Schrock carbenes transition metal complex bonding

Figure 1.1 Bonding in Fischer, Wanziick-Arduengo and Schrock carbene transition metal complexes.

Electrophilic and nucleophilic phosphinidene complexes have been related to the corresponding carbene complexes of which the Fischer-type is usually considered as a singlet-singlet combination and the Schrock-type as a triplet-triplet combination. However, both the strongly preferred triplet state of R-P and the M=P bond analysis suggest this schematic interpretation to be less appropriate for transition metal complexed phosphinidenes. 

In contrast, Schrock carbenes are electron deficient [10 to 16 valence electrons (VE)] early transition metal complexes with the metal atom in a high oxidation state and carbene substituents that are limited to alkyl groups and hydrogen [131]. Their bonding situation can be described in terms of the interaction of a triplet carbene with a triplet metal fragment resnlting in a covalent double bond [132], Tantalum complexes like [(np)3Ta=CHBu ] and [Cp2(Me)Ta=CH2] are representative of Schrock carbenes. 

There are two types of transition metal carbene and carbyne complexes low-valent (so-called Fischer type)i" i" and high-valent (so-called Schrock type). The two classes of compounds are quite different in their chemical behavior. Such different chemical reactivity is sometimes rationalized on the basis that the metal-carbene and metal-carbyne bonds in Fischer-type complexes have donor-acceptor character, whereas the bonding in Schrock-type complexes is more typical for a normal multiple bond. 

S. F. Vyboishchikov and G. Frenking, /. Am. Chem. Soc., submitted for publication. Structure and Bonding of Low-Valent (Fischer-Type) and High-Valent (Schrock-Type) Transition Metal Carbene Complexes. 

Carbenes, generated by several methods, are reactive intermediates and used for further reactions without isolation. Carbenes can also be stabilized by coordination to some transition metals and can be isolated as carbene complexes which have formal metal-to-carbon double bonds. They are classified, based on the reactivity of the carbene, as electrophilic heteroatom-stabilized carbenes (Fischer type), and nucleophilic methylene or alkylidene carbenes (Schrock type). 

Conventionally, carbene complexes are grouped into two classes Fischer [2] and Schrock [3] carbenes. Fischer carbenes may be identified by their 7r-donor substituents on the carbene carbon and having 7r-acceptor ligands bonded to a low-oxidation state, late-transition metal. These ligands may be thought of as singlet carbenes that donate a... 

Iron porphyrin carbenes and vinylidenes are photoactive and possess a unique photochemistry since the mechanism of the photochemical reaction suggests the Hberation of free carbene species in solution [ 110,111 ]. These free carbenes can react with olefins to form cyclopropanes (Eq. 15). The photochemical generation of the free carbene fragment from a transition metal carbene complex has not been previously observed [112,113]. Although the photochemistry of both Fischer and Schrock-type carbene has been investigated, no examples of homolytic carbene dissociation have yet been foimd. In the case of the metalloporphyrin carbene complexes, the lack of other co-ordinatively labile species and the stability of the resulting fragment both contribute to the reactivity of the iron-carbon double bond. Thus, this photochemical behavior is quite different to that previously observed with other classes of carbene complexes [113,114]. 

A number of bimetallic complexes which contain bridging alkylidene and vinylidene ligands semi-bonded to palladium are known, and as these also contain bridging carbonyl ligands, they are discussed in Section 8.04.1.8. In these dimers, the carbene ligand is more closely associated with the early transition metal, for which Schrock-type carbene complexes are well known. 

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