Silver, complex with ethylene

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... 

Although non-ZiEGLER catalysts are involved, there is excellent evidence for free radical polymerization of yr-complexed monomer. Bier et al (348) have shown that ethylene forms complexes with silver salts in neutral aqueous solution at 10—40° C. and 5—50 atmospheres ethylene. Initiation by peroxides produced high molecular weight, branched... 

A further smdy of the effect of the nature of the alkyl substituent in monoalkyl ethylenes on the change in chemical shift of the vinyl protons upon complexation with silver ion will be discussed in terms of the change in the dipole moment of the molecule. 

Let us now consider metal complexes. Transition metal complexes of olefins, alkynes, and arenes and other organic 7T systems have become quite common in both the textbook and research literature some exotica of this type will be discussed in a later section. We start with an ethylene complex with the simplest metal ion, Li+. The cation is definitionally electrophilic, the olefin has an energetically accessible pair of electrons, and the complex is really quite sensible. We may recognize it as analogous to the better known olefin-silver complexes... 

Particular examples of such facilitation are given below. When a gas mixture of an olefin (e.g. ethylene) and paraffin (e.g. ethane) is exposed to an aqueous membrane containing silver nitrate, ethylene complexes with the silver ion (see Section 4.1.9.3) ... 

The use of silver fluoroborate as a catalyst or reagent often depends on the precipitation of a silver haUde. Thus the silver ion abstracts a CU from a rhodium chloride complex, ((CgH )2As)2(CO)RhCl, yielding the cationic rhodium fluoroborate [30935-54-7] hydrogenation catalyst (99). The complexing tendency of olefins for AgBF has led to the development of chemisorption methods for ethylene separation (100,101). Copper(I) fluoroborate [14708-11-3] also forms complexes with olefins hydrocarbon separations are effected by similar means (102). 

Olefin Complexes. Silver ion forms complexes with olefins and many aromatic compounds. As a general rule, the stabihty of olefin complexes decreases as alkyl groups are substituted for the hydrogen bonded to the ethylene carbon atoms (19). 

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... 

Replacement of silver nitrite by inexpensive sodium or potassium nitrite enhances the utility of this process. Treatment of alkenes with sodium nitrite and iodine in ethyl acetate and water in the presence of ethylene glycol gives conjugated nitroalkenes in 49-82% yield.63 The method for generation of nitryl iodide is improved by the treatment of iodine with potassium nitrite complexed with 18-crown-6 in THF under sonication, as shown in Eq. 2.32s4... 

The shift in the C=C frequency, vi, for adsorbed ethylene relative to that in the gas phase is 23 cm-1. This is much greater than the 2 cm-1 shift that is observed on liquefaction (42) but is less than that found for complexes of silver salts (44) (about 40 cm-1) or platinum complexes (48) (105 cm-1). Often there is a correlation of the enthalpy of formation of complexes of ethylene to this frequency shift (44, 45). If we use the curve showing this correlation for heat of adsorption of ethylene on various molecular sieves (45), we find that a shift of 23 cm-1 should correspond to a heat of adsorption of 13.8 kcal. This value is in excellent agreement with the value of 14 kcal obtained for isosteric heats at low coverage. Thus, this comparison reinforces the conclusion that ethylene adsorbed on zinc oxide is best characterized as an olefin w-bonded to the surface, i.e., a surface w-complex. 

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. 

Noteworthy evidence for a Langmuir—Hinshelwood mechanism is provided by Force and Bell [114,116]. Apart from demonstrating that ethylene does not adsorb on silver completely covered with oxygen, the authors show by IR analysis at 220° C that the adsorbed complex has a structure which is analogous to ethylene, coordinated with isolated silver ions, viz. 

During their work on the arylation of aromatic compounds by substitution, Fujiwara, et al. observed biaryl formation when aromatic compounds were placed in the presence of olefin-palladium complexes and silver nitrate.80 Developing this reaction as a method for biphenyl synthesis, these authors showed that the more stable the olefin-palladium complex was, the lower the yield. Ethylene dichloropalladium proved to be the best choice, when used with silver nitrate. However, the reaction required stoichiometric amounts of both catalysts (Scheme 10.47). Benzene derivatives substituted by electron-donating or -withdrawing groups reacted as well, but a mixture of regioisomers was produced, except for nitrobenzene, which only gave m,m -dinitrobiphenyl. 

Similar cts-bis-carbene chelate complexes of palladium(Il) [327,330,331], but without the hydroxy functional groups on the wingtips, were used by the same research group for the copolymerisation of ethylene and CO. Once again, chelating bisphosphane complexes inspired the synthesis and application of their NHC counterparts [332,333]. The actual, defined catalyst precursors were the cationic complexes formed after haUde abstraction with silver salts in acetonitrile as donor solvent. 

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... 

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