Carbynes Alkylidynes

As in the case of carbene complexes, 13C NMR spectroscopy is particularly useful in that the carbyne carbon typically resonates to low field (240 and 360 ppm), with heteroatom substituents shifting this to higher field. As noted above for carbene complexes, X-ray crystallography reveals that carbyne complexes have very short metal-carbon bonds, typically the shortest of any metal-carbon multiple bond, but lengthened if heteroatom substituents are present. 

Free carbynes, CR , are exceedingly reactive short-lived species and 

CF3C02 is a better leaving group than CH3C02 . 

The stability may be increased by employing amino (thermodynamic stabilization) or sterically demanding (kinetic stabilization) carbyne substituents. 

The reactions of carbyne complexes will be discussed, albeit more briefly, along the lines developed for alkylidene complexes, to emphasize the parallels. Once again, as with alkylidenes, early generalizations suggested two classes of complexes, Fischer-type and Schrock-type, which have subsequently been recognized as extremes in a tunable reactivity continuum. Furthermore, within the chemistry of tungsten, Fischer-type 

---

Molybdenum and tungsten carbyne (alkylidyne) complexes frequently undergo 2+2 cycloaddition reactions with alkynes to give the corresponding metallacyclobutadiene... 

The oldest route to carbyne (CR) complexes involves a Lewis acid (A+) attack on Fischer-type carbenes (equation 15). In this way, several Re carbyne derivatives under the general formula [[Re](CR)]° + have been prepared. The leading examples are [ReCp(CO)2(CR)]+ (R = Ph, SiPhs) because the analogous alkyl carbynes (alkylidynes) are much less stable due to facile deprotonation as elegantly shown by the spontaneous near-quantitative proton loss shown in equation (16). ... 

Carbene (alkylidene) Carbyne (alkylidyne) Cyclopropenyl (cycto-CgHj)... 

One route to a carbyne (alkylidyne) complex was reaction 23.40 equation 23.91 illustrates the initial method of Fischer. The abstraction of an ot-H atom from a Schrock-type carbene yields the corresponding carbyne complex (equation 23.92). 

Early -block metal complexes containing one or two Qt-hydrogen atoms (see 24.47) may undergo a-hydrogen abstraction to yield carbene (alkylidene, 24.49) or carbyne alkylidyne, 24.50) complexes. The solid state structure of the product of reaction 24.45 confirms differences in the Ta—C bond lengths 225 pm for Ta—Caikyi and 205 pm for Ta Ccarbene-... 

Fischer was also the first to synthesize a carbyne (alkylidyne), an organometaiiic compound having a M=C triple bond, as shown in Figure 18.26. A filled sp-hybridized orbital on the C atom donates electron density to an empty orbital on the metal. r-backbonding from filled orbitals on the metal can occur into two perpendicular sets of r MOs. Thus, the carbyne acts as a tr-donor and two r-acceptors. Because it has two ar-acceptors and only one a- donor, the ligand is formally... 

The following types of carbene (alkylidene) and carbyne (alkylidyne) ligands are known " ... 

Carbyne (alkylidyne) complexes are implicated in the metathesis of alkynes (p. 377). 

CO insertion p-hydrogen ehmination agostic M H C interaction a-hydrogen abstraction carbene (alkylidene) carbyne (alkylidyne) sandwich complex metallocene half-sandwich complex... 

Fischer synthesized his carbyne (alkylidyne) complexes upon attempting to make halogenocarbene complexes using the following reaction ... 

---