Stability of Olefin Complexes

Stability constants for many olefin complexes are currently known/ A variety of experimental methods are used to determine these constants potentiometry, polarography, thermometric titration, and others/ 

Generally, the n olefin orbital energy is more sensitive to the influence of substituents than the n orbital energy (Table 6.9). Therefore, the dependence of the equilibrium constant on the lowest n unoccupied orbitals is often linear. This type of relationship is observed for the following reaction 

These considerations do not involve Ag(I) and Cu(I) complexes with olefins containing both electron-accepting and electron-donating substituents such complexes are less stable compared to the ethylene complex (Table 6.11). 

The stability of Ag(I) complexes with cyclic olefins (Table 6.11) decreases according to the series C5 C7 C6 C8, which is caused almost exclusively by enthalpy changes the entropy changes are almost the same for all olefins. The stability of Ag(I) coordination compounds with diolefins of the type CH2CH(CH2) CHCH2 is maximum for = 2. Therefore, the chain containing six carbon atoms is probably optimal for the formation of a chelate with silver. 

The metal-olefin bond energy data are scarce. Such energies for group 8 transition metals are usually about 20-30% lower than bond energies of terminal carbonyl groups. The metal-olefin bond energies for some iron, nickel, and rhodium complexes are given in Table 6.12. 

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

At this point we seem to have come a long way from the chemistry of Joe Chatt. Is there really a connection between the stability of olefin complexes of platinum(ii) and the semi-conducting properties of GaAs In this short review I have tried to show that there is a direct relationship between the two. The class... 

The relative stabilities of olefin complexes of rhodium(I) have been compared 94> by following the displacement of ethylene from the thermodynamically stable 2,4-pentanedionatobis(ethylene)rhodium(I), (acac)Rh(C2H4)2, using i.r. spectrophotometry ... 

The stability of olefin complexes is also sensitive to steric effects. The binding of ethylene is stronger than that of a-olefins in nearly all cases. However, the magnitude of the steric effect on the binding of an olefin depends on the other ligands. For example, the ethylene complex of bis(amine)PdMe in Equation 2.11b is 10-fold more stable than the propylene or hexene complex, while flie ethylene complex of PtClj" is ortiy two-fold more stable than the corresponding propene complex. ... 

Compounds Containing Two-Electron n-Ligands 7. STABILITY OF OLEFIN COMPLEXES... 

Olefins, e.g., ethylene or styrene, do not displace the hydroxyacetylene from these complexes. The stability of the complexes depends on the group R. of the acetylene RMeC(OH)-C. C-C(OH)Me2 it decreases in the order R = methyl > phenyl > hydrogen. Methylation of the hydroxyl groups also lowers the stability of the complexes 36). 

Substituted olefins also complex with CuCl. The halide dissolves in allyl alcohol owing to complex formation 183, 184). Other unsaturated alcohols give complexes of the type (alkenol)CuCl and [(alkenol)Cu] + when added to solutions of CuCl in aqueous HCl 185). Similar complexes are formed with various unsaturated carboxylic acids, and the stability of the complexes is influenced by the stereochemistry about the double bond 13). Maleic acid, however, forms the anionic complex [CuCl(02C -CH=CH.C02H)] in addition to the neutral and cationic complexes 11, 12). 

These studies have indicated that (i) there is usually only one olefin molecule coordinated at each silver ion (621), (ii) alkyl substitution at the double bond decreases the stability of the complex (288, 416, 621), (in) with endo-cycloolefins, the stability constant increases with increasing ring strain (578), (iv) more stable complexes are formed with cis-than with diene complexes are formed by the 1,5-diene systems (416), and (vi) deuteration of an olefin increases the complex stability (154). 

Wakatsuki and co-workers 287) have prepared vinyl ether complexes of Pd(II) and Pt(II) of composition [CI2ML] (M = Pd or Pt L = vinyl ethers, propenyl ethers, 1,2-dimethoxyethylene) by displacement of PhCN from Pd(PhCN)2Cl2- They are fairly stable at 25°C. Since these ethers would be good a donors but poor tt acceptors, the stability of these complexes suggest that a donation is most important with tt complexes of Pd(II) and Pt(II). This conclusion is in agreement with earlier IR studies, as well as a recent NMR study of Pt olefin and acetylene complexes 35). Furthermore, a study of complex formation... 

A decisive role may be played by the strength of the bond between olefin and the catalyst (4,47,57). This view is indirectly supported by the degree of stability of organometallic complexes of transition metals with olefinic ligands, which decreases with increasing isomerization ability of the given metal. It may be assumed that the relative adsorptivity of an olefin on the surface of various metals corresponds to the stability of compounds formed by the olefin with salts of the respective metals (62-64). 

Whilst the synthesis of new transition metal-olefin and -acetylene complexes continues unabated, only a relatively small amount of data has accumulated on the thermodynamic stability of these complexes and these are restricted almost exclusively to complexes of the unsatured species acting as monodentate ligands. Metals able to coordinate strongly with unsaturated ligands are restricted to those in a small triangle around the centre of the periodic table, and designated class (b) acceptors by Ahrland et al., 0>. Class (b) acceptors include Cu(I), Rh(II), Ag(I), Pt(II) and Hg(II). However the majority of such metals form inert complexes which are either very readily oxidised or involve solubility problems. If thermodynamic stability constants are to be measured reliably, the equilibrium should be reached reasonably quickly, the reaction should be clean and the stoichiometry should be known or easily deduced. Furthermore, the equilibrium must be followed by means of suitable electrodes or changes in some physical property of the reaction mixture. The solvent is therefore important. 

Three main factors were assumed to be responsible for the variation in stability of silver ion-olefin and -acetylene complexes, namely electronic, steric and structural strain effects but their relative importance was not known, although it is virtually impossible to separate them. Increasing the electron density at the double bond, by exchanging a hydrogen for a methyl group, should increase the stability of the complex, if the olefin is regarded as an electron donor. However, as the opposite is found, the steric influence of substituents would appear to be very important or the ir-bond makes a significant contribution. 

It has recently been shown 90> that the olefin tetracyanoethylene, (CN)2C= C(CN)2, can displace phenylacetylene from the bis-triphenylacetylene from the bis-triphenylphosphine platinum(O) complex, the first time such a reaction has been achieved. Preparative work on olefin complexes of platinum(0) of the same type has also shown that electron-withdrawing substituents on the olefin increase the stability of the complex, e.g. trans-4,4-dinitrostilbene forms a more stable complex than trans-stilbene itself 86L... 

A correlation of a high thermal stability with a high ionization potential for a olefin has been observed with iron (O)-olefin complexes of the type Fe(CO)4-(olefin) 96> i.e. poor donor but good acceptor properties increase the stability of the complex. The acrylonitrile complex is one of the most stable, the ethylene complex the least stable and the complexes of styrene or vinyl chloride are of intermediate stability. 

The photochemistry of Fe(CO)s with dienes or olefins in polytetrafluoro-ethylene has been studied. I.r. evidence for (diene)Fe(CO)3, (dieneljFeCO, and for (ethylene)Fe(CO)4 is presented, and it has been found that the stability of these complexes towards air oxidation is much higher in the polymer matrix than in solution. 

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