Acetylene complexes properties

The compounds (VIII) and (IX) display their quasi-complex properties less clearly than do (VI) and (VII). Their trans isomers lose acetylene much more easily than do the cis ones. 

Whilst the synthesis of new transition metal-olefin and -acetylene complexes continues unabated, only a relatively small amount of data has accumulated on the thermodynamic stability of these complexes and these are restricted almost exclusively to complexes of the unsatured species acting as monodentate ligands. Metals able to coordinate strongly with unsaturated ligands are restricted to those in a small triangle around the centre of the periodic table, and designated class (b) acceptors by Ahrland et al., 0>. Class (b) acceptors include Cu(I), Rh(II), Ag(I), Pt(II) and Hg(II). However the majority of such metals form inert complexes which are either very readily oxidised or involve solubility problems. If thermodynamic stability constants are to be measured reliably, the equilibrium should be reached reasonably quickly, the reaction should be clean and the stoichiometry should be known or easily deduced. Furthermore, the equilibrium must be followed by means of suitable electrodes or changes in some physical property of the reaction mixture. The solvent is therefore important. 

Temkin and co-workers have investigated the thermodynamic properties of the soluble complexes of unsaturated hydrocarbons with various metal salts with particular reference to their role in catalytic reactions. Using a potentio-metric technique, they were able to calculate the thermodynamic data shown in Table 6 for the silver(I)-acetylene complexes 30) and the silver(I)-ethylene complex 31). The results obtained for acetylene have been related to the low activity of silver salts as catalysts for the hydration of acetylene. For the sil-ver(I)-ethylene complex, the relationship between the ionic concentrations and... 

Acetylenes are a priori electron-donating ligands with only weak acceptor properties and their n complexes should be relatively labile. The initial step of at least some of the reactions could be a conversion of the complexed acetylene in its ground state to a biradical with sp hybridized carbon atoms. This activated acetylene complex may then stabilize itself in many ways, depending largely on the type of the additional coordinated ligands in the transition state. 

It is well known that Rh(I) complexes can catalyze the carbonylation of methanol. A heterogenized catalyst was prepared by ion exchange of zeolite X or Y with Rh cations.126 The same catalytic cycle takes place in zeolites and in solution because the activation energy is nearly the same. The specific activity in zeolites, however, is less by an order of magnitude, suggesting that the Rh sites in the zeolite are not uniformly accessible. The oxidation of camphene was performed over zeolites exchanged with different metals (Mn, Co, Cu, Ni, and Zn).127 Cu-loaded zeolites have attracted considerable attention because of their unique properties applied in catalytic redox reactions.128-130 Four different Cu sites with defined coordinations have been found.131 It was found that the zeolitic media affects strongly the catalytic activity of the Cd2+ ion sites in Cd zeolites used to catalyze the hydration of acetylene.132... 

In summary, the Zn-Al and Zn-Ga based-hydrotalcites were found as very effective supports for docking a Rh(TPPTS)3Cl complex. The process was carried out via ionic exchange and occurred without any damage of die support structure or of the complex. The resulted catalysts were found to catalyze the cyclization of acetylenic carboxylic acids to the corresponding 5-membered ring heterocycles in good to excellent yields. The basic properties of the support also allow a clean and selective reaction of unsubstituted acetylenic carboxylic acids. 

A characteristic feature of contemporary investigations in the held under consideration, is the interest in cycloaddition reactions of nitrile oxides with acetylenes in which properties of the C=C bond are modified by complex formation or by an adjacent metal or metalloid atom. The use of such compounds offers promising synthetic results. In particular, unlike the frequently unselec-tive reactions of 1,3-enynes with 1,3-dipoles, nitrile oxides add chemo-, regio-and stereoselectively to the free double bond of (l,3-enyne)Co2(CO)6 complexes to provide 5-alkynyl-2-oxazoline derivatives in moderate to excellent yield. For example, enyne 215 reacts with in situ generated PhCNO to give 80% yield of isoxazoline 216 (372). 

TABLE III. Some Properties of Cobaltacydopentadiene Complexes, [Co—CR =CR2—CR3=CR4(n5-C5H5)(PPh3)], from Two Different Acetylenes. 

Numerous investigations have been undertaken on the reactions of ruthenium carbonyls with olefins and acetylenes. Two complex types [55] and [56] result from the reaction of Ru3(CO)j2 with ethylene and other Simple olefins (127). The complexes [56], which belong to the products of reaction (1) (Chapter 2.5.) are also formed from Ru3(CO)i2 and diphenyl acetylene (183). [55] and [56] show interesting fluxional properties, and four different types of ligand scrambling are possible (163). 

Full papers have appeared on the formation and reactivity of the compounds ML(CNR)2 (M = Ni, Pd, Pt L = Oz, azobenzene, olefin, diazo-fluorene, acetylene) (231-237) (see also Sections IV,D,2 and V,D). Complexes of the type Ni(olefin)(CNBu )2 have been prepared for a large range of olefins (234, 237). The isocyanide stretching frequencies have been measured and related to the electron-withdrawing properties of the olefin. Other unsaturated molecules such as imines, diazenes, ketones, nitroso compounds, and acetylenes have been similarly studied. The effect of substituent change has been found to be cumulative and an empirical relationship has been developed to predict v(NC) (237). 

Monosubstituted acetylenes, polymerization, 11, 559 Monosubstituted ferrocenes, properties, 12, 222 Monothiolato compounds, in mono-Cp Ti(IV) complexes,... 

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