Classification of transition metal-carbene complexes

Carbenes are defined as species containing divalent carbon [1], and they may display either electrophilic or nucleophilic reactivity depending on whether the two unshared electrons on the carbon center are unpaired (triplet carbene) or paired (singlet carbene). Metal-carbene complexes can be classified in a similar way based on their reactivity toward electrophiles and nucleophiles. The resonance forms shown in Fig. 4.1 define the limiting structures, and the formal charge on the carbene carbon indicates the preferred reactivity. Those that are nucleophilic at carbon are called Schrock-type complexes or alkylidenes, and they generally 

Current Methods in Inorganic Chemistry, Volume 3 Editors H. Kurosawa and A. Yamamoto 2003 Elsevier Science B.V. All rights reserved 

At the other extreme, metal carbenes that are electrophilic at carbon are called Fischer-type complexes, and they generally contain jt-donating heteroatom substituents [4], Fischer reported the first example in 1964 [5], In these cases, the metal-carbene interaction can be represented by three resonance structures, the first with a formal M=C double bond, the second with a M-C single bond and charge separation, and the third with additional multiple bond character between the carhon and the heteroatom substituent. 

The nucleophilic-electrophilic/Schrock-Fischer distinctions have been extremely useful throughout the development of metal-carbene chemistry because they provide a way to categorize metal carbenes and rationalize their reactivity patterns [6]. Yet, as an increasing variety of complexes are studied, it is becoming clear that these classifications represent only the prototypical complexes that were inihally discovered. We now know of many examples with intermediate characteristics and reactivity profiles, such as electrophilic species that lack heteroatom stahilizahon and even complexes like (Cp)(CO)2Re=CHR that display ambiphilic reachvity, meaning that this rhenium carbene reacts with both nucleophiles and electrophiles (Eq. 4.1) [7]. 

Transition Metal-Carbene Complexes in Olefin Metathesis 

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