From Metal Alkynyls

In contrast to the alkynyl anion, coordination to a metal center results in Co, being electron-poor and subject to frontier-orbital controlled nucleophilic attack, while the 

The alkynyl-metal complexes are strong carbon bases, with measured pK values for M(C=CBu )L2Cp being 13.6 [ML2 = Fe(CO)(PMe3)j and 20.8 [ML2 = Ru (PMe3)2] [170], 

While protonation affords the vinylidene expected by H migration from the original 1-alkyne, use of other electrophiles provides a convenient route to disubsti-tuted vinylidenes. The stereospecificity of this reaction with Re(C=CR)(NO)(PPh3) 

Cp has been discussed [170b]. Alkylation with haloalkanes (often iodoalkanes), triflates (alkyl, benzyl, cyclopropyl), or [RsO] (R = Me, Et) is often the best entry to vinylidenes of any particular system. Other common electrophiles, such as halogens (Cl, Br, I), acylium ([RCO] ), azoarenes ([ArN2] ), tropylium ([C7H7] ), triphenylcarbenium (trityl, [CPhs] ), arylthio (ArS) and arylseleno (ArSe) have also been used. 

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Fig. 16.30. Pd(0)-catalyzed arytation of a copper acetytide at the beginning of a three-step synthesis of an ethynyt aromatic compound. Mechanistic details of the C,C coupling Step 1 formation of a complex between the catalytically active Pd(0) complex and the arylating agent. Step 2 oxidative addition of the arylating agent and formation of a Pd(II) complex with a cr-bonded aryl moiety. Step 3 formation of a Cu-acetylide. Step 4 trans-metalation the alkynyl-Pd compound is formed from the alkynyl-Cu compound via ligand exchange. Step 5 reductive elimination to form the -complex of the arylated alkyne. Step 6 decomposition of the complex into the coupling product and the unsaturated Pd(0) species, which reenters the catalytic cycle anew with step 1.

F. Mixed-Metal Clusters Derived from (l-Alkynyl)carbyne Complexes 232... 

Typical modes for the preparation of (l-alkynyl)carbene complexes, as well as spectroscopic and structural data, are briefly summarized. Alkoxy-and amino(l-alkynyl)carbene complexes are available from metal carbonyls and metal isocyanides, respectively. 

The purpose of this review is to attempt to discern, from consideration of the full range of data from these techniques, which metrical and vibrational-spectroscopic parameters may be meritoriously applied to the discussion of the electronic structures of metal-alkynyl complexes, and what conclusions may be drawn from them. Because this review aims to extract the intrinsic parameters that characterize M—C=C—R bonding, the discussion is limited in scope to compounds possessing terminal (rj ) alkynyl ligands bound to a single metal center (2) complexes, clusters, oligomers. 

We begin with a brief overview of the findings from the limited number of theoretical calculations, photoelectron and electronic-spectroscopic studies, and other physical measurements on terminal metal-alkynyl complexes in order to provide a context for discussing the results of X-ray... 

In addition to their obvious role in establishing connectivity and stereochemistry, the data from structural studies of metal-alkynyl complexes have been used to evaluate the M-C bond order, the extent of M CCR... 

The problem with applying this or related equations to the calculation of M-C bond lengths for metal-alkynyl complexes of the type ML (CCR) is that one typically knows neither /-(ML ) nor x(ML ), either or both of which may differ significantly from the tabulated values for bare M or If, however, M-C distances for two ML R (R = alkyl, alkynyl) complexes are available, then r(ML ) can be factored out and the bond-length difference Ai/(M-C) = [J(M-C,p3) - tf((M-Ci.p)] can be expressed as a function of x(ML ) [Eq, (3)] if an experimental value of Ad(M-C) for a pair of ML R complexes gives a chemically reasonable value of X(ML ), then M-C rr-bonding need not be invoked, even if AJ(M-C) exceeds 0.08 A. 

Unless one completely discounts the bond-ionicity arguments set out earlier, it appears that there are few instances where comparisons between metal-alkynyl and metal-alkyl M-C bond lengths provide unambiguous evidence for M-CCR 7r-bonding interactions, and thus interpreting structural data from such a standpoint is probably well grounded only if one has independent spectroscopic (or other) evidence for these interactions in the compounds under consideration. 

Viewed as a whole, it appears that the results of the structural, spectroscopic, theoretical, and other physical studies on metal-alkynyl complexes cannot be interpreted within a single, simple picture of the nature of metal-alkynyl bonding. In view of the fact that understanding the electronic-structural basis of the reactions of metal-alkynyl complexes and the physical properties of the advanced materials developed from them requires a clear picture of this bond, further work in this area would be highly desirable. Of particular value would be studies that applied a combination of the experimental techniques described above and theoretical calculations to a series of archetypal metal-alkynyl complexes from across the transition series, so as both to gauge the effect of the metal on the nature of the M-CCR bond and to clarify the benchmark parameters for each technique within different metal-alkynyl bonding regimes. 

The history of routinely interpreting the structures and spectra of metal-carbonyl and -cyano complexes from a rr-backbonding perspective tempts one to carry this approach over to metal-alkynyl complexes as well. But however successful this may appear to be for individual or limited series of metal-alkynyl compounds, consideration of the whole body of metrical and vibrational-frequency data associated with the M—C=C—R linkage makes it clear that, rather than a particular datum or series of data demonstrating the presence of 7r-backbonding, they are... 

The bis(propynyl)beryllium compound [Be(C=CMe)2NMe3]2 is unusual in crystallizing with two types of dimeric molecule in the lattice, one of which has a diamond-shaped (Be-C)2 ring very similar to that of 31. The other, structure 32, has a nearly rectangular (Be-C)2 ring, explicable in terms of donation of charge from the alkynyl triple bond into the available metal orbital. 

The nature of the metal-alkynyl bond has long been a matter of debate, especially with regard to the metal-C(sp) bond order. Multiple bonding could in principle arise from interactions between occupied alkynyl k levels and empty d-orbitals at the metal or from backbonding between filled metal d-orbitals and unoccupied 71 levels on the alkynyl ligand. Early photoelectron spectroscopy work on... 

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