Theoretical studies acetylene complexes

A few theoretical studies have been carried out to clarify the mechanism of the metal-assisted acetylene to vinylidene tautomerization. The first work done by Silvestre and Hoffmann using extended Hiickel calculations on the complex... 

The alkyne-to-vinylidene tautomerization processes on various transition metal centers have also been discussed. Three different pathways for the formation of vinylidene from p -acetylene on electron-rich transition metals were the most theoretically studied. Most studies suggested that the favorable pathway proceeded via an intermediate with an agostic interaction between the metal center and one C—H bond followed by a 1,2 hydrogen shift (the bl+b2 pathway shown in Scheme 4.5). The reverse process, the vinylidene-to-p -acetylene tautomerization, was also discussed. It was found that complexes with electron-poor metal centers were able to mediate the reverse process. 

Metal cluster complexes containing vinylidene ligands have been considered as models of species present when olefins or alkynes are chemisorbed on metal surfaces (114). Vinylidene has been detected in reactions of ethylene or acetylene with Fe(100), Ni( 111), and Pt(l 11) surfaces (115), and was shown to be an intermediate by theoretical studies on a manganese surface (116). The facile cleavage of C-H bonds which occurs in these systems, together with hydrogen addition or abstraction, also occurs on metal clusters. Typical of the reactions considered is the hydrogen transfer reaction... 

The adsorption mode of acetylene on a Cu(llO) surface is more complex and the available experimental information concerning this system, is fairly scarce. In a recent paper [9], an experimental and a theoretical study of this adsorption system was presented. Using HREELS and ARUPS the authors concluded that acetylene adopts a low symmetry (most likely C ) adsorption geometry on Cu(llO) (IX on Fig. 

There have been extensive experimental and theoretical studies devoted to the structural and bonding characterization of weakly bound van der Waals complexes of acetylene. Structures of these complexes can often be determinated experimentally by means of Fourier transform microwave and infrared spectroscopic techniques. On the theoretical side, advanced treatments are required to understand the complex nature of the weak bonding in terms of the relative contributions of polarization and dispersion interactions, interactions of multiple moments, and electrostatic interactions involved in these completes. To determine the interaction energy in a weak complex, it is necessary to use large basis sets with the inclusion of electron correlation interactions. Theoretical calculations have been reported for van der Waals complexes of acetylene with COj [160], CO [161, 162], AICI3 [163], NH3 [164], He [165], Ar [166], H2O [167], HCN [168], HF [169-172], HCl [173, 174], and acetylene itself in the forms of non-covalent dimer [175-180], trimer [175,181], tetramer [175, 182, 183], and pentamer [175]. These calculations are very useful for the determination of multiple isomeric forms of the complex. For example, calculations at the MP2/6-31G level along with IR spectra indicate that the HCN-acetylene complex exists in a linear form in addition to the T-shaped structure observed previously by microwave studies (see Fig. 1-5) [168]. 

The metal ion-acetylene complexes are calculated in the bridging structure geometry. No asymmetric structures were examined. Theoretical studies on symmetric Cu" - and Ag -acetylene complexes have recently been reportedThe only experhnental value of the dissociation energy for the three systems studied here is for CUC2H2 where an upper bound of 30.5 kcalmoL has been estimated. The QCISD(T)/PP//MP2/PV calcu-... 

The mechanism of the processes where the alkene or alkyne is functionalized by two ER groups necessarily involves the insertion of the substrate into one M-ER bond. The actual mechanism depends on the specific reaction, type of substrate, and catalyst. Eor diboration or disilation of alkenes, theoretical studies have found this step rate determining when the catalyst is a Pt complex [176,179]. The insertion of acetylene into Pt-BR2 bonds is faster than the insertion of ethylene, and this step is not rate determiiung for diboration of alkynes [178,179]. As was pointed out before (see Section 6.4.1 (a)), insertion of an alkyne into the Pd-SnRs bond is preferred over insertion into the Pd-SiRa in the silylstannation of alkynes [177]. 

The absorption of acetylene in the vacuum-UV spectral region has also received extensive study, both experimentally [363-370] and theoretically [371-374]. The spectral region 155-200 nm displays an abundance of complex, diffuse vibronic structure [365]. Much of this absorption, which is substantially stronger than that associated with the A—X system at longer... 

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