Radical stability metal hydrides

The first quotation is of use to those studying kinetics—one of the first signs that a radical process is going on is a rate variance from one day to the next, when background levels of trace radical initiators are at different levels. The second stresses the importance of removing trace radical impurities in synthesis. Very pure metal hydrides (or other complexes) may show a surprising stability provided they are free of all radical contaminants. 

Use of metal complexes with weaker Mt -H bonds (i.e., leading to a more stabilized metal-based radical) led to chain transfer catalytic activity in MMA polymerization. The penta-phenylcyclopentadienyl compound (C5Ph5)Cr (CO)3, XXXVIIIa, is a stable radical (no tendency to form dimer XLa) and the corresponding hydride complex (C5Ph5)Cr (CO)3H, XXXIXa, is also available (Figure 29). The reaction of XXXIXa with a large excess of MMA produces low-Mn PMMA, rather than hydrogenated MMA. The same process is initiated by thermal AIBN... 

Hydride Abstraction from Organic Ligands The removal of a hydride ion from an organic radical is an important method of generating carbonium ions stabilized by metal carbonyl systems. Dauben and Honnen (61) in 1958 were the first to exploit this method by use of the powerful hydride abstractor, triphenyl methyl (or trityl) carbonium ion, which is converted thereby into triphenylmethane. 

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