The Structure of Carbenes

We made two important observations earlier regarding the structure of carbenes that we will now return to and seek an explanation for firstly, we said that the X-ray crystal structure of this stable, crystalline carbene shows that the bond angle at the carbene C is 102° and, secondly, we said that many carbenes can be observed by ESR—in other words, they have unpaired electrons. 

You may be somewhat surprised that the structure of carbenes can be investigated by ESR—after all, we explained in Chapter 39 that ESR observes unpaired electrons, and you might expect the six valence electrons of a carbene all to be paired. Indeed, in some carbenes they are, but in many they are not. This is an important point, and we will discuss it at length later in the chapter. 

In addition to the syntheses of metal-carbene complexes from compounds not containing the carbene ligand, a variety of methods for modifying the structure of carbene complexes have been developed. Three different reactive centers in a transition metal-carbene complex can be utilized in designing a synthesis of a new carbene complex (see Scheme 4). For example, (CO)s-CrC(OCH3)CH3 can be converted into a wide range of carbene complexes by (a) nucleophilic attack at the carbene carbon atom followed by loss of... 

Carbenes are disubstituted carbon atoms bearing two electrons (which may be paired or unpaired). The parent species is methylene, CH2. The structures of carbenes are based on sp hybridization, and there are two possible arrangements of the electrons within the... 

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