Silver complexes olefin

None of the theories proposed before 1951 to explain the nature of the bonding in metal-olefin complexes was entirely satisfactory (35). Chatt (S3) suggested that, in addition to the ordinary coordinate bond, some sort of bond involving the filled d-orbitals of the metal atom was essential for coordination of the olefin, but such a bond was difficult to formulate until Dewar (64) described it in terms of molecular orbitals. The structure which he proposed for the silver-olefin complexes, and that subsequently proposed for the platinum-olefin complexes by Chatt and Duncanson (35) are shown schematically in structures (I) and (II). The type bond, which has also been called a ji-bond (64, 4), is formed by the overlap of the filled bonding... 

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). 

Silver ions react readily with olefins, forming a silver-olefin complex according to the reaction ... 

The olefin fraction of the feed gas crosses the membrane and reacts reversibly with silver ions to form a soluble silver-olefin complex... 

Electrolysis at copper electrodes of cupric perchlorate with 1,5-cyclooctadiene in methanol yields well-defined crystals of an air-stable cuprous complex (CgHi2)2CuC104 (390). The C=C stretching band at 1660 cm in the diene is replaced by two bands at 1638 and 1595 cm" in the complex, suggesting nonequivalent coordination of the double bonds. As with the silver-olefin complexes (492) there is a dependence... 

BFr< SbFg. This order parallels that observed for olefin solubility in concentrated silver salt solutions (40, 193). Structural investigations of crystalline silver-olefin complexes have shown a nearly covalent bond between the silver and the nitrate ions (28, 399), but an electrostatic bond only between silver and fluoroborate ions (537). Consequently, the differing complex stability may be largely attributable to the differences in the energy required for the expansion which permits incorporation of the olefin molecule into the salt lattice. These differences will depend upon the anion composition (537), geometry, and size. Similarly, the degree of silver ion-anion association in concentrated solutions will vary with the anion and a similar explanation can account for the dependence of olefin solubility on the anion. In dilute solutions, however, the silver ion environment and thus the olefin solubility may be essentially independent of the anion (193). 

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... 

Table 8. Thermodynamic data for silver-olefin complex formation MX(cryst) + n-olefin(gJ [M(olefin)nX] (cryst)...

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>-... 

As in silver-olefin complexes, charge transfer into anti-bonding ligand orbitals should result in a lengthening of the C=C bond and a decrease in vC C. 

Poly acrylonitrile/silver nanoparticles were synthesized by Zhang et al. [2001] by the UV irradiation technique for polymerization and reduction of Ag ions of AgNOs, simultaneously. Here the silver nitrate directly dissolves in acrylonitrile monomer, form silver-olefin complex. 

Lucas and Winstein studied silver-olefin complexes [99]. 

In the infrared spectrum of this complex obviously no silver atom-ligand frequency can be assigned. This is in accord with the observation that in silver-olefin complexes such a frequency cannot usually be attributed to a particular band. 

Despite thdr lability, many of the silver-olefin complexes may be isolated as thdr nitrate or perchlorate salts and, in most cases, the ratio of co-ordinated C=C groups to the metal atom is 1 1. However, the stoicheiometry of the isolated olefin complexes does not necessarily indicate the nature of the species in solution. For example, in solution norbomadiene forms a 1 1 complex with Ag(I), whilst the solids isolated from the solution may have the stoicheiometry C7H8(AgNOj)2 [30] or C7H AgNOj [53,30]. 

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