Metal double bonds

IN THE PURE IN THE CASEOUS IN THE SOLID METALLIC DOUBLE-BOND IONIC... 

TaCl2(PMe3)2]2(U-Cl)2 (13). The short metal-metal bond length of 2.710(2)A has been interpreted as a formal metal-metal double bond on the basis of molecular orbital arguments (13,14). An unusual feature of this complex is that it reacts with molecular... 

Rapid rotation of the end groups and/or bridging hydrides is required to account for apparent magnetic equivalencies. The molecule does, however, have built in pseudo-cylindrical symmetry, i.e., one set of metal TT-type orbitals binds the bridging hydrides and the other set forms the TT-component of the metal-metal double bond. 

Today, however, carbene complexes covering a broad range of different reactivities have been prepared. Often it is no longer possible to predict whether a carbene complex will behave as an electrophile or as a nucleophile. Thus, a reactivity-based nomenclature would be difficult to apply consistently. For this reason in this book compounds with a carbon-metal double bond will be called carbene complexes or alkylidene complexes , terms not associated with any specific chemical behavior. 

Particularly stable are coordinatively saturated, 18-electron carbene complexes of the type (CO)5M=C(X)R (M W, Cr X OR, NR2 R H, alkyl, aryl). These complexes are often referred to as Fischer-type carbene complexes, in honor of E. O. Fischer, who prepared these compounds for the first time in 1964 [61]. Since then these compounds have attracted broad interest, and many hundreds of heteroatom-substituted carbene complexes have been synthesized. Thereby valuable new insights were gained into the nature of the carbon-metal double bond. These complexes are also becoming increasingly important for organic synthesis, both as reagents and as catalysts. 

In Figure 2.2 the most important synthetic approaches to alkoxy- or (acy-loxy)carbene complexes from non-carbene precursors are sketched. Some of these strategies can also be used to prepare amino- and thiocarbene complexes. These procedures will be discussed in detail in the following sections. In addition to the methods sketched in Figure 2.2, many complexes of this type have been prepared by chemical transformation of other heteroatom-substituted carbene complexes. Because of the high stability of most of these compounds, many different reactions can be used to modify the substituents at C without degrading the carbon-metal double bond. The generation of heteroatom-substituted carbene complexes from other carbene complexes will be discussed in Section 2.2. 

Five-membered carbo- or heterocycles can be prepared with the aid of heteroatom-substituted carbene complexes in several different ways. In the following sections the focus will be on cyclization reactions in which the carbon-metal double bond plays a decisive role. 

In most of the reactions of heteroatom-substituted carbene complexes with alkynes the first event is insertion of the alkyne into the carbon-metal double bond. If vinylcarbene complexes undergo insertion reactions with alkynes, (1,3-butadien-l-yl)carbene complexes result (Figure 2.27). 

Fig. 3.3. Possible mechanisms of carbon-metal double-bond formation by a-abstraction.

A further way of generating carbon-metal double bonds is based on the [2 -I- 2] cycloreversion of metallacyclobutanes. This method has proven particularly useful for the generation of synthetically valuable titanium and zirconium carbene complexes (Section 3.1.7). 

Fig. 3.6. Generation of carbon-metal double bonds by intermolecular a-deprotonations and a-eliminations [403,421-423].

Conjugate hydride abstractions have also been used for the generation of carbon-metal double bonds. An interesting reaction sequence, in which a (thermally unstable) cationic, non-heteroatom-substituted tungsten carbene complex is prepared by conjugate hydride abstraction, is shown in Figure 3.9. 

A further synthetic approach to carbon-metal double bonds is based on the acid-catalyzed abstraction of alkoxy groups from a-alkoxyalkyl complexes [436 -439] (Figure 3.11). These carbene complex precursors can be prepared from alk-oxycarbene complexes (Fischer-type carbene complexes) either by reduction with borohydrides or alanates [23,55,63,104,439-445] or by addition of organolithium compounds (nucleophilic addition to the carbene carbon atom) [391,446-452]. 

Halides are, not surprisingly, also suitable leaving groups for the generation of carbon-metal double bonds by a-abstraction. a-Haloalkyl complexes can be converted into carbene complexes either thermally [459,483] or by treatment with Lewis acids [180], The vinylogous variant of this reaction has also been reported (Figure 3.17). 

Fig. 3.17. Generation of carbon-metal double bond.s by a-abstraction of halides [459 490 -492].

The mechanism of metathesis is used in several variants, either to polymerize, degrade, etc. The various reaction types are summarized in Table 1.2. The metathesis reaction is catalyzed by metal-carbene complexes. The mechanism, exemplified with cyclopentene is shown in Figure 1.5. In the first step, the complex reacts with a monomer to regenerate the carbon metal double bond. This double bond is able to react further with another monomer thus increasing the size of the molecule. 

Complexes with metal-metal double bonds 991... 

Since in C2-unit hydrogenolysis both carbon atoms of the C—C bond to be broken must be primary or secondary (isobutane cannot cleave in C2-unit mode through adsorption of its tertiary carbon atom), Anderson formulated the cleavage of neohexane according to Eq. (11.80) involving carbon-metal double bonds (1,2-dicarbene mechanism) 267... 

Another synthetic approach to p.-methylene complexes originates from the very simple idea that there might be an analogy with olefin cyclo-propanation (146) in organometallic chemistry if one had starting materials at hand that contain metal-metal double bonds (147). This type... 

The weakness of most metal-metal bonds compared with metal-ligand bonds makes cleavage of metal-metal bonds by nucleophiles a common process (15). In the case of metal-metal double bonds this corresponds to nucleophilic addition to the metal-metal bonded systems. Since unsaturated clusters exist which can be considered to contain metal - metal double bonds, this should be an important aspect of substrate activation by clusters. 

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