Silver ethylene complexes

It seems reasonable to assume that biligand silver - ethylene complex formed above 230 K represents the reactive catalytic complex which promotes further transformations of ethylene molecules. 

Fig. 4. Geometry for silver ethylene complexes proposed by Urch (224).

The longest established silver(III) complexes are the red to brown bi-guanides, like the ethylene bis(biguanide) shown in Figure 4.14 persulphate oxidation of Ag+ in the presence of this ligand gives a silver(III) complex with essentially square planar coordination. 

Other polydentate ligands are polyamines and related ligands. Stability constants of silver(I) complexes with polyamines in dimethyl sulfoxide,419 A-methyl-substituted 4-methyldiethylene-triamines,420 or ethylene- or N- or C-methylated ethylenediamine in aqueous solution have been reported.421 The structure of the silver 1,3-diaminopropane complex, [Ag NH2(CH2)3NH2 ]-C104,422 and complex formation with 1,4-diaminobutane and 1,5-diaminopentane have been reported.423 A dinuclear silver(I) compound with ethylenediamine [(enH)Ag(en)Ag(enH)2]4+ has... 

Chemical considerations suggest that metal-olefin back donation will be less important for silver(I) than for platinum(II), and Basch s ab initio calculations on [Ag(C2H4)]+ (75) have confirmed this view. These calculations suggest that most of the electronic rearrangement of the ethylene unit in this complex ion can be accounted for by the polarization effects induced by the positive charge on the silver atom. Indeed, the bonding metal-olefin molecular orbital has only 6.5% Ag 5s orbital character. This result agrees nicely with recent ESR studies on y-irradiated silver-olefin complexes which estimate a 5s spin density of 4.6% for this molecular orbital 92, 93). 

Copper(I) and silver(I) complexes are exceptions of the general trend in stability constants with electron-donating or attracting substituents. Thus most known 7i-complexes of silver and copper are less stable than their respective ethylene complexes (154 156). The steric hindrance introduced by the substituents seems to have a major effect in those systems. 

The gas-phase oxidation of ethylene to ethylene oxide over a supported silver catalyst was discovered in 1933 and is a commercially important industrial process. Using either air or oxygen, the ethylene oxide is produced with 75% selectivity at elevated temperatures (ca. 250 °C). Low yields of epoxides are obtained with propylene and higher alkenes so that other metal-based catalysts are used. A silver-dioxygen complex of ethylene has been implicated as the active reagent.222... 

The following procedures for the preparation of ethylene-bisbiguanidinium hydrogen sulfate and silver(III) complexes of ethylenebisbiguanide are taken from RAy and Chakravarty.1,4... 

Ethylene, complexes with platinum-(II), 6 210, 211, 214, 215 Ethylenebisbiguanide complexes with silver (III), 6 74 Ethylenebisbiguanidesilver(III) hydroxide, 6 78 Ethylenebisbiguanidesilver(III) nitrate, 6 78... 

In contrast, the reaction of tributyltin enol ethers and nitrosobenzene in the presence of a 1 2 mixture of BINAP and AgOTf in ethylene glycol diethyl ether afforded the N adduct predominantly with high enantioselectivity (Table 9.13). Momiyama and Yamamoto have determined the structures of silver-BINAP complex by an X-ray analysis and a 31P NMR study3015. 

The gas phase oxidation of ethylene to ethylene oxide over silver catalysts has been studied extensively.49 la-c It has been suggested that epoxide formation involves transfer of oxygen from a silver-oxygen complex to the olefin on the catalyst surface.4913 Silver-on-silica also catalyzes the liquid phase oxidation of cumene to cumene hydroperoxide. A mechanism that involved insertion of coordinated oxygen into a C—H bond was proposed630 ... 

The marked difference between vc=c iid t for ethylene in the silver ion complex and in complexes with the other transition metals suggests either a much weaker coordinate bonding to silver or at least a smaller TT component in the coordinate bond. In this instance the t value is somewhat below that of free ethylene indicating a net transfer of charge from olefin to metal as opposed to the apparent net charge transfer from metal to olefin with the other metals. As has been suggested previously (152, 232, 403) the a component of the coordinate bond may predominate in... 

The stability constants of silver ion complexes have been evaluated by classical partition techniques (130,131) and more recently by the measurement of dissociation pressures (132,133) and the methods of gas-solid (134) and gas-liquid chromatography (135,136). The use of supported solutions of silver nitrate as a stationary phase for the separation of olefins is now quite general. Enthalpies of formation have been recorded for olefin complexes of silver borofluoride (132), for silver nitrate complexes with cyclic olefins (131) and for silver nitrate-butadiene complexes (133) the results are summarized, together with some values for the ethylene-silver ion complex, in Table XXXVIII. 

The most complete study of silver olefin complexes was made by Muhs and Weiss 21), who used a simple, rapid gas chromatographic technique. The method made use of the relationship between the complexing equilibrium constants and the retention times for olefins, using a silver nitrate-ethylene glycol stationary phase on the column. The reaction followed was... 

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... 

Copper (I) complexes of olefins have been less widely studied but have been found to be analogous to silver(I) complexes in several ways. It was shown 54>, that solid cuprous chloride absorbed ethylene, propylene and isobutylene and solid cuprous bromide absorbed ethylene to give 1 1 complexes, while diole-fms (butadiene and isoprene) and acetylenes were reported S6> to form complexes with a 2 I copper olefin (or acetylene) stoichiometry. Andrews and... 

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