1. On the Molecular Structure of Di-t-butyl Beryllium.
2. The terms “persistent” and “stable” have been applied to long-lived radicals. A persistent radical has arelatively long lifetimeunder the conditions it is generated, whereas a stable radical is inherently stable as an isolated species and shows no sign of decomposition under an inert atomsphere at room temperature. In this review, the use of the description “stable” will be confined to radicals that can beisolatedunder ambient conditions and can be stored without decomposition, either indefinitely or for long periods, at room temperature in the absence of air or 807 moisture. Thus, the Gomberg radical, ·CPh3,9has been often describeda,bas a stable radical, but it exists in equilibrium with its head-to-tail dimer in solution and is also dimerized as a solid. Similarly, the phosphinyl radical :Ṗ{CH(SiMe3)2}2(see later refs 25, 151, 155, and 156), which has a half-life of ca. 1 year in solution at room temperature, associates to form a weakly P−P bonded dimer in the crystal phase. Thus, ·CPh3and :Ṗ{CH(SiMe3)2}2are persistent rather than stable radicals. In contrast, the boron radical [Li(12-crown-4)2][BMe3] is isolable as crystals (ref 43) at room temperature, where it is indefinitely stable in the absence of air and moisture. As a result, [BMes3]•-can be described as a stable radical. What constitutes a “relatively long lifetime” in the context of the description “persistent”? One definition is that it is sufficient time to allow the radical to be prepared in solution or in the solid state at room temperature under an inert atmosphere and to allow it to be spectroscopically and chemically characterized with conventional techniques.bThese restrictions require a half-life of the order of minutes. However, as will be seen, several of the new main-group radicals have half-lives of at least several months or are indefinitely stable under such conditions. It seems probable that some of these persistent radicals will be isolated as stable crystalline species in the future. (a) Smith, M. B.; March, J.March's Advanced Organic Chemistry;Reactions, Mechanisms and Structure; Wiley: New York, 2001; p 238. (b) Forrester, A. R.; Hay, J. M.; Thomson, R. H.Organic Chemistry of StableFree Radicals; Academic Press: London, 1968; p 1.