Ag-olefin complexes

Duncanson (i) of the molecular orbital bonding concepts of Dewar (2), which he developed to explain the structure of Ag+-olefin complexes, led to the suggestion that ethylene is symmetrically coordinated to the metal. Platinum, atomic number 78, has the electronic configuration of the xenon core (Is 2s 2p 35 3p 3d Z = 54), then... 

As mentioned in the introduction, TT-bonded Ag-olefin complexes are paradigms in the thermochemistry of organometallic species. Nonetheless, there are comparatively few relevant quantitative data. 

Silver-olefin complexes have been extensively investigated, and have been reviewed in some detail (67, 138, 176, 183). Distribution studies have shown the existence in solution of complexes of the types [Ag(olefin)]+,... 

Anhydrous silver-olefin complexes are readily dissociable, low-melting, and variable in composition 92a, 176, 183). Cyclic olefins and polyolefins form stable complexes with silver nitrate or perchlorate, but again the Stoichiometry of the complexes varies considerably, sometimes depending on the conditions of preparation. The following types have been isolated [Ag(un)2]X (un = e.g., cyclohexene, a- and /3-pinene) ISO), [Ag(diene)]X diene = e.g., dicyclopentadiene 220), cyclo-octa-1,5-diene 50, 130), bi-cyclop, 2,1 ]hepta-2,5-diene 207), and cyclo-octa-1,3,5-triene 52), and [Ag2(diene)]X2 (diene = e.g., cyclo-octa-1,3- and -1,4-diene 180), bi-cyclo[2,2,l]hepta-2,5-diene 1) and tricyclo[4,2,2,0]-decatriene 10)). Cyclo-octatetraene (cot) forms three adducts with silver nitrate 52), viz., [Ag(cot)]NOs, [Ag(cot)2]N03, and [Ag3(cot)2](N03)3. On heating, the first two lose cyclo-octatetraene and all three decompose at the same temperature. From the stoichiometry of the above complexes it appears that the... 

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). 

II(S)) and/or to a different reaction rate of the two diastereomeric 7r-olefin complexes to the corresponding diastereomeric alkyl-rhodium complexes (VI(s) and VI(R)). For diastereomeric cis- or trans-[a-methylbenzyl]-[vinyl olefin] -dichloroplatinum( II) complexes, the diastereomeric equilibrium is very rapidly achieved in the presence of an excess of olefin even at room temperature (40). Therefore, it seems probable that asymmetric induction in 7r-olefin complexes formation (I — II) cannot play a relevant role in determining the optical purity of the reaction products. On the other hand, both the free energy difference between the two 7r-olefin complexes (AG°II(S) — AG°n(R) = AG°) and the difference between the two free energies of activation for the isomerization of 7r-com-plexes II(S) and II(R) to the corresponding alkyl-rhodium complexes VI(s) and VI(R) (AG II(R) — AG n(S) = AAG ) can control the overall difference in activation energy for the formation of the diastereomeric rhodium-alkyl complexes and hence the sign and extent of asymmetric induction. 

The model can be identified with the diastereoisomeric 7r-olefin complexes, and thus AG° should, at least qualitatively, control the sign of the asymmetric induction. In this case, if AGhi(S) > AG ii(R), AG° must be larger than AAG. If AG°nasymmetric induction will correspond... 

Silver has three synthetically useful oxidation states Ag(I), Ag(II), and Ag(III).16 Free Ag(III) is very unstable, and most currently known Ag(III) complexes are stablized with electron-donating and/or sterically demanding ligands.17 It is known that Ag(I) can be oxidized to Ag(II) with strong oxidants such as persulfates. Nitrogen-based ligands such as pyridines are commonly used to stabilize high-valence metal ions.18 In 2003, He and coworkers utilized a pyridine-supported silver catalyst and reported the first silver-catalyzed aziridination of olefins.19... 

Abstract The manuscript describes the methods that are most often used in the preparation of N-heterocyclic carbene (NHC) complexes. These methods include (1) insertion of a metal into the C = C bond of bis(imidazolidin-2-ylidene) olefins (2) use of carbene adducts or protected forms of free NHC carbenes (3) use of preformed, isolated free carbenes (4) deprotonation of an azolium salt with a base (5) transmetallation from an Ag-NHC complex prepared from direct reaction of an imidazolium precursor and Ag20 and (6) oxidative addition via activation of the C2 - X (X = Me, halogen, H) of an imidazolium cation. 

Silver olefin compounds have been studied extensively for over 30 years. The majority of the compounds are stable only in solution and many have been characterized via stability constant studies. Solid relatively unstable salts have been prepared having the stoichiometry Ag ,(olefin)nX , where X is usually the nitrate, tetrafluoroborate, or perchlorate anion, m = 1,2, and n = 1,2, 3.1 Recently, relatively stable, nonionic, monomeric complexes of the type y diketonato(olefin)silver(I) have been characterized.2... 

Olefin Complex Configuration No. ofC=C per Ag+ Shortest silver contacts Ag-C Ag-0 > Ref. 

Until about 1957, only metals toward the end of the transition series, such as Pd, Pt, Cu, Ag, and Hg, were known to form mono-olefin complexes. In 1959 Schrauzer 219, 220) prepared the first olefin complex of nickel, starting with nickel carbonyl and acrylonitrile ... 

For the reader desiring more information on mono-olefin complexes of the transition metals, a number of excellent reviews are available. Early studies on olefin-metal complexes (primarily of Pt, Pd, Ag, and Hg) have been reviewed by Keller Chatt 27), and Douglas 70). Two more... 

This somewhat incomplete picture can be coordinated with known facts of homogeneous catalysis. We may say that the monoolefin complexes of Ni are too unstable to be reactive, while the simple analogues of Pt are too stable optimum reactivity therefore resides in complexes of Pd and Rh where these opposing effects balance properly. We must defer for the present a discussion of the effects which other coordinated groups e.g., PR3, GO, SnGl.r) have on the reactivity of coordinated olefins since much more systematic information is needed for this to be feasible. The limited catalytic activity of olefin complexes e.g., Gu Ag ) is presumably caused by the absence from these complexes of any vacant orbitals at which attack by another reactant can occur. 

Olefin Olefin Ag+ in complex Anion Method — AH (kcal. mole i) Kef. 

A series of five-coordinate, dicationic Rh(ll) and lr(ll) olefin complexes stabilized by N-donor ligands has been investigated in detail. Species of the type [lr°(iV4-ligand)(efliene)] + (162,179,180) [Rh (N3-ligand)(nbd)] + (181) (nbd = norbomadiene) and [M (iV3-ligand)(cod)] (182,183) have been obtained from their M(l) precmsors by one-electron oxidation using either [Fc] or Ag as an oxidant (Fig. 62). 

A non-dependence of the thermodynamic equilibrium constant on the solvent for two different types of diols was found 34>, which indicated that Ag+ as well as undissociated AgN03 formed complexes with olefins, comparable with mercury salt-olefin complexes 35>. Further formation constant investigations 36> by gas chromatography of silver complexes of cyclo-olefins had shown that methyl substitution at the double bond markedly reduced the stability and... 

Dewar s theory is still the most acceptable explanation of the bonding in silver-olefin complexes the relative contributions of the component M -ol (a o-type bond) and M- ol (a jr-bond) co-ordinate bonds is, however, uncertain. Spectroscopic evidence tends to suggest that the o-bonding is more important than the 7r-bonding. However the chemical shifts in H n.m.r. studies may be explained by either polarization effects or oH-M charge transfer (o-bond formation). Transfer of 0.1 to 0.2 electrons could account for the entire chemical shifts found but, since polarization effects cannot be calculated accurately, conclusions on the importance of the o-bonding can be only tentative I28>. As-symetric substitution at the double bond results in a non-symmetric orientation of the Ag+ ion (from H n.m.r.) but the shifts found could result from steric as well as electronic effects 127>-... 

Substituent groups on the olefin with a +1 effect (e.g. Me) will tend to strengthen the M- -ol o-bond and weaken the 7r-bond while -I substituents should have the opposite effect. The tendency for the thermodynamic stability of Ag+—ol complexes to increase as the +1 nature of substituents increases supports the claim that o-bonding is more important than ir-back-bonding. However, there is also some evidence that the jr-acceptor properties of a ligand are more important than its o-donor properties 75). 

Copper(I)-, Ag(I)- and Au(I)-olefin complexes may be prepared either by dissolving the metal salt in the liquid olefin or by passing a stream of gaseous olefin through a solution of the metal salt. In this way olefin complexes of Ag(I) salts with nitrate , perchlorate , tetrafluoroborate, hexafluoroacetylacetonate , jS-diketonate and trifluoromethylsulfonate anions have been prepared. Similarly Cu(I) complexes with trifluoroacetate and trifluoromethylsulfonate anions have been prepared. These ry -olefin complexes have varying stabilities and compositions. The Cu(I) complexes disproportionate to Cu(0) and Cu(II). The most stable complexes of both Cu(I) and Ag(I) are those with the trifluoromethylsulfonate anions . The [Cu(olefin)][CF3S03] complexes are prepared by a two-step method ... 

The > -olefin complexes of Cii(I), Ag(I) and Au(I) dissociate in solution. The loss of the olefinic ligand can be prevented by incorporating it into a multidentate phosphine or arsine ligand . Thus o-CH2=CHCH2CgH4PPh2 reacts with Cu(I) halides to form a 1 1 complex ... 

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