Thermodynamic lead-ligand interactions

The thermodynamics of lead-ligand interactions provide key insights both into the probable mechanisms of lead s toxicity and into the design of improved chelation therapy agents. Various constants can be used to describe the affinity of a metal ion for a given ligand described by the net equilibrium (300, 301) ... 

From the broad range of studies that have been conducted to date on both the kinetics and thermodynamics of lead-ligand interactions, we draw the following conclusions ... 

More studies are needed on the kinetics and thermodynamics of lead-ligand interactions in complex systems (including mixed-phase systems) so that we can better understand lead speciation in vivo and in the environment. 

The final stereochemistry of a metathesis reaction is controlled by the thermodynamics, as the reaction will continue as long as the catalyst is active and eventually equilibrium will be reached. For 1,2-substituted alkenes this means that there is a preference for the trans isomer the thermodynamic equilibrium at room temperature for cis and trans 2-butene leads to a ratio 1 3. For an RCM reaction in which small rings are made, clearly the result will be a cis product, but for cross metathesis, RCM for large rings, ROMP and ADMET both cis and trans double bonds can be made. The stereochemistry of the initially formed product is determined by the permanent ligands on the metal catalyst and the interactions between the substituents at the three carbon atoms in the metallacyclic intermediate. Cis reactants tend to produce more cis products and trans reactants tend to give relatively more trans products this is especially pronounced when one bulky substituent is present as in cis and trans 4-methyl-2-pentene [35], Since the transition states will resemble the metallacyclobutane intermediates we can use the interactions in the latter to explain these results. 

Very recently, the kinetics and thermodynamics of a variety of axial ligation reactions have been investigated with Fe11 and Co11 porphyrins involving the small molecules CO, 02, and NO as ligands 27-30, 40, 92, 93). These experiments lead to the conclusion that the dynamic trans effects observed in these systems cannot alone be explained by the interaction models D and F (Fig. 1). Especially imidazole and its derivatives do not hold the place in various series of trans effects that they should take on the ground of their proton basicities. Therefore, besides their usual a-donor-tr-acceptor function, these unsaturated molecules are ascribed an additional 7r-donor function. 

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