Trivalent group 14 radicals
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A trivalent group 14 radical (also known as a trivalent tetrel radical) is a molecule that contains a group 14 element (E = C, Si, Ge, Sn, Pb) with three bonds and a free radical, having the general formula of R3E•. Such compounds can be categorized into three different types, depending on the structure (or equivalently the orbital in which the unpaired electron resides) and the energetic barrier to inversion. A molecule that remains rigidly in a pyramidal structure has an electron in a sp3 orbital is denoted as Type A. A structure that is pyramidal, but flexible, is denoted as Type B. And a planar structure with an electron that typically would reside in a pure p orbital is denoted as Type C. The structure of such molecules has been determined by probing the nature of the orbital that the unpaired electron resides in using spectroscopy, as well as directly with X-ray methods.[3] Trivalent tetrel radicals tend to be synthesized from their tetravalent counterparts (i.e. R3EY where Y is a species that will dissociate).
While the trivalent triphenylmethyl radical, which was the first organic radical described, has been known for over 100 years,[4][5][6] characterization of transient, persistent, or stable radicals of heavier tetrel compounds have been only accessible in recent years (from the 1960s to the present). The most recent large advance has been the characterization of the first stable trivalent lead radical, as described in 2007.[3][7]
Such developments have only been made in recent years because these compounds tend to be highly reactive (with respect to reactions such as dimerization and radical chain reactions). There have been two main approaches for stabilization. Firstly electronic stabilization, the tetrel is connected to an electron-rich atom such as oxygen, nitrogen, or fluorine. Secondly steric stabilization, the tetrel is surrounded by bulky ligands (such as -Y(SiMe3)2 (Y = N, CH), -Si(SiMe3)2Et (-Ebt), or -Si(SiMe3)3 (-Hyp)). It has become convention to describe a radical that can persist long enough for spectroscopic or chemical analysis as persistent and a radical that can persist indefinitely as stable.[7] Trivalent tetrels can also synthesized in a cyclic structure (e.g. Ar3Ge3•). This class of molecules tends to be slightly more stable than the acyclic analogues as there is a stabilization through the delocalization of the unpaired electrons within the π-system.[8]
