Effect of Coordination

The first reported instance of stereoselective polymerization was probably the cationic polymerization of isobutyl vinyl ether in 1947 [Schildknecht et al., 1947]. A semicrystalline polymer was obtained when the reaction was carried out at —80 to —60°C using boron tri-fluoride etherate as the initiator with propane as the solvent. The full significance of the polymerization was not realized at the time as the crystallinity was attributed to a syndiotactic structure. X-Ray diffraction in 1956 indicated that the polymer was isotactic [Natta et al., 1956a,b], (NMR would have easily detected the isotactic structure, but NMR was not a routine tool in 1947.) 

Isobutyl vinyl ether BF3 etherate in propane at —80 to —60°C Isotactic 

Methyl methacrylate )MgBr (3(TC) or n-C4H9Li ( 78 to 0°C) in tjtCHs Isotactic 

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The base peak in the mass spectrum of the LM free metal-ligand ion and the fragmentation patterns of this parent ion are of particuliar significance since they illustrate the effect of coordination upon the properties of the thiazole ligand. The free thiazole fragments upon electron impact by two major routes (Scheme 86 also cf. Section II. 6). 

Metal-Induced Cycloadditions. The effect of coordination on the metal-iaduced cyclo additions of maleic anhydride and the isostmctural heterocycles furan, pyrrole, and thiophene has been investigated (47). Each heterocycle is bound to an Os(II) center in the complex... 

An interesting approach to studies of the effects of coordination on the reactivity of lithium polydienes in hydrocarbon solvents was developed by Erussalimski and his colleagues 151 154 The polymerization of lithium polyisoprene in hexane is accelerated by the addition of TMEDA152), the rate levels off at a value of R = [TMEDA]/[li-thium polyisoprene] of 8, its final value giving the absolute rate constant of propagation of the polyisoprene coordinated with TMEDA, namely 0.17 M7l s at 20 °C. 

The colours of these compounds are associated with charge-transfer transitions involving the dye moiety. Thus, when the steric and/or electronic effects of coordination cause variation in the energy of these transitions, then the induced spectral changes may be used to monitor complex formation. 

Wehrli, B. and W. Stumm, 1988, Oxygenation of Vanadyl(IV). Effect of coordinated surface hydroxyl groups and OH. Langmuir 4,753-758. 

The effect of coordination may be illustrated by the vibrational properties of the four modifications of MgSi03 [16]. Above 1000 K, the entropy of these modifications of MgSi03 decreases in the order... 

Anions can promote hydrolysis of complex cations by producing ion pairs of enhanced reactivity (see 2.178). Usually however, ligands accelerate the removal of a coordinated ligand by entering the metal coordination sphere with it and thereby labilizing it towards hydrolysis. We have already seen the effect of coordinated OH on the enhanced labilities of Fe(III) and Cr(III). Dissociative mechanisms and considerable acceleration are promoted by CFlj, CN , SOj and other groups on inert Cr(III), Co(III) and Pt(IV) complexes. Nitrate ions, for example, reduce the half-life for replacement of water in Cr(H20)g by dmso from —380 h to 10 s ... 

The slope (p = 1.93) is slightly smaller than that for uncoordinated nitriles (p = 2.31). Since C nmr shows the effects of coordination on the nitrile carbon chemical shift are independent of substitutents, the observed difference in p must be an effect associated with the activated complex. 

Figure 2. Effect of coordination geometry on V XANES spectrum V naphthenate (----), (---). ...

The dimethylaminomethyl group (entry 9) is easily introduced by a Mannich reaction, and lithiation occurs readily at -78°C (88JOC5685). After reaction with a variety of electrophiles, hydrolysis can be performed directly with aqueous acid to give 2-substituted imidazoles in good yield. However, the 2-lithio anion 47 was found to be quite basic, despite the base-weakening effect of coordination with the amino substituent, and thus it was capable of deprotonating the 2-butyl derivative 48 as it was formed by reaction with 1-bromobutane (Scheme 42). No such side-reac-... 

Fig. 13. Calculated effect of coordination number on the ion selectivity of electrically neutral model ligands...

The current chapter focuses on the electrochemistry of the ionic forms of copper in solution, starting with the potentials of various copper species. This includes the effect of coordination geometry, donor atoms, and solvent upon the electrochemical potentials of copper redox couples, specifically Cu(II/I). This is followed by a discussion of the various types of coupled chemical reactions that may contribute to the observed Cu(II/I) electrochemical behavior and the characteristics that may be used to distinguish the presence of each of these mechanisms. The chapter concludes with brief discussions of the electrochemical properties of copper proteins, unidentate and binuclear complexes. 

Dale Margerum Ralph Wilkins has mentioned the interesting effect of terpyridine on the subsequent substitution reaction of the nickel complex. I would like to discuss this point—namely the effect of coordination of other ligands on the rate of substitution of the remaining coordinated water. However, before proceeding we should first focus attention on the main point of this paper-which is that a tremendous amount of kinetic data for the rate of formation of all kinds of metal complexes can be correlated with the rate of water substitution of the simple aquo metal ion. This also means that dissociation rate constants of metal complexes can be predicted from the stability constants of the complexes and the rate constant of water exchange. The data from the paper are so convincing that we can proceed to other points of discussion. 

Because of the small concentration of the 2 1 complex the last term can be ignored. From the extreme rate values in the absence of zinc and with an excess of zinc, 2i and 22 are determined as 2.4 X 104 min.-1 and 1.57 min.-1 respectively. These values can be combined with the trend in the rate constants to give the stability constant of the reactive complex, presumably Zn(OR)(OAc), as 3 X 107. For the simple zinc complex in water the literature values of the stability constant for the 1 1 complex vary from 2.5 X 108 to 6.3 X 108. The diazo coupling reaction of the complex indicates the smaller effect of coordination vis a vis protonation since this reaction is very sensitive to such effects and does not proceed with phenols. Unfortunately the choice of cations for such a reaction is restricted since the cation should not interfere with the analytical methods used to obtain the kinetic data nor should it introduce additional reactions such as occur with transition metal cations which can catalyze the decomposition of the diazonium salt via a redox process. 

The rate of ethane formation was used to obtain kinetic data on this reaction. The effect of coordination on the reaction is primarily to vary the substrate and perhaps retard the reaction somewhat by a slight depletion of electron density from the ortho position. 

What about the effect of coordination number on rate The iron-sulfur cluster core dendrimers above are four-armed dendrimers. We compared these with... 

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