Ligand dissociation reactions

The rhodium dimer has two Lewis acidic sites and thus the catalyst could coordinate to two substrate molecules under saturation kinetics, which would make the Michaelis-Menten plots complicated. This does not happen and the second site becomes less acidic once the other site is occupied by the substrate. What does happen, though, is that other Lewis bases compete with the substrate, as might be expected. The ligand dissociation reaction may be part of the rate equation of the process. Coordination of one Lewis base reduces already the activity of the catalyst. The solvent of choice is often anhydrous dichloromethane. The polar group may also be part of one of the substrates and in this instance one cannot avoid inhibition. 

This has been followed by a similar study of the heat and kinetics of thermal decomposition of the complexes (NH4)2[U02L2X], where HL = cupferron and X = CO2- or C204-.115 The ligand dissociation reactions were found to be of zero order in these latter cases, compared with first in the former cases with monovalent chloro or fluoro substituents. 

Figure 5-25. A ligand dissociation reaction may allow small amounts of free amine to be present in solution. This free amine may then react with electrophiles.

In conclusion, whereas Co(NH3)63+ is useless because it undergoes a fast photodecomposition reaction, the analogous Co(sep)3+ complex may be employed as an electron transfer photosensitizer because of its intrinsic stability in the excited state and in the reduced form. In the same way, one can think to use cage-type polypyridine ligands for Ru complexes, so as to prevent ligand dissociation reactions. 

We will first consider ligand dissociation reactions. When coupled with addition reactions, dissociation reactions can be useful synthetically, providing an avenue to replace ligands such as carbon monoxide and phosphines by other ligands. 

The equilibrium constants for the ligand dissociation reaction NiL4 - NiL3 + L... 

Figure 20. Feixoin immobilization rates versus metal substitution rates. Immobilization kinetics were measured by competition with ligand dissociation reactions, induced by the addition of excess Ni (24 equiv) to sol—gel materials containing silylated ferroin complexes during the immobilization step. Treatment of the data according to a rate law based on competing parallel reactions afforded first-order immobilization rate constants in the range 3 x X 10- ...

When sufficient thermal energy is supplied to dissociate at least one metal-nitrogen chemical bond in tire glassy complex Mo(C=0)4(CH3NH2)2, this coordi-natively saturated crosslinked structure reverts to a 5-coordinate complex above the glass-transition temperatrrre. The ligand dissociation reaction is... 

The following generic ligand dissociation reaction is proposed to analyze the onset of Tg in macromolecule-metal complexes with enhanced glass-transition temperatures relative to the imdiluted polymers ... 

Predictions for Co " complexes with poly(L-histidine), based on the ligand dissociation reaction in this section, reveal that LFSE for pseudo-tetrahedral complexes in the glassy state is 6.6kJ/mol larger than that for 3-coordinate complexes in the molten state. However, these poly(L-histidine)/metal complexes were prepared in aqueous solution and lattice waters should be retained in the glassy state structure. Pseudo-octahedral glassy complexes that revert to 5-coordinate complexes via the onset of Tg represent a better model, so A(LFSE) of 1.9kJ/mol from Table 12 is employed on the horizontal axis of Figure 8 for poly(L-histidine)/Co + instead of 6.6 kJ/mol calculated in Table 14. 

B. The Energetics of Ligand Dissociation Reactions in Model Systems Comparison with Experimental Enhancements for d-Block and f-Block Complexes... 

The majority of ligand-dissociation reactions are thought to occur on the ultrafast time scale. A relatively rare case of a slow ligand dissociation from a relaxed excited state takes place in some amino complexes of Rh(III) and Ir(III), or in Co(II) polycyano complexes. In these complexes, the lowest ligand-field (dd) state is formed upon t2g eg excitation, and undergoes ligand dissociation and consequent ligand substitution reaction on a nanosecond time scale. 

Although the early photochemistry of bimetallic carbonyl complexes centered on metal-metal bond homolysis and ligand dissociation reactions, another class of reactions that can occur involves photodisproportionation between the two metal... 

A basic concept in ligand dissociation reactions is in the idea that the reaction should be facile when the reactants are sterically strained. The release of steric strain on ligand dissociation can be measured by the difference in MM steric energy between the products and reactants, AU. Such AU values should provide a way to predict the influence of steric effects on the kinetics of ligand dissociation. 

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