Cation-insertion process

In contact with aqueous alkaline media, metal oxide electrochemistry is dominated by hydroxylation processes. However, in contact with acidic media, proton and cation insertion processes occur, eventually leading to complicated responses where reductive or oxidative dissolution processes frequently take place (Scholz and Meyer, 1998 Grygar et al., 2002 Scholz et al., 2005). As far as such processes involve disintegration of the porous structure of the material, electrochemically assisted dissolution processes will be taken only tangentially here. 

During electrochemical reduction (charge) of the carbon host, lithium cations from the electrolyte penetrate into the carbon and form a lithiated carbon Li rCn. The corresponding negative charges are accepted by the carbon host lattice. As for any other electrochemical insertion process, the prerequisite for the formation of lithiated carbons is a host material that exhibits mixed (electronic and ionic) conductance. 

Since group 3 metallocene alkyls are isoelectronic with the cationic alkyls of group 4 catalysts they may be used as olefin polymerization initiators without the need for cocatalysts. The neutral metal center typically results in much lower activities, and detailed mechanistic studies on the insertion process have therefore proved possible.216-220 Among the first group 3 catalysts reported to show moderate activities (42 gmmol-1 h-1bar-1) was the yttrocene complex (77).221... 

Insertion of CO is therefore always kinetically controlled. When an alkyl palladium species has formed, the open site will be occupied by a coordinating CO molecule. Carbon monoxide coordinates more strongly to palladium than ethene, even when the palladium centre is cationic. The reason for this is steric the cone angle of ethene is much larger than that of CO and the steric hindrance in the ethene complex is therefore much larger. If the barriers of activation for the insertion processes of ethene and CO are of the same order of... 

Otherwise, the reactions of indenyl-ruthenium(II) allenylidenes [RuCty -CgHy) =C=C=C(R )Ph (PPh3)2][PF6] (R = H, Ph) with ynamines R C CNEtj (R = Me, SiMea) have been reported to yield the alkenyl(amino)allenylidene complexes 41 via insertion of the ynamine into the Cp=Cy allenylidene bond (Scheme 10) [52, 53], This insertion process involves an initial nucleophilic addition of the ynamine at Cy atom of the cumulene, which leads to the cationic alkynyl intermediate complexes 39. Further ring closing, involving the Cp atom, generates the [2+2]... 

The mechanism for the stereoselective polymerization of a-olefins and other nonpolar alkenes is a Ti-complexation of monomer and transition metal (utilizing the latter s if-orbitals) followed by a four-center anionic coordination insertion process in which monomer is inserted into a metal-carbon bond as described in Fig. 8-10. Support for the initial Tt-com-plexation has come from ESR, NMR, and IR studies [Burfield, 1984], The insertion reaction has both cationic and anionic features. There is a concerted nucleophilic attack by the incipient carbanion polymer chain end on the a-carbon of the double bond together with an electrophilic attack by the cationic counterion on the alkene Ti-electrons. 

The electrochemical insertion process includes - charge transfer (red-ox reaction) by electrons and ions and -> diffusion of ions in the host to theirs sites. The inserted species can be cations (upon cathodic polarization), anions (upon anodic polarization), atoms, and molecules. Insertion of ions can be accompanied by co-insertion of solvent molecules from the -> solvation shell of the ions, which are dragged to the host together with the ions as the electrical field is applied. Insertion electrodes are highly important for the filed of - Li batteries, as most of the electrodes used in Li/Li-ion batteries are in fact insertion electrodes. Thereby, this entry concentrates mainly on Li ion insertion electrodes. 

Alternative approaches exist to explaine the mechanism of chemical interaction. It is accepted that water is formed as a result of mechanochemical interaction, solid reagents are dissolved in water, and the reaction proceeds via the dissolved state (hydrothermal-like process). On the other hand, it is assumed that the initial stages of the process involve the interaction between acid and base centers present at the surfaces in contact, and the next stages are connected with the process of calcium cations insertion into aluminum hydroxide lattice. 

Redox conductivity of conducting polymers involves anion insertion associated to oxidation processes (p-type doping) and cation insertion coupled with reduction ones (n-type doping). Efficient polymer-based electronic devices require lowering of the redox potential of the involved electron transfer processes, and electrochemical reversibility is necessary to allow repetitive charge/discharge processes. In the... 

A typical example of anodic ECMs is iridium oxide which may be rapidly and reversibly coloured in aqueous solutions containing sulphuric acid. The exact type and mechanism of the ion insertion process in this oxide are still uncertain [17]. In aqueous H2SO4, the most commonly used electrolyte, two mechanisms have been proposed, i.e. a cation mechanism involving coloration via proton extraction ... 

The only isolated metal-hydroxo complex that has been shown to react with olefins to form products from transfer of hydroxide is Cp Ir(PMe3)(Ph)(OH). However, this formal insertion process does not occur by a migratory insertion mechanism. Instead, the reaction with olefin is catalyzed by trace amounts of Cp Ir(PMe3)(Ph)(OTf) and appears to involve replacement of tiiflate with ethylene to generate the cationic [Cp Ir(PMe3)(Ph)(CjH )], which undergoes attack by the separate iridium hydroxido complex, as shown in Scheme 9.11. 

Entropy is the major force for membrane formation. If enough elements of a membrane itself were incorporated in a model cation-channel-former, the normally feeble entropic force might assist the assembly and/or insertion process. If so, then the compound illustrated schematically could be reduced by two covalent linkages to that shown below. It, in turn, could exist in a fully extended conformation as shown. 

---