Structure of Metal Carbenes

The word carbene derives from the name given to free, disubstituted carbon compounds with general structure 3 (X and Y same as in l).2 Because the central carbon atom does not possess an octet of electrons, free carbenes are electron deficient and extremely reactive. They are so reactive that some carbenes insert themselves into normally inert alkane C-H bonds (equation 10.1) or react with alkenes to form cyclopropanes (equation 10.2), a synthetically useful transformation. 

Transition metal-carbene complexes are not normally produced from free carbenes 3 moreover, they do not produce free carbenes. It is, however, useful to think of metal-carbene complexes as a construct of a free carbene and a metal 

2We will call such compounds free carbenes to distinguish them from metal complexes possessing divalent carbon ligands. 

3There is one exception to this statement. One route to the synthesis of AMieterocyclic carbene complexes involves complexation of the free carbene with a metal salt. Such complexes will be discussed later in this section. 

Free carbenes exist in two different electronic states, singlet and triplet,4 which are represented in structures 4 and 5. The singlet state has one lone electron pair, whereas the triplet state has two unpaired electrons. 

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Several approaches exist to understanding the structure of metal carbenes. Perhaps the simplest and most familiar is resonance theory. Structures 10-13 represent several possible contributing resonance structures of metal carbenes. Structures 10-12 seem to be important contributors for Fischer carbene complexes, as indicated by experiment and calculations. 

IV. MODIFICATION AND ELABORATION OF THE STRUCTURE OF METAL-CARBENE COMPLEXES... 

Strassner T (2004) Electronic Structure and Reactivity of Metal Carbenes. 13 1-20 Strong LE, see Kiessling LL (1998) 1 199-231... 

These observations are compatible with the model for the carbene complex presented in Section II,A. Both metal and w-donor substituents compete to donate electron density to unfilled carbenepz orbitals, and with good 7r-donors such as nitrogen, the metal is less effective. In terms of resonance formalism, the resonance hybrid 39 makes a more significant contribution than 40 to the structure of the carbene ligands in these compounds. Similar conclusions are reached when the structures of Group 6, 7, and other Group 8 heteroatom-substituted carbene complexes are considered. 

Generally phenol formation is the major reaction path however, relatively minor modifications to the structure of the carbene complex, the alkyne, or the reaction conditions can dramatically alter the outcome of the reaction [7]. Depending on reaction conditions and starting reactants roughly a dozen different products have been so far isolated, in addition to phenol derivatives [7-12], In particular, there is an important difference between the products of alkyne insertion into amino or alkoxycarbene complexes. The electron richer aminocarbene complexes give indanones 8 as the major product due to failure to incorporate a carbon monoxide ligand from the metal, while the latter tend to favor phenol products 7 (see Figure 2). 

Understanding the structure and dynamics of metal carbenes related to Fischer carbenes provides insights into their reactivities and selectivities. For example, their stability decreases when electron-donating substituents are replaced by hydrogen or, especially, electron-withdrawing substituents ... 

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