ABSTRACT
Carbene complexes [1] contain a divalent carbon species (“carbene”) coordinated to a transition metal center via a formal carbon to metal double bond. The interaction with the metal accounts for a dramatic increase in stability of the carbene species which can be generated, stored and handled at ambient temperature. Based on the nature of the metal, its oxidation state and its coligand sphere, two types of carbene complexes are to be distinguished: Fischer-type metal carbenes typically contain a late transition metal (groups 6 to 8) in a low oxidation state, which is additionally coordinated by ligands featuring good π-acceptor properties (e.g. carbon monoxide). The carbene carbon atom behaves as an electrophilic center and is usually stabilized by π-donation from heteroatoms (e.g. oxygen or nitrogen). In contrast, atypical Schrock-type metal carbene (see chapter 7) is characterized by an early or middle transition metal (groups 3 to 6) in a higher oxidation state bearing efficient donor-coligands. This results in a reversed polarity of the carbene to metal bond and imposes considerable nucleophilicity onto the carbene carbon atom. As indicated in Figure 1, Fischer carbene complexes are known for a wide variety of metals. The broadest application in organic synthesis based on stoichiometric reactions, however, have gained the group 6 metal carbenes, especially the pentacarbonyl chromium complexes which are easy to prepare and-in some caseseven commercially available.
