Metal-Alkyne Bonds

The metal-alkyne bonding parameters of molecules of the type CpML-(RC=CR)X are similar to those of other classes of four-electron donor alkynes in Table I. Both cationic and neutral derivatives with cyclopen-tadienyl (Cp = t75-C5H5) or indenyl (tj5-C9H7) illustrate the structural regularities of these molecules. The preponderance of molybdenum derivatives relative to tungsten reflects more extensive and diverse synthetic chemistry with molybdenum to date. 

Sulfur dioxide reacts generally with transition metal alkyl, aryl, and a-allyl complexes to give sulfinate complexes. The reaction, first described in 1964 by Wojcicki and Bibler, resembles well-known insertion reactions of CO, C2F4, SnCl2, tetracyanoethy-lene, and other unsaturated species into metal-alkyl bonds, but there are important stereochemical and mechanistic differences Sulfur dioxide insertion into metal-alkene and metal-alkyne bonds have not been reported. However, PdCl2 has been used as a catalyst for copolymerization of ethylene and SO2 to polysulfones and insertion into a Pd-ethylene bond is a conceivable reaction step. 

H NMR shows that the alkyne ligand in 242 (R = H) rotates about the metal-alkyne bond axis with an activation barrier of only 50 kJ/mol. The structure of the diphenylacetylene complex 242 was determined by X-ray crystallography. The reaction shown in Eq. (183) was expected to generate alkylidyne alkyne complexes such as W(CPh)CI(PhC2R)(CO)-(PMe3)2. Such species are probably intermediates in the reaction however, a n conflict between the adjacent C=C and M=C triple bond systems... 

Canonical forms for metal-alkyne bonds where the metallacyclopropene shown at right is the largest contributor to the resonance hybrid because of the large extent of -backbonding in these complexes. 

Relatively few data are available concerning NMR spectra of protons connected to the alkyne carbon atoms in acetylene metal complexes. In contrast to olefins, coordinated acetylenes have their proton signals shifted to lower t values. The shift to lower fields generally equals 2.5-4 ppm for coordinated acetylenes. Therefore, acetylene protons in alkynes bonded to the central atom have chemical shifts which are typical for olefin hydrogen atoms. This is in agreement with theoretical predictions. X-ray data, IR spectra, and the metal-alkyne bond model. The chemical shift of protons for some alkyne complexes are given in Table 6.21. 

Fig. 7.10 Possible bonding interactions in a metal—alkyne bond.

Metal-alkyne bonding generally is considered to be dominated by two orbital interactions (Figs, la and lb) which act synergistically (Dewar, 1951 Chatt and Duncanson, 1953 Maitlis, 1971a,c Nelson and Jonassen, 1971). 

Chromium.— The acetylene ligand in [(A -C5H6)Cr(NO)(CO)(C2H2)] adopts a preferred conformation at low temperatures ( < 223 K). Above ambient temperature, rotation about the metal-alkyne bond takes place and the two low-temperature n.m.r. signals coalesce. The free activation energy (51.6—... 

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