Alkynes 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. 

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. 

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—... 

Organometallic compounds which have main group metal-metal bonds, such as S—B, Si—Mg,- Si—Al, Si—Zn, Si—Sn, Si—Si, Sn—Al, and Sn—Sn bonds, undergo 1,2-dimetallation of alkynes. Pd complexes are good catalysts for the addition of these compounds to alkynes. The 1,2-dimetallation products still have reactive metal-carbon bonds and are used for further transformations. 

The subsequent insertion of the alkyne into the metal-carbene bond affords the (r]1 r]3)-vinylcarbene complex D, which may exist either as a (Z)- or an ( )-metallatriene. This intermediate maybe considered as a branching point in the benzannulation reaction as three diverging routes starting from this point have been explored. 

It is useful to consider the possible formulations of alkyne and allyl bonding to metals in terms of Green s MLX formalism.64 Coordination of an alkyne in a simple dative two-electron fashion is denoted ML, whereas the limit of metallacyclobutene formation is denoted MX2. For the allyl ligand, three imaginable coordinations are possible simple q1 coordination is denoted MX, butq3 coordination can encompass both MLX (one a bond plus a dative alkene coordination) and MX3 (three M—C a bonds). 

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