The Chemistry of Aluminum(I), Silicon(II), and Germanium(II)

Although tetrameric Al(I) compounds have been known for a long time, the monomeric Al(I) compounds that are analogous to carbenes are very recent entrants in Al(I) chemistry. They possess novel structural features and exhibit distinct reactivity. This has resulted in the isolation and characterization of various unusual aluminum(III) compounds such as the aluminatetrazoles and aluminacyclopropenes. In comparison to the recent emergence of monomeric aluminum(I) compounds, stable silylenes and germylenes (carbene analogues of silicon and germanium) were recognized much earlier. This led to the evolution of the Si(II) and Ge(II) chemistry that at times surpasses the sophistication achieved in divalent carbon chemistry. Thus, while carbon lacks an example of a stable chlorocarbene (LCCl), for silicon there is one example of LSiCl, and for germanium there are a fair number of LGeCl compounds. While reactivity studies on LSiCl are anticipated, the utility of RGeCl as a synthon is well documented. Exotic compounds such as a germanethioacid chloride, a germanium(II) hydride, and a germanium(II) hydroxide are some of the examples that were derived from LGeCl. Recent results from our laboratory at Göttingen have helped in the development of these interesting areas of research, and the present account summarizes our contributions to the chemistry of Al(I), Si(II), and Ge(II).