Synthesis, Structure and Bonding in Schrock Carbenes

Carbene complexes not containing hetero-atoms X and or Y but of the form, M=CHR, are known as alkylidene complexes and were first made by Schrock in 1975. A number of routes have been developed one of the most interesting is by a-elimination. Equation 17, 

Another route is by incorporation of carbenoid moieties, Equation 18, 

In these alkylidene complexes the M=C bond is significantly shorter than normal M-C single bonds, and the bonding is illustrated by formula (I). The partial double bond character confers special properties and such carbenes are intermediates in olefin metathesis reactions 

In addition to complexes of terminal alkylidenes, many complexes containing alkylidenes bridging two metal atoms have been made these are commonly known as p-methylene complexes. A wide range of complexes containing other metal-carbon bonds are known such as the carbyne complexes (for example, Cr = CPh(CO)5) which contain metal-carbon triple bonds. While such species may well be involved in some catalytic reactions, their importance in industrially catalyzed processes still has to be evaluated. 

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