Dissociation and substitution

If one end of a chelate ring on an octahedral complex is detached from the metal, the five-coordinate transition state can be considered as a fluxional molecule in which there is some interchange of positions. When the chelate ring reforms, it may be with a different orientation that could lead to racemization. If the chelate ring is not symmetrical (such as 1,2-diaminopropane rather than ethyl-enediamine), isomerization may also result. For reactions carried out in solvents that coordinate well, a solvent molecule may attach to the metal where one end of the chelating agent vacated. Reactions of this type are similar to those in which dissociation and substitution occur. 

Alternatively, bi- or multidentate ligands can also be used for support. As an additional benefit, the latter offer greater stability for the coordinatively bound metal center against leaching through ligand dissociation and substitution reactions. The first, somewhat remarkable, approach to this is shown in Figure 42.11, based on numerous examples of the support of bidentate phosphines on polymers [1-5]. 

I propose to develop and apply such methods, based on ultrafast X-ray absorption spectroscopy, to study the ultrafast molecular motions of organometallics in solutions. In particular, initial studies will focus on photo-induced ligand dissociation and substitution reactions of transition metal carbonyls and related compounds in various solvent systems. 

Nickel(II) complexes with cryptands are still rare. In general the encapsulation of nickel(II) in this type of macrocyclic ligand makes the complexes extraordinarily resistant to dissociation and substitution reactions. 

For a localized adsorption the concentration of the adsorption sites has to be taken into account [4], In addition, a reversible change of the chemical state of the adsorbate in the chromatography process has to be considered, e.g. dissociative and substitutive adsorptions as described in Part II of this chapter (Part n, Section 2.3, Equations 52-54). The reaction enthalpy and entropy has to be introduced into the calculations [5-8] as well as the... 

Ligand association, dissociation, and substitution processes are facile, so the exact number of ligands on a metal center is usually not a major concern when mechanisms and catalytic cycles involving transition metals are drawn. 

Kinetic studies of H2 dissociation and substitution rates show that the potential energy surfaces for these reactions vary dramatically even with minor changes in ancillary ligands or for isomers (Table 7.7).5,69 Electronic effects, especially the influence of the trans ligand, appear to be more important than steric factors. For the Ir system, the ds isomer with H2 trans to Cl has a strongly bound H2 (dHH = 1.11 A) while the trans isomer with H2 trans to H contains a weakly bound H2 that dissociates nearly 10s times faster (see also Section 4.7.1). One of the few comprehensive quantitative studies of H2 substitution reactions shows displacement of H2 by L (MeCN, PhCN, ) fromCMHCHjXP) (M = Fe, Ru, Os) is first-order in concentration of complex and zero order in L, Le., a dissodative mechanism.70... 

Mechanisms and Rates of Lead-Chelate Dissociation and Substitution Reactions... 

More recent mechanistic studies have been able to distinguish between pathways (b) and (c), and all results indicate that (c) is operative [87]. The initial ligand dissociation and substitution steps have been studied using NMR magnetization transfer experiments, NMR and UV-vis kinetics, and mass spectrometry [87,88]. These investigations indicate that both phosphine/phosphine (Fig. 4.31a) and phosphine/olehn (Fig. 4.31b) substitution reactions in (L)(PR3)(X)2Ru= CHR complexes proceed by a dissociative mechanism involving a 14-electron intermediate (L)(X)2Ru=CHR (A). Although this proposed intermediate has not been observed in solution, presumably due to its low concentration, it has been identihed in the gas phase [88]. 

Reactions involving the gain or loss of a ligand These reactions deal with the addition or subtraction of a ligand to or from the metal center and include ligand dissociation and substitution, oxidative addition, reductive elimination, and nucleophilic displacement. 

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