Metal-ligand cleavage, homolytic

DR. JAMES ESPENSON (Iowa State University) I should like to raise a slightly different aspect of substitution reactions, an aspect which relates to a different kind of mechanism. This involves a change from the conventional heterolytic process of metal ligand bond cleavage to one dealing with homolytic bond cleavage. 

A second way to activate the peroxide ligand for oxygen transfer to C = C bonds could be via homolytic 0-0 cleavage of the peroxide by coordination to a second metal center. From this point of view, the peroxide ligand is converted into the [M(/z-0)]2 moiety in which the oxide bridges are now electrophihc and then each oxygen atom from the M2(/z-oxo)2 core is transferred to a close C = C bond in a cis position (Scheme 8). As this conclusion is based on only two reported examples, it should be taken as a working hypothesis. 

First-Order Kinetics, K[A] Unimolecular processes, such as ligand dissociation from a metal center or a simple homolytic or heterolytic cleavage of a single bond, provide a straightforward example of a first-order reaction. The kinetics of this simple scheme, Equation 8.5, is described by a first-order rate law, Equation 8.6, where A stands for reactants, P for products, [A]0 for initial concentration of A, and t for time. The integrated form is shown in Equation 8.7 and a linearized version in Equation 8.8. 

Alkyl ligands are strong a -donors, and coordination of such ligands to metal centers such as rhenium(I) and ruthenimn(II) would perturb the electronic structures of the metal centers and give complexes with remarkable photochemical properties. Irradiation of the complexes [Re(CO)3(R)(4,4 -Me2-bpy)j (R = Ft, Tr, Bz) into their visible MLCT absorption bands (ca. 400-500 nm) resulted in extremely efficient homolytic cleavage of the Re-R bonds with the formation of the radicals [Re(CO)3(4,4 -Me2-bpy)]- and R-.i Tiss -pjjg methyl analogue underwent a similar photochemical reaction, but with a quantum yield of only 0.4. It was proposed that... 

Although alkoxide hgands are often stable and fairly unreactive ancillary ligands, particularly when bonded to early transition metals, the C-0 bond in an alkoxide ligand can undergo special types of reactions when a related oxo complex is accessible [114]. Two mechanisms of both homolytic and heterolytic fashions for C-O bond cleavage in alkoxide hgands have been discussed. 

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