Ligand interchange mechanism

Figure 1. Ligand interchange mechanism as modified from Reference 48, The circles represent the outer ligand spheres and Y may he either solvent or other reagent. Heavy dots, M X, are to represent the greater importance of bond-breaking relative to bond-making, M - Y...

In inert systems such as technetium and rhenium, ligand substitution reactions-including solvolysis-proceed under virtually irreversible conditions. Thus, the nature of the reaction center, the nature of the leaving group, and the nature and position of the other ligands in the complex affect the rates and activation parameters in a complicated manner. Most substitution reactions take place via interchange mechanisms. This is not too surprising when the solvent is water - or water-like - and where, in order to compete with the solvent,... 

All A mechanisms must be associatively and all D mechanisms must be dissociatively activated. The interchange mechanisms (I) include a continuous spectrum of transition states where the degree of bondmaking between the entering ligand and the complex ranges from very substantial (Ia mechanism) to negligible (Id mechanism) and inversely... 

Formation of L-lysinehydroxamato-iron(III) complexes occurs by an interchange mechanism formation and dissociation (acid-catalyzed) are significantly affected by charge repulsion (the ligand is H3N+(CH2)4CH(NH3 )CONHOH (335). Rate constants for complex formation between Feaq and two synthetic chelators of the dicatecholsper-midine family are, at 450 and 500M-1s-1 (336), similar to that for desferrioxamine. 

It is not surprising that the substitution processes on seven-coordinate 3d metal ions follow an interchange, rather than a limiting dissociative mechanism. In the case of seven-coordinate Fe(III) complex we even found an associative interchange mechanism for the substitution of solvent molecules as a result of the high n-acceptor ability of the fully conjugated pentadentate ligand system present in its equatorial plane 44). 

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