Dimetallacyclopropane

This section deals with alkylidene complexes L M=CR2 and vinylidene complexes LnM=(C)n,=CR2 in which the metal-bound carbon atom bears only hydrogen, alkyl, or aryl groups, but neither heteroatoms (halogen, nitrogen, oxygen, or sulfur) nor electron-withdrawing groups. Dimetallacyclopropanes and ketene complexes will not be discussed. 

Metal carbene and carbyne complexes can act as donors analogous to alkenes and alkynes. When gold is the acceptor such interactions can lead to dimetallacyclopropanes or dimetaUacy-clopropenes, as illustrated by the reaction (81).748... 

In practice, this approach to the syntheses dimetallacyclopropanes or -propenes has been very successful (27, 86-93). Complexes 42-47 are... 

Despite their thermodynamic stability, dimetallacyclopropanes proved to be quite reactive when treated with appropriate reagents, specifically, unsaturated hydrocarbons and protic acids. Again, the nature of the metals and coligands distinctly influence the reactivity. 

Scheme 1. Alkene-and alkyne-metal complexes may be formulated with metallacyclo-propane (A) or -propene (B) structures, respectively. Similarly, complexation of a metal atom with an alkylidene or alkylidyne group gives rise to a dimetallacyclopropane (C) or -propene (D) ring system.

This view is supported by calculated fragmentation energies of all combinations of dimetallacyclopropanes... 

The most widely studied dimetallacyclopropane is iron complex (55) because it is air stable and readily available <80JA1752>. Diferracyclopropane (55) and diosmacyclopropane (56) are both known to undergo reversible CO insertion into the metal-carbon bond <88JA7868>. This insertion reaction provides a basis for an efficient synthesis of acetate esters <8iJOM(2l9)C5> from alcohols and (55) at 60 °C. An advanced intermediate in this reaction has been isolated and characterized by x-ray diffraction <84AG(E)432>. 

The -methylene complexes of the type L M( i-CH2) ML are quite stable. Photolytic and thermal stability of dimetallacyclopropane compounds is exceptional. 

Carbenes bridging two transition metals that are usually bonded to each other, synthesized by W.A. Herrmaim , are very robust, because there is no double bond M=C. They can be formulated as dimetallacyclopropane and have been considered as molecular models of the interaction of carbenes with metal surfaces. For... 

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