Metal complex types carbyne

The wealth of empirical information collected for transition metal carbene and carbyne complexes may be best interpreted within the framework of sound theoretical models for these compounds. Perhaps the most significant contribution made by the theoretical studies of carbene and carbyne complexes concerns an understanding of the reactivity patterns they display. In this section the relationship between bonding and reactivity is examined, with particular emphasis being given to the ways in which studies of Ru, Os, and Ir compounds have helped unify the bonding models applied to seemingly diverse types of carbene and carbyne complexes. 

A major breakthrough in the synthesis of transition metal methylene and methylidyne complexes has been achieved by Stone and his group it originates from the simple idea that M=C double bonds in Fischer-type carbenes and M=C triple bonds in carbyne systems should add to low valent metal complexes as do C=C and C=C linkages, respec-... 

Finally, if one considers molecules with a carbon-carbon triple bond of the type that exists in alkynes, one realizes that there also are three possible paths to metal-containing derivatives (Fig. lc). One may construct synthesize complexes (11) in which the two metal-ligand bonds exist at a right angles to each other. Let us finally consider a cleaved triple bond in which one-half is replaced by a metal complex fragment thus we come to the carbyne complexes about which I shall report in the second part of this review. 

E. O. Fischer, G. Kreis, C. G. Kreiter, J. Muller, G. Huttner, and H. Lorenz, trans-Halo[alkyl(aryl)carbyne]tetracarbonyl Complexes of Chromium, Molybdenum and Tungsten. A New Complex Type with Transition Metal-Carbon Triple Bond, Angew. Chem. Int. Ed. Engl. 12, 564-565 (1973). 

These play an enormous role in organometallic chemistry and will be mentioned frequently. The nature of the M to C bonding is very much dependent on the nature of M and R. At one extreme there are compounds in which M is in a high valence state and the R group(s) not of jr-donor character. In these cases, the bonds are comparable to those just discussed for M=NR and M=N. For these types of compounds, the terms alkylidene (M=CR2) and alkylidyne (M=CR) have been favored. On the other hand, when the metal is in a low valence state and the substituents on carbon are n donors, the M—C bonds are not full double or triple bonds and the systems are rendered stable by the migration of charge from the substituents (such as OR or NR) onto the carbon atom, as shown in resonance terms in (16-VI). For these compounds it is customary to use the terms carbene complex and carbyne complex. The chemistry is qualitatively different for the two classes. 

Metal alkylidyne complexes undergo a variety of oxidation and reduction reactions as well as redox-induced transformations of the alkylidyne ligands. A method for the direct transformation of Fischer-type carbyne complexes into Schrock-type alkylidyne complexes was developed in our laboratory. Bromine oxidation of the /ra/7, -carbyne bromo tetracarbonyl complexes 49 of molybdenum and tungsten in the presence of dimethox-yethane affords the dme-stabilized alkylidyne tribromo metal complexes 50 [Eq. (42)] (81). For alkyl-substituted complexes (R = Me, CH2CMe3)... 

Alkylidyne-metal complexes have traditionally been divided into two categories, according to the oxidation state of the metals, in a manner directly analogous to the classification of the very large number of known alkylidene-metal species (19a,b). Hence Fischer-type alkylidyne complexes involve metals in low oxidation states, while Schrock-type complexes generally involve more electropositive metals with higher oxidation states (13). However, the properties of some of the numerous carbyne-metal complexes that have been characterized since the early days have in many cases blurred the distinction between the two classes (12a). 

The bonding interactions of a carb)me ligand are essentially those of a metal carbene complex, but with an additional ir-bond (Figure 2.16). One orbital of cr-symmetry and two of iT-symmetry overlap with three metal orbitals of appropriate symmetry. When considered trianionic, all three orbitals of the ligand fragment contain two electrons. Theoretical studies of heteroatom-substituted or "Fischer-type" carbyne complexes - indicate that the HOMO predominantly consists of the metal fragment, and the LUMO consists of one of the TT -orbitals of the metal-carbon bond. This result explains the tendency of nucleophiles to attack the carbyne carbon in carbyne complexes, just as they attack the carbene carbon in Fischer carbene complexes. 

The pattern of chemical reactions observed for these compounds clearly sets them apart from "Fischer-type" carbyne complexes of GroupVI e.g., W(hCR)X(CO)4. Whereas the "Fischer-type" complexes typically react with nucleophiles at the carbyne carbon all of the reactions observed for the five coordinate mthenium and osmium complexes, including the cationic examples, are electrophilic additions to the MsC bond. The following sections deal individually with, protonation, addition of halides of the coinage metals, addition of chlorine and chalcogens, and finally an attempted nucleophilic addition where the nucleophile is directed to a remote site on the aryl ring of the carbyne substituent. 

Fischer-type carbyne complexes react with nucleophiles to form metal carbene compounds. 

There are two types of transition metal carbene and carbyne complexes low-valent (so-called Fischer type)i" i" and high-valent (so-called Schrock type). The two classes of compounds are quite different in their chemical behavior. Such different chemical reactivity is sometimes rationalized on the basis that the metal-carbene and metal-carbyne bonds in Fischer-type complexes have donor-acceptor character, whereas the bonding in Schrock-type complexes is more typical for a normal multiple bond. 

Raubenheimer HG, Kruger GJ, Lombard AA, Linford L, Vdjoen JC. Sulfar-containing metal complexes. 12. Reactions of a-thio carbanions with carbene complexes of the type [M(CO)5 0(alkyl)Ar ] and with the carbyne [(f -MeC5H4)-Mn(CO)2(CPh)][BCl4]. Organometallics. 1985 4 275-284. 

Metal-carbene and carbyne complexes occupy a central place in synthesis and catalysis. Metal-carbene or alkylidene eomplexes can have an eleetrophilic singlet carbene (Pettit type) or a nucleophilic triplet carbene (Sehrock type, often d ). 

Metal-carbyne complexes M CR are less known than metal carbenes. The carbyne can also be nucleophilic (Schrock type) or electrophilic (Fischer type). Fischer-type metal-carbynes are obtained by reaction of BF3 on a neutral Fischer-type metal-carbene complex, whereas Sehroek earbynes are often obtained by deshydrohalogenation of a Schrock-type metal-carbene eomplex. They catalyze alkyne metathesis and, in particular, give heterocycles with unsaturated substrates. 

The same dichotomy of bonding models is also found for carbyne complexes that have a formal triple bond M=CR. There are metal-carbyne bonds that belong to the donor-acceptor type (the Fischer car-... 

Phosphines, as nucleophiles, add to many unsaturated substrates giving metal-lated ylides. Scheme 17 collects some representative examples of the addition of phosphines to carbyne complexes, giving (57) [132], to allenylidenes (58) [133], a-alkenyls (59) [134], or a-alkynyls (60) [135]. Moreover, reaction of phosphines with 7i-alkenes [136] and 71-aIkynes (61)-(64) [137-140] have also been reported. It is not possible to explain in depth each reaction, but the variety of resulting products provides an adequate perspective about the synthetic possibihties of this type of reactions. 

An impressive series of carbene- and carbyne-bridged complexes of platinum have been formed by reaction of metal carbene complexes with a platinum complex (equation 141). These complexes have been verified by X-ray crystallography, and the synthetic method appears to be one of some generality. Among the complexes of this type formed from carbenes are ones having metal frameworks with Pt—W,409-413 Pt—Mn414-418 and Pt—Cr413,417 bonds. 

Reactions of this type proceed via metal carbyne complexes (equation 4). The most direct evidence is that such complexes (Mt = Mo, W, Re) can act as initiators for the... 

Carbyne complexes, for example 151 and 152, belong to another type of complexes with multiple a-carbon-metal bond [73,259a-c] ... 

The reversible [2+2] cycloaddition of metal alkylidyne or Fischer-type metal carbyne complexes remains the only general methodology for the synthesis of metallacyclobutadiene complexes. Recent literature revolves principally around the heavier group 6 metals and the investigation of intermediates in catalytic alkyne metathesis (Scheme 25 Equation 45) <1996CHEC-II(lb)887> (W <2005OM4684>, Mo <2003JOM56>). 

For this new type of compounds we would like to propose the name carbyne complexes for two reasons (i) in analogy with carbene complexes, (ii) because similarly to alkyne, on the basis of the diamagnetism of these compounds, a formal metal-carbon triple bond had also to be postulated. 

In this way, a general route to complexes of type P, containing i -ri1 r 2-C2 M L ligands, becomes available. An alternative mode of bonding is via a p3-carbynic carbon as in Q. If the carbon chain is longer than two, uncoordinated carbons separate the cluster-bonded fragment from the metal center. 

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