Structures of Carbene and Carbyne Complexes

The TT-backbonding in alkylidene and alkylidyne complexes is evidenced by considerably shorter M —C distances compared to the metal-carbon distances in compounds containing crM —C bonds. The metal-carbon distances in compounds containing M —C single bonds are usually ca20pm longer than the metal-carbene carbon distances and even up to 40 pm longer compared to the metal-carbyne carbon distances. 

Therefore, in alkylidyne complexes, there is a triple bond M = CR, and in alkylidene complexes, the multiplicity of the M — C bond considerably exceeds one. The M-CPh distance in f(phenyIcarbyne )tetracarbonyl(pentacarbonylrhenio )molyb-denum is 183.5 pm. Thus, within the limits of experimental error, it equals the sum of the radii of the triple bonds Mo = Mo and C = C (122.4 + 60.5= 182.9 pm). In carbene complexes containing oxygen as the heteroatom, the multiplicity of the carbon-oxygen bond is so great that isomers exist  

There is no n bond between the phenyl and carbene carbon atom. In these, as in other 

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