Metal carbon bonds, homolytic

Alternative paths for decomposition of the metal carboxylate can lead to ketones, acid anhydrides, esters, acid fluorides (1,11,22,68,77,78), and various coupling products (21,77,78), and aspects of these reactions have been reviewed (1,11). Competition from these routes is often substantial when thermal decomposition is carried out in the absence of a solvent (Section III,D), and their formation is attributable to homolytic pathways (11,21,77,78). Other alternative paths are reductive elimination rather than metal-carbon bond formation [Eq. (36)] (Section III,B) and formation of metal-oxygen rather than metal-carbon bonded compounds [e.g., Eqs. (107) (119) and (108) (120). Reactions (36) and (108) are reversible, and C02 activation (116) is involved in the reverse reactions (48,120). 

The homolytic decomposition in the absence of scavengers for the radical R is often followed by the reaction of the radical "R with the transient complex with the metal-carbon bond ... 

Isocyanide ligands appear to be radical probes and hence activate or initiate radical reactivity. For example, the addition of TCNE to [Co(CNMe)s]+ is radical in nature (46) and homolytic fission of the metal-carbon bond in [(>/, ff-CgHl3)Ru(CNR)4]+ complexes has been observed (id), and it is possible that radical stabilization can occur through metal-... 

Comparing the reactants and the products, the reactions are apparently nonredox processes. Using a spin-trapping EPR technique it was shown [114] that irradiation of the complexes leads to an alkyl radical formation (CH3 or C2Hj). The efficiency of the homolytic metal-carbon bond splitting depends on the electronic properties of the other axial ligand. The ostensibly non-redox photoinsertions are thus a product of two redox reactions. As far as the photoreactive excited state is concerned, the metal-carbon bond is either indirectly activated by a ir-nt excitation localized on the tetrapyrrole ring [112] or there is an... 

A widely accepted mechanism of coenzyme Bir dependent rearrangements encompasses, as the initial step, the homolytic cleavage of the carbon-cobalt bond to generate the 5 -deoxyadenosyl radical. The thermodynamic and kinetic aspects of this and related processes involving the homolysis of transition metal-carbon bonds are discussed. 

When polymerization occurs at a transition metal-carbon bond, this type of complexing weakens the metal-carbon bond and facilitates homolytic cleavage (202) with migration of the polymer radical to one... 

Our results indicate that the autoreduction cannot occur by a conventional outer sphere mechanism because of the gross mismatch of the electrochemical potentials. Experimental data available at this time are consistent with homolytic iron-carbon bond cleavage which may or may not involve a simultaneous nucleophilic attack on the coordinated cyanide. The homolytic metal-carbon bond cleavage may serve as a model for similar processes reported for vitamin Bi2 (26). 

Electrochemical oxidations of alkylmetal donors are irreversible in all cases studied owing to rapid homolytic cleavage of the metal-carbon bond at the cation-radical stage (Eq. 3) ... 

Carbon-carbon bond formation by reductive elimination from Ni, Pd, or Pt complexes is widespread. In many cases it is presumed to occur as the final step in a catalytic cycle, whereby the organic product is expelled from the metal center, but in others it is a well-defined, mechanistically studied reaction. Elimination takes place from Ni, Pd, and Pt complexes in their - - 2 or + A oxidation states, and it may be promoted by thermolysis, by photolysis, or by nucleophilic attack at the metal center. The reaction may proceed by heterolytic or homolytic metal-carbon bond cleavage, reductive elimination, or dinuclear elimination, and more than one mechanism may operate. 

Further evidence for oxygen insertion by the homolytic dissociation of a metal-carbon bond has been obtained ... 

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... 

Barker, P. J., Winter, J. N., Homolytic Cleavage of Metal-Carbon Bonds, Chem. Met.-Carbon Bond 2 [1985] 151/218. 

The subsequent experimental verification of these results indeed proved that IPrAu-alkyl compounds start to decompose only at temperature of about 180 °C. In analogy with organomercury compounds, this decomposition could be based on a homolytic metal-carbon bond cleavage. 

Moving from RCo(III) and RCr(III) compounds to RCo(IV), one finds that the metal-carbon bond can break in a variety of ways. Electrochemical studies by an East-West partnership on the attack of pyridine on organocobalt(IV) chelate complexes indicate three, possibly four, main decomposition routes 5n2, involving a conventional remote side attack on the a-carbon atom Sf i, in which there is an initial internal transfer of R from the metal to the chelate ligand homolytic and possibly 5n1 (see Table 7.2). 

However, the possibility of a hydride mechanism is not excluded. Irradiation of [6-27] may homolytically cleave the metal-carbon bond to form a 7r-allyl complex [6-28]. Hydride ion addition to the 7r-complex [6-29] gives an isopropyl complex, C5H5Fe(CO)3—CH(CH3)2 This process is designated as a n-a rearrangement. Analogous 7r-allyl and olefin complexes of molybdenum and of tungsten are known. 

In the presence of efficient scavengers for M"Lm and/or R the mechanism of decomposition of complexes with metal-carbon -bonds, which decompose heterolytically under routine conditions, can be shifted towards the homolytic decomposition (49,50,124). 

Some interesting chemistry has appeared relating to the ability of the isocyanide ligand to stabilize unusual oxidation states. A series of palladium metal - metal bonded complexes has been synthesized by redox reactions involving two metal complexes in different formal oxidation states (33 -35). Similar ruthenium(I) and osmium(I) dimers have been prepared by an unusual homolytic fission of a ruthenium-carbon bond (36) or by singleelectron oxidation of Os(CNXylyl)5 (18). 

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