Elimination of ligands

The removal of the oxygens of surface Cr(Vl) may be classified as a reductive elimination of ligands. If from the beginning the metal is applied in a low valence state the formation of the c.u.s. requires just the (thermal) removal of the outer ligands from the anchored surfaee eompound  

Fixation and ligand desorption usually are not separated processes, but are charaeter-ized by an overlap that can be studied by analysing the exhaust of a TPD experiment. In Fig. 18.2 this is shown for FeCl2. The reaction can be earned out in an inert, i.e. oxygen-free, gas stream (Ar) or under high vacuum conditions. Cr and Mo as well may be also applied to the support in low valence state as Cr(III) [42-47] or Cr(II)/Mo(II) [23]. 

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Among the many reactions of organometallic compounds, ones involving insertion and elimination of ligands are important in applications to synthesis and catalysis. An example of a carbonyl insertion is ... 

Oxidative addition is a reaction where the metal undergoes formal oxidation and atoms, groups of atoms, or molecules are added to the metal center. Reductive elimination is the exact opposite of oxidative addition—the metal ion is formally reduced with elimination of ligands. A few examples are shown in Fig. 2.6. In all these examples the forward reactions are oxidative addition... 

The oxidative addition and its reverse, reductive elimination, of ligands are two of the most fundamental steps in reactions occurring at metal centers in organometallic and inorganic complexes and are extremely important in organic synthesis and catalysis [3, 4, 10, 11]. In typical oxidative addition reactions... 

Schene 3-6. Ihe stoichiometry of the product of a redox condensation reaction may greatly differ from the one expected for a simple condensation by elimination of ligands. 

Although thermal induced elimination of ligands is a very convenient method to perform in the laboratory, it is very difficult to control the extent of the elimination. Such reactions are therefore rarely specific leading to the formation of a variety of products whose distribution normally depends on their relative stabilities. Reaction conditions are rather critical, hence for obtaining good yields of a given product they must be carefully selected ... 

The synthetic accessibility of ligands designed by de novo methods. This topic typically is not considered. Some compounds with unstable or reactive function-ahties are eliminated by applying simple rules, but these rules are still insufficient. 

Another method for the hydrogenoiysis of aryl bromides and iodides is to use MeONa[696], The removal of chlorine and bromine from benzene rings is possible with MeOH under basic conditions by use of dippp as a ligand[697]. The reduction is explained by the formation of the phenylpalladium methoxide 812, which undergoes elimination of /i-hydrogen to form benzene, and MeOH is oxidized to formaldehyde. Based on this mechanistic consideration, reaction of alcohols with aryl halides has another application. For example, cyclohex-anol (813) is oxidized smoothly to cyclohexanone with bromobenzene under basic conditions[698]. 

It is well known, that in aqueous solutions the water molecules, which are in the inner coordination sphere of the complex, quench the lanthanide (Ln) luminescence in result of vibrations of the OH-groups (OH-oscillators). The use of D O instead of H O, the freezing of solution as well as the introduction of a second ligand to obtain a mixed-ligand complex leads to either partial or complete elimination of the H O influence. The same effect may be achieved by water molecules replacement from the inner and outer coordination sphere at the addition of organic solvents or when the molecule of Ln complex is introduced into the micelle of the surfactant. 

For elimination of intramolecular energy losses, we have synthesized ligands with high hydrophobisity - perfluoro-P-diketones R -CO-CH -CO-R, (R = CgF j or CgF R = phenyl or a-thienyl), that without second ligand eliminate completely water molecules from the inner coordination sphere. These ligands we have used in analysis at determination of Sm, Eu, Nd, Yb microamounts in high-purity lanthanide and yttrium oxides. 

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