be interconverted (uv irradiation in the forward direction, heat in the reverse direction). Tetrakis(trimethylsilyl)tetrahedrane Tetrakis(trimethylsilyl)tetrahedrane can be prepared by treatment of the cyclobutadiene precursor with tris(pentafluorophenyl)borane and is far more stable than the tert-butyl analogue. The silicon–carbon bond is longer than a carbon–carbon bond, and therefore the corset effect is reduced. Whereas the tert-butyl tetrahedrane melts at 135 °C concomitant with rearrangement to the cyclobutadiene, tetrakis(trimethylsilyl)tetrahedrane, which melts at 202 °C, is stable up to 300 °C, at which point it cracks to bis(trimethylsilyl)acetylene. The tetrahedrane skeleton is made up of banana bonds, and hence the carbon atoms are high in s-orbital character. From NMR, sp-hybridization can be deduced, normally reserved for triple bonds. As a consequence the bond lengths are unusually short with 152 picometers. Reaction with methyllithium with tetrakis(trimethylsilyl)tetrahedrane yields tetrahedranyllithium. Coupling reactions with this lithium compound gives extended structures. A bis(tetrahedrane) has also been reported. The connecting bond is even shorter with 143.6 pm. An ordinary carbon–carbon bond has a length of 154 pm. Tetrahedranes with non-carbon cores In tetrasilatetrahedrane features a core of four silicon atoms. The standard silicon–silicon bond is much longer (235 pm) and the cage is again enveloped by a total of 16 trimethylsilyl groups, which confer stability. The silatetrahedrane can be reduced with potassium graphite to the tetrasilatetrahedranide potassium derivative. In this compound one of the silicon atoms of the cage has lost a silyl substituent and carries a negative charge. The potassium cation can be sequestered by a crown ether, and in the resulting complex potassium and the silyl anion are separated by a distance of 885 pm. One of the Si−–Si bonds is now 272 pm and its silicon atom has an inverted tetrahedral geometry. Furthermore, the four cage silicon atoms are equivalent on the NMR timescale due to migrations of the silyl substituents over the cage. The dimerization reaction observed for the carbon tetrahedrane compound is also attempted for a tetrasilatetrahedrane. In this tetrahedrane the cage is protected by four so-called supersilyl groups in which a silicon atom has 3 tert-butyl substituents. The dimer does not materialize but a reaction with iodine in benzene followed by reaction with the tri-tert-butylsilaanion results in the formation of an eight-membered silicon cluster compound which can be described as a dumbbell (length 229 pm and with inversion of tetrahedral geometry) sandwiched between two almost-parallel rings. In eight-membered clusters of in the same carbon