Schrock alkylidene moiety

Compared to molybdenum- or tungsten-based Schrock catalysts, the reactivity of ruthenium-based systems is different. While reactivity slightly increases in the order Ielectron-withdrawing, groups [171]. The thermodynamics and in par-... 

Oxoalkylidene complexes of the type (W=CH-CH=CPh2)(0)[0CMe(CF3)2]2 L (16) were obtained by Grubbs and utilized for ROMP of norbomene [159]. His synthetic strategy employed 3,3-diphenylcyclopropene for the synthesis of the alkylidene moiety [159,160]. This method was later adapted for the synthesis of the first well-defined ruthenium metathesis initiators [161]. Furthermore, (0Ar)2W(CH-/-Bu)(0)(PMe3) (17) was synthesized by Schrock et al. and reported to polymerize 2,3-dicarbomethoxynor-bornadiene in a living manner [162]. 

These two greatly different properties of metal carbene complexes reflect a crucial difference in the metal-carbon bonding. Single carbene moieties (present in Fischer and Casey complexes) form a donor-acceptor metal carbene bond involving a a donor bond and a % re-donor bond. On the other hand, triplet carbene moieties (appearing in Osborn and Schrock complexes) form a covalent metal alkylidene double bond (a, n) via spin pairing of the sp2-hybridised carbon... 

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