiguity in the electron count for carbene complexes

Ifwe now consider the ionic model, the electronic octet of the carbon must be completed, which means that we must consider the dianionic form (CR2) (4-36c). Since the two extra electrons were supplied by the metal, we obtain the same oxidation state as that yielded by the covalent model with an X2 ligand. In the ionic description, the dianion (CR2) is a strong tt-donor, due to its doubly occupied orbital. 

It is important to be aware of these different possible points of departure, since the ambiguity in the calculation of the oxidation 

State appears very frequently in experimental articles about these complexes. An example is given in 4-37, where we see that the complex [Cp2Ta(CH3)(CH2)] can be described as a Ta(III) complex, if CH2 is 

This apparently complicated situation (which is at least fairly complicated in reality) arises from the different possible choices for the electron distribution within the CR2 ligand (L or X2 ) or between the ligand and the metallic fragment (a neutral or a dianionic ligand ). The initial distribution is in fact somewhat arbitrary no matter which choice is made, and the situation becomes clearer if one considers the electronic structure of the complex as a whole, rather than that of the separate fragments. 

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