Other Syntheses of Carbene Complexes

Since the time that our first paper about metal carbene complexes was published in 1964, this area of research has expanded quickly. Today there are available several major review articles (40-43) dealing with the chemistry of carbene complexes. Therefore I want now only to single out particularly interesting syntheses. 

In our laboratory in 1968, Ofele (44) treated 1, l-dichloro-2,3-diphenyl-2-cyclopropene with disodium pentacarbonylchromate and, with the concomitant elimination of sodium chloride, he obtained pentacarbony 1(2,3-diphenylcyclopropenylidene) chromium(0). This compound is stable up 

X-Ray structural analysis (45) showed that the three C—C distances of the ligand are not identical the distance between the 2 carbon atoms bearing the phenyl substituents is shorter than the other two distances. The carbene carbon-chromium distance of 2.05 A lies within the range of values found for our carbene complexes, thereby indicating that in this case as well an authentic carbene complex exists. 

A very interesting synthetic method was published in 1969 by Richards and co-workers (46). They found that, in the reaction of alcohols with certain isocyanide complexes sueh as those of platinum(II) ], an addition of the alkoxy group to the carbon atom as well as of the hydrogen to the nitrogen atom of the isocyanidc ligand occurs, and one thus obtains the corresponding carbene complexes  

This procedure has led subsequently to many compounds of this kind. The relationship between the complex chemistry of isocyanides and of carbon monoxide comes to mind at this point. 

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