Terminal metal-alkynyl complexes

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... 

The number of X-ray crystal structures of terminal metal-alkynyl complexes has increased ten-fold since the last review 13 years ago (7). Metrical data for the M—C C—R fragments of terminal metal-alkynyl complexes are set out in Table I 34-151). Bridging alkynyl complexes of the type L M—C=C—ML (and higher L M——ML analogs) are not included in this survey these complexes have been the subject of recent reviews (4b, 10). 

Metrical Data for Terminal Metal-alkynyl Complexes... 

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. 

Alkynyl complexes contain metal-carbon bonds in which the metal is bound to the sp-hybridized carbon at the terminus of a metal-carbon triple bond. The materials properties of these complexes have been investigated extensively. The properties of these complexes include luminescence, optical nonlinearity, electrical conductivity, and liquid crystallinity. These properties derive largely from the extensive overlap of the metal orbitals with the ir-orbitals on the alkynyl ligand. The M-C bonds in alkynyl complexes appear to be considerably stronger than those in methyl, phenyl, or vinyl complexes. Alkynyl complexes are sometimes prepared from acetylide anions generated from terminal alkynes and lithium bases (e.g., method A in Equation 3.42), but the acidity of alkynyl C-H bonds, particularly after coordination of the alkyne to the transition metal, makes it possible to form alkynyl complexes from alkynes and relatively weak bases (e.g., method B in Equation 3.42). Alkynyl copper complexes are easily prepared and often used to make alkynylnickel, -palladium, or -platinum complexes by transmetallation (Equation 3.43). This reaction is a step in the preparation of Ni, Pd, or Pt alkynyl complexes from an alkyne, base, and a catalytic amoimt of Cul (Equation 3.44). This protocol for... 

Two analogous dimeric palladium(l) complexes have been reported where an alkynyl moiety binds both palladium atoms, interacting <7-bonded to one of the metals and in an 77 -fashion with the other. Equation (25) shows the preparation by a comproportionation reaction of the allyl, alkynyl-bridged complex.A butadiene alkynyl complex has been prepared from a bis-butadiene Pd(i) dimer and a terminal alkyne. 

In place of the commonly used metal halides as the metal precursors, transition metal complexes with labile leaving groups have also been used for the preparations of metal alkynyls (Scheme 10.8). The presence of good leaving groups, for example, N2, MeCN or H2O ligands, would provide potential sites for ligand substitutions. These kinds of reactions are usually carried out in the presence of sodium hydroxide or trialkylamine which act as a base to deprotonate the terminal alkynes. 

Exploration of the constmction of mixed-metal luminescent materials using rhenium(i) diimine alkynyl complexes in a terminal mode has also been of interest... 

Table XI contains data for other organometallic complexes. The chloro complexes are of interest because they were reacted with terminal alkynes to afford alkynyl complexes, and the greater nonlinearity of the alkynyl complex eompared to the sum of the precursor alkyne and chloro complex suggests the importance of electronic communication between ligated metal and alkynyl ligand. The aryl complexes have a donor-bridge-acceptor composition, but nonlinearities are...

Already 20 years ago, Antonova et al. proposed a different mechanism, with a more active role of the transition metal fragment [3], The tautomerization takes place via an alkynyl(hydrido) metal intermediate, formed by oxidative addition of a coordinated terminal alkyne. Subsequent 1,3-shift of the hydride ligand from the metal to the P-carbon of the alkynyl gives the vinylidene complex (Figure 2, pathway b). 

Alkynes react readily with a variety of transition metal complexes under thermal or photochemical conditions to form the corresponding 7t-complexes. With terminal alkynes the corresponding 7t-complexes can undergo thermal or chemically-induced isomerization to vinylidene complexes [128,130,132,133,547,556-569]. With mononuclear rj -alkyne complexes two possible mechanisms for the isomerization to carbene complexes have been considered, namely (a) oxidative insertion of the metal into the terminal C-Fl bond to yield a hydrido alkynyl eomplex, followed by 1,3-hydrogen shift from the metal to Cn [570,571], or (b) eoneerted formation of the M-C bond and 1,2-shift of H to Cp [572]. 

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