Homolytic Ligand Dissociation

Heterolytic and homolytic ligand dissociation processes occur, and are of importance in the context of the catalytic roles of metal complexes, e.g. ... 

Whereas homolytic ligand dissociation is not commonly observed for inorganic complexes, it has been identified as an important process in organometallic chemistry where it is favored by the characteristic weakness of transition metal-alkyl tr-bonds. Recent determinations yield metal alkyl band dissociation energies for CH3— Mn(CO)s (ca. 120 kJ/mol) and for several alkylcobalt complexes (ca. 80-100 kJ/mol) . Homolytic dissociation of such complexes results in the formation of free radicals and in the opening up of free radical catalytic pathways, e.g., for hydrogenation". Important biochemical examples of free radical catalytic mechanisms, initiated by the homolytic dissociation of a transition metal-carbon bond (i.e., the 5 -deoxyadenosy 1-cobalt bond of coenzyme 8,2) are provided by the coenzyme B,2-promoted rearrangements (see Section... 

Ligand dissociation can involve either heterolytic or homolytic processes. Most biological processes involve the former, although examples of the latter are known. For example, adenosylcobalamin (vitamin B-12 coenzyme), in association with an appropriate apoenzyme, catalyzes a variety of rearrangements via a 1,2-hydride shift (see Chapter 13). The hrst step in these reactions is thought to be homolytic dissociation of a Com—alkyl bond as shown ... 

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. 

This paper is concerned with certain aspects of the thermodynamics and kinetics of transition metal-alkyl homolytic bond dissociation processes, notably of stable, ligated complexes in solution (L M-R, where L is a ligand and R = alkyl, benzyl, etc.)(l ). The metal-alkyl bond dissociation energy of such a complex (BDE, strictly bond dissociation enthalpy) is defined as the enthalpy of the process represented by Equation 1. 

Based on the analysis of the reactions in Scheme 3 and on previous studies (46, 47), a mechanism for the reaction was proposed in which the /x-peroxo complex, 16, may simultaneously abstract two hydrogen atoms from iso-propyl groups on the pyrazolyl ligands. Alternatively, because of the weak 0—0 bond, 16 may homolytically dissociate to form two Tp"Co(0-) oxo-radical moieties, and these species would then abstract hydrogen from the iso-propyl groups. In either case, the resulting carbon-centered radical can either react with solvent, as was observed for the Tp complex (46), or with another carbon-centered radical so as to regenerate the Tp"Co(OH) complex and produce a derivative of the Tp" complex with an iso-propenyl substituent, 18. Ultimately, either route would produce the (/x-OH)2 complex, 17. 

The primary photochemical reaction of the [MM (CO)i0] (M, M =Mn, Re) complexes in solution has been shown to result in (1) homolytic fission of the metal-metal cr-bond to form M(CO)s radicals, (2) dissociation of a CO ligand to form [MM (CO)9], and (3) non-dissociative relaxation to the electronic ground state [86]. Similar behaviour is shown by other dinuclear carbonyl complexes [87]. 

Several directly measured values of AH° for homolytic dissociation of a metal-metal-bonded carbonyl in solution have been obtained (9). This was for the complexes [(n3-C3H5)Fe(CO)2 )2 where L = CO or a number of different P-donor ligands. The low value AH = 56.5 kJ mol-1 when L = CO was not unexpected for such a sterically crowded molecule. The P-donor substituents increased the steric crowding and displaced the equilibria in favor of the monomers but the effect seemed to be controlled more by AS° than AH°. In general metal-metal bond energies, however they may have been estimated, are too large to allow for direct measurement of equilibrium constants in solution in this way. 

For example, the cis stereochemistry in reaction (i) is maintained in the product enedione. The reactions may involve concerted addition to the n system of the organic ligand. Further evidence for oxygen insertion by the homolytic dissociation of a... 

A quantum-mechanical study of the model compounds TiCl3N=PH3 and Re03N=PH3 reveals that the metal-ligand bond dissociates homolytically. Variation of the metal-nitrogen-phosphorus angle influences the potential energy surface only to a small extent." ... 

Similarly, photolysis of [(h -C HPjEfCHpj in the presence of pyridine, acetonitrile, or PMe Ph leads to [(h -C5H5)jW(CH3)L, (h -C,H,),W(H)(CH I.)p (L = PMCjPh) and [(h -CjHPjWHfL)] , (L = pyridine, CHjCN). These reactions proceed via homolytic dissociation of a CHj ligand followed in some cases by a-hy-dride elimination from the resultant ((h -C5H5)2W(CHj) intermediate to give a hydridomethylene complex ... 

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