Alkylidyne compounds

Alkyldene and alkylidyne compounds have also been made [200],... 

A.J. Carty, Guelph-Waterloo Centre What are the frequencies of the metal-carbon (v(M=C)) stretching frequencies in the tungsten alkylidyne compounds One might expect these to be quite high in view of the x-ray data showing short M=C bonds and evidence of multiple bond reactivity. 

Thus (2 ) is prepared by adding [Rh(C2Hlt)2(acac)] to (6), the latter being preformed by treating the mononuclear tungsten alkylidyne compound with [Rh(C0)2(acacl] (6). Similarly, step-wise addition of [Rh(C0)2(n-C9H7)] to r(n-C5H5)(0C)2W = CR] yields ( )(9). 

A number of reactions between M2(OR)6 compounds and alkynes have been found to give alkylidyne compounds by a metathesis of the M=M and C=C bonds (see Sections 15.3.6.4) and evidence has been presented275 for an equilibrium between a ditungstatetrahedrane and a methyl-idyne-tungsten complex supported by BulO ligands (equation 72). 

It is important to note that this reaction works only with W2(OCMe3)6 and not with any other M2(OR)6 molecule and that while it appears to be general for aliphatic R it does not proceed for R = QH5. It goes under very mild conditions and is virtually quantitative. Since the (Me3CO)3W=CR compounds are alkyne metathesis catalysts, still other tungsten alkylidyne compounds, not obtainable in the above reaction, can be obtained by the following reaction ... 

The nitrido compound is very stable and only slightly soluble and thus this reaction provides access to some alkylidyne compounds not readily accessible by the reaction with alkynes, e.g., (Me3CO)3W=CPh. The solid (Me3CO)3W=N has a linear polymeric structure167 in which the WNWN chains have alternating W-N distances of 1.74(1) A and 2.66(1) A. 

Electrophilic attacks can give alkylidyne compounds (Chapter 16)... 

Several alkyls and aryls with M=0 (or M=NR) groups have been characterized. A simple methyl, OsOMe4 is obtainable from 0s04. The dioxo alkyl, 0s02(CH2Bu )2,125 can be converted into alkylidene and alkylidyne compounds,... 

The formation of alkylidene and alkylidyne compounds was noted above. Quite a number of these are now known. The use of electron rich alkenes to make carbene complexes was pioneered by Lappert131 as in the reaction... 

Molybdenum, tungsten, and rhenium alkylidyne compounds similar to the complexes involved in alkene metathesis can metathesize alkynes via metallacyclobuta-diene intermediates ... 

The reactivity of alkylidyne compounds has also been widely studied, particularly by Seyferth and co-workers who have carried out a great variety of reactions with cobalt complexes (303-312). Other groups... 

The bridging alkylidyne compounds (Me3SiCH2)4M2(/u-CSiMe3)2 supported on silica catalyze the exhaustive hydrogenation of a variety of aromatic substrates. Hydrogenation of benzene at 1200-1400 psi of H2, for example, gave 100% cyclohexane. ... 

The rhodium compound [Rh6(/i-AsBu2)2(iW4-AsBu )(/i-CO)2(CO)9] is probably the only example of a Rh cluster in which the Rhg framework is a pentagonal pyramid. Recently, the mixed metal cluster [Co6Ni2(C2)(CO)i6] having the structure of a biprism sharing a rectangular face (the carbon-carbon distance is 149.4 pm) was prepared.An important class of clusters are alkylidyne compounds of the type [Co3CR(CO)9]. 

Although containing metal-carbon a bonds and possessing sp hybridization of carbon atoms, the /z-alkylidene and 3-alkylidyne compounds... 

In general, screening of carbyne carbon atoms is smaller compared to carbene carbon atoms. The 5( C) NMR shifts for alkylidyne complexes are in the 250-350 ppm range, and therefore are higher than shifts for sp hybridized carbon atoms in acetylenes by about 200 ppm or more. The presence of only one signal for carbonyl groups in complexes of the type MX(CR)(CO)4 (M = Cr, Mo, W) proves that they have trans geometry. For WX(CR)(CO)4, 5C(CO) is lower than for carbene compounds such as W(C0)5(CXR), which indicates (like IR spectra) that the W-CO k back bond is weaker in alkylidyne compounds compared to alkylidene complexes (Table 5.3). 

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