Aromatic Silicon Benzene has long intrigued chemists on account of the energy stabilization, termed aromaticity, which arises from π-electron delocalization around its ring framework. A persistent question has been how such stabilization would be impacted were the carbons to be replaced by heavier atoms such as silicon. Abersfelder et al. (p. 564 ) have prepared a benzene analog with Si atoms in place of all six-ring carbons, but a slightly altered bonding framework in which substituents outside the ring are no longer evenly distributed. Instead, the substituents pair up at two Si sites, leaving two other ring sites with no external appendages. The resulting compound no longer has a continuous network of π-electrons, but retains a degree of aromatic stabilization involving sigma and nonbonding electrons.

Singlet diradicals are usually not energy minima. As observed by femtosecond spectroscopy, they readily couple to form final sigma bonds. Substituent effects allow lifetimes to increase into the microsecond range. Taking advantage of the properties of hetero-elements, a diradical has been prepared that is indefinitely stable at room temperature. The availability of diradicals that can be handled under standard laboratory conditions will lead to further insight into their chemical and physical properties, raising the likelihood of practical applications, especially in the field of molecular materials such as electrical conductors and ferromagnets.

Silicon shuffle minimizes energy: Isomerization of the dismutational isomer of hexasilabenzene (see structure; R=2,4,6‐iPr3C6H2) produces the Si‐bridged propellane, a stable representative of the global minimum on the Si6H6 energy surface, which, despite its mass, can be distilled without decomposition. Its halogenation proceeds in the bridgehead positions. Unprecedented 29Si NMR data of the new Si6R6 isomer is explained with magnetically induced cluster currents.

The reactivity of two stable Si(6)R(6) clusters (4 and 5, R = 2,4,6-(i)Pr(3)C(6)H(2)) with unsymmetrical substitution patterns (including Si, SiR, and SiR(2) vertices) is reported. In order to account for the importance of such clusters as model systems for transient intermediates in the deposition of elemental silicon, we here propose the term "siliconoids" for silicon clusters with unsaturated valencies. With the hexasilaprismane 8a, a saturated-i.e., non-siliconoid-Si(6)R(6) isomer is accessible from a suitable Si(3) precursor. Thermal redistribution of the substituents converts 1,1,2-trichlorocyclotrisilane 6 into the corresponding 1,2,3-derivative 7 prior to the requisite reductive coupling step leading to 8a. On the other hand, a stable expanded Si(11)-siliconoid 9 was isolated as a minor side product of the thermal isomerization of 4 to 5, thus providing a first example of siliconoid cluster expansion in the condensed phase. In the solid-state structure, the two unsubstituted vertices of 9 strongly interact in a staggered propellane-like fashion. Oxidative cluster contraction of a siliconoid scaffold is observed upon treatment of siliconoid 5 with a large excess of iodine in refluxing toluene, thus providing access to a highly functionalized hexaiodocyclopentasilane 11 in high yield. Conversely, chlorination of the isomeric 4 with BiCl(3) as a mild source of Cl(2) results in a complex mixture of products from chlorination of the unsubstituted vertices as well as σ-bonds of the cluster framework of 4. The main product, 1,2-dichlorotricyclo[2.2.0.0(2,5)]hexasilane 12, undergoes thermal cluster contraction to give tricyclo[2.1.0.0(2,5)]pentasilane 14 with an exohedral chlorosilyl group.