Silver Olefin Cationic Complexes

High pressure mass spectrometry studies by Guo and Castleman44 led to values of the equilibrium constant of reaction 19 over the temperature range of ca 650-740 K. A van t Hoff plot afforded Ar//°(19) = (135.6 6.3) kJmol-1 and Ar5°(19) = 126.4 kJ mol-1 K 1). Based on a single value of the equilibrium constant of reaction 19 at 750 K (together with an estimated entropy change of 92.5 k.l mol 1 K), the value Ar//°(20) = (141.0 12.6) kJ mol-1 was derived. 

The enthalpies of reactions 19 and 20 may be identified with the first and second Ag+—C2H4 bond dissociation enthalpies in Ag(C2H4)2+(g), respectively. It is a long extrapolation from ca 700 K to 298 K. 

Chen and Armentrout45 used guided ion beam mass spectrometry on the ion (C2H4Ag)+ and, while they could show it is the aforementioned Ag-(C2H4)+(g), they could only derive a lower limit of (22.2 6.8) k.l mol 1 for equation 20 at 0 K. How can the two numerical values of ArH° (20) be reconciled the Guo/Castleman one is seemingly too high and/or the Chen/Armentrout one is too low. 

Guo and Castleman s value for the bond enthalpy is consistent with their value for equation 19. Chen and Armentrout s value is consistent with other values in their paper (see Table 5 for a collection of these and other values). From analysis of radiative association kinetics of their Fourier transform ion cyclotron resonance results, Klippenstein, Dunbar and their coworkers46 derived first Ag+ 2-pentene and isoprene binding energies of (154 29) and (174 29) kJmol-1, and second energies of (164 29) and 

TABLE 5. Gas phase data (in kJmol 1) for silver and gold organometallic species 

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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). 

A paper dating from 1976 investigated the reaction of olefins or alcohols with cationic copper and silver complexes of carbon monoxide 49 The remarkable outcome of these kinetic measurements is that there is no difference between the catalytic behavior of Cu(CO) + and Ag(CO)2 +. 

Also in the 1930s, detailed studies about the thermodynamic stability of adducts of silver(I) with olefins were carried out by Howard Lucas and coworkers, who determined the equilibrium constants between the hydrated Ag+ ion and the corresponding cationic olefin silver(I) complex in dilute aqueous solutions of silver nitrate [25]. In the context of this work, Saul Winstein and Lucas made an initial attempt to describe the interaction between Ag+ and an olefin by quantum mechanics [26]. Assisted by Linus Pauling, they explained the existence of olefin silver(I) compounds in terms of resonance stabilization between the mesomeric forms shown in Fig. 7.4. Following this idea, Kenneth Pitzer proposed a side-on coordination of Ag+ to the olefin in 1945 and explained the stability of the corresponding 1 1 adducts as due to an argentated double bond , in analogy to his concept of the protonated double bond [27]. He postulated that the unoccupied s-orbital of silver(l) allowed the formation of a bond with the olefin, similar to the s-orbital of the proton. 

There are factors other than olefin basicity and the steric effects of substituent groups on the olefin which can affect the stability of the silver ion complex. These include the energy required to displace solvate molecules from the coordination sphere of the metal ion and the degree of association between the cations and anions, especially in concentrated solutions or in solid salts. 

It has been independently observed (296) that the different stabilities of the silver complexes seem to depend upon the strength of the interaction between the cations and anions and suggested that the inability of silver chloride and sulfate to form olefin complexes may be explained by assuming that in these compounds the anions have a very strong affinity for the silver ions. 

The only complex having olefinic double bonds alone in the coordination sphere of the silver ion which has been subjected to a complete structural analyses (404) is the bullvalene (532) complex (CioHjo)sAgBF4. The structure consists of discrete (CioHio)3Ag+ cations and BF4 anions (Fig. 12). The three bullvalene molecules in the complex cation are... 

The hydrogenation activity which was very low for the Tt-chloro-bridged neutral rhodium(I) complexes 3 could be enhanced tremendously by reaction with silver tetrafluoroborate according to Fig. 3, transforming them into cationic species 4 possessing two additional free coordination sites to bind both the substrates, olefin and hydrogen, in the transition state during the catalytic reaction. 

Silver ions cause perturbation of the (E)-(Z) photoisomerization pathway for both stilbene and azobenzene . The efficiency of silver ions in this respect is compared with the effect of Nal which can only induce a heavy atom effect. Ag+ clearly forms complexes with both compounds. Observation of cis-trans conversion in olefin radical cations shows that electron transfer can bring about isomerization of stilbene derivatives. The efficiency of such processes obviously depends on the presence and nature of any substituents. Another study deals with photochemical generation, isomerization, and effects of oxygenation on stilbene radicals. The intermediates examined were generated by electron transfer reactions. Related behaviour probably occurs through the effect of exciplex formation on photoisomerization of styrene derivatives of 5,6-benz-2,2 -diquinoyE. ... 

Formation constants for silver(I)-olefin complexes have been obtained in aqueous solution by potentiometric methods by Hartley and Venanzi 45>. The ligands allylammonium perchlorate, but-2-enyl ammonium perchlorate, allyl alcohol and but-2-en-l-ol, were shown to have affinities for silver(I) comparable with acetate and fluoride ions the complexes are much weaker than the corresponding platinum(II) complexes, possibly due to the non-directional characteristic of the silver component of the a-bond (5s-orbital). The small difference between the formation constants for the allylammonium and but-2-enylammonium complexes and the larger difference between the alcohol complexes was assumed to be an enthalpy effect, arising from an electrostatic repulsion in the case of the unsaturated ammonium cations. 

A review of the photochemical properties of copper complexes includes a survey of the photocatalysed reactions of copper-olefin complexes. The addition of acetonitrile to norbornene may be induced by irradiation in the presence of silver ions. The reaction appears to involve excitation of a LMCT excited state of the norbomene-silver complexes and the formation of norbornene radical cations. 

Dissolution of transition-metal salts in an IL improves the 1-hexene/ n-hexane separation via the formation of a metal-olefin complex with silver nitrate outperform the other tested materials. Therefore [BM1M][N03] plays an important role in the separation process, due to its ability to isolate the silver cation fi om the nitrate anion, thereby producing a large amount of silver ions for the metal-olefin complexation, resulting in improved performance. 

When copper(i) oxide is treated in benzene with trifluoromethanesulphonic anhydride, the complex (Cu0 S02 CF3)2(C H,), which releases benzene only above 120 °C, is obtained. The trifluoromethanesulphonate ion does not compete with olefins for co-ordination sites on the copper cf. ref. 28, p. 2%) and cationic Cu complexes are formed. Oxidation of alkyl radicals by copper(ii) trifluoromethanesulphonate in acetic acid gives more of the alkyl acetate, by oxidative solvolysis, and less olefin, than oxidation by copper(ii) acetate, the difference in product ratio being due to the greater dissociation of the copper trifluoromethanesulphonate. Silver(ii) complexes Ag (bipy)2-(O3S CF,) (bipy = 2,2 -bipyridyl) may be prepared from the silver(i) complex either by electrolysis or by treatment with silver(ii) oxide and trifluoro-methanesulphonic acid. ... 

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... 

If olefins can act as donors, one might expect them to act as ligands to metal ions. Complexes of this kind have indeed been known for many years, the classic examples being the complexes formed from olefins and silver cations (63) and platinum derivatives such as Zeise s salt (64). 

Solutions of cationic olefin complexes of silver exhibit NMR spectra that support the conclusion of IR data that the Ag -olefin interaction is weak. The olefinic protons here are observed at t 3.9, about 0.7 ppm lower than the region observed for resonances of the corresponding free olefins. In addition, no spin-spin coupling between Ag and H was present. 

The crystal structure of an ethylene sorption complex of a partially decomposed, fully silver cation exchan zeolite A was determined. One type of unit cell consisted of an arrangement of one six-ring Ag+, in the sodalite unit, with seven other silver cations recessed into the large zeolite cavity, where each forms a lateral c-complex with ethylene. These silver cations are in a near tetrahedral environment [Ag—C and Ag—O 2.54(8) and 2.49(1) A, respectively]. Co-ordination complexes AuCl,L (L = olefin) are monomeric and non-disso-ciated in CHCls solution, in contrast with the mixed valency complexes, Au2-Cl4,L ( = 2 or 3), which exist as L Au+, AuCl4. (Norbomadien) Au2Cl4 (n = 1), unlike the complexes with r = 2 or 3, is also not dissociated. ... 

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