Tetrahedrane
Tetrahedrane is a hypothetical hydrocarbon with chemical formula Template:Carbon4Template:Hydrogen4 and a tetrahedral structure. Extreme angle strain (carbon bond angles deviate considerably from the tetrahedral bond angle of 109.5) prevents this molecule from existing.
In 1978 Gunther Maier prepared a stable tetrahedrane with 4 tert-butyl substituents. These substituents are very bulky and completely envelop the tetrahedrane core. Bonds in the core are prevented from breaking because this would force the substituents closer together (corset effect) resulting in Van der Waals strain.
Tetrahedrane is one of the possible platonic hydrocarbons.
The IUPAC name of tetrahedrane is tricyclobutane.
In Tetra(trimethylsilyl)tetrahedrane (I) the tert-butyl groups have been replaced by trimethylsilyl groups. Remarkably this compound is far more stable than the tert-butyl analogue. The silicon to carbon bond is longer than a carbon carbon bond and therefore the corset effect is reduced. On the other hand the trimethylsilyl group is a sigma donor which explains the increased stabilization of the tetrahedrane. Whereas the tert-butyl tetrahedrane melts at 135 C at which temperature decomposition to the cyclobutadiene starts,  the trimethyl silyl tetrahedrane melts at a much higher temperature of 202 C and is even stable up to 300 C and this compound reverts back to the acetylene starting material.
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 1.52 angstroms (152 picometers). The latest development is the organic synthesis and characterization of the tetrahedrane dimer (II). The connecting bond is even shorter with 1.436 A (143.6 pm). An ordinary carbon carbon bond has a length of 1.54 A (154 pm).
In tetrasilatetrahedrane the carbon atoms in the tetrahedrane cage are replaced by silicon. The standard silicon silicon bond is much longer (235 pm) and the cage is again enveloped by a total of 16 trimethylsilyl groups. This makes the compound thermally stable. The silatetrahedrane can be reduced with potassium graphite to the tetrasilatetrahedranide potassium salt. 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 captured by a crown ether and in the resulting complex potassium and the silyl anion are separated by a distance if 885 picometer. One of the Si- - Si bonds is now 272 picometer 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 4 so-called super silyl groups in which a silicon atom has 4 tert-butyl substituents. The dimer does not materialize but a reaction with iodine in benzene followed by reaction with the tri-tert-butyl sila anion results in the formation of an eight membered silicon cluster compound which can be described as a Si2 dumbbell (length 229 picometer and with inversion of tetrahedral geometry) sandwiched between two almost parallel Si3 rings.
In known eight-membered clusters of in the same carbon group,  tin Sn8R6 and germanium Ge8R6 the cluster atoms are located on the corners of a cube.