Vinylidene Metal Complexes

Preparation and Stoichiometric Reactivity of Mononuclear Metal Vinylidene Complexes... 

Much of the chemistry of metal-vinylidene complexes has been summarized in several reviews [11-14] and the following will merely summarize the main preparative methods and survey the reactions of many of the metal complexes so obtained. Complexes of most transition metals have been described, although most work has been developed using electron-rich ruthenium derivatives, which have been used in... 

There is not sufficient space to discuss all vinylidene complexes which have been reported, for example over 200 crystal structures are listed in the CCDC. Consequently, this article largely concentrates on the chemistry of metal vinylidene complexes which has been described since 1995. Vinylidene complexes are generally available for the metals of Groups 4—9, with several reactions of Group 10 alkynyls being supposed to proceed via intermediate vinylidenes. However, few of the latter compounds have yet been isolated. This chapter contains a summary of various preparative methods available, followed by a survey of stoichiometric reactions of vinylidene-metal complexes. A short section covers several non-catalytic reactions which are considered to proceed via vinylidene complexes. The latter, however, have been neither isolated nor detected under the prevailing conditions. 

The main synthetic approaches to metal-vinylidene complexes will be discussed under the following headings ... 

Table 1.1 Some metal-vinylidene complexes, L M=C=CRR, prepared from 1-alkynes.

The stoichiometric reactivity of metal-vinylidene complexes will be covered in the following sequence ... 

Many stoichiometric reactions, usually involving alkynes, have been reported to proceed via metal vinylidene complexes which have not been detected nor isolated. This section does not survey the extensive field of reactions catalyzed by metal vinylidene complexes. 

Reactions Supposed to Proceed via Metal Vinylidene Complexes 43... 

Heavier group 14 analogs of metal vinylidene complexes... 

Figure 4.1 Simplified 7i-orbital interaction diagram for metal vinylidene complexes of the Fisher-type.

Although metal vinylidene complexes are quite abundant [4-18], heavier group 14 analogs of metal vinylidene complexes containing M=C=E or M=E=C moiety (E = heavier group 14 elements) have not yet been found. A recent theoretical study of the stability of the heavier Group 14 analogs of vinylidene complexes... 

In this chapter, we first analyzed the electronic structures of metal vinylidene and allenylidene complexes. The electronic structures allow us to understand the reactivities of these complexes. For metal vinylidene complexes of the Fischer-type, nucleophilic attack usually occurs at the a-carbon and electrophilic attack at the P-carbon. For the corresponding metal allenylidenes, electrophilic attack occurs at the P-carbon and/or the metal center. Then we briefly reviewed the theoretical study of the barriers ofrotation ofvinylidene ligands in various flve-coordinate complexes M (X) C1(=C=CHR)L2 (M = Os, Ru L = phosphine). The study showed that 7t-acceptor ligands (X), electron-withdrawing substituents and lighter metals gave smaller barriers. 

In this chapter, we summarized the theoretical studies carried out on metal vinylidene complexes. Special emphasis was placed on aspects of their electronic structures, reactivities and their roles in organic reactions. Theoretical studies on the related metal allenylidene complexes have been quite limited. More theoretical studies on various aspects of these complexes, particularly on their metathesis reactivities, are clearly necessary. 

The authors proposed mechanism for dimerization involves initial formation of metal vinylidene complex 9 via 1,2-H-migration. A second molecule of arylacetylene acts as a dienophUe in a formal [4 + 2] Diels-Alder cycloaddition with 9. A subsequent... 

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