Stable isomers of sila- and germadodecahedrane. A semiempirical (AM1) investigation of the structure of 4/6 and 4/5/6 ring containing E-20 (E=Si,Ge) systems

AM1 molecular orbital calculations with full geometry optimisation of 4/6, 5/5 and 4/5/6 isomers of E-20 (E=Si,Ge) show a clear preference of germaniu m to be accommodated in four membered rings and to achieve coordination num bers greater than 3 (V A). As a consequence, unlike C-20 or Si-20, the dode cahedral form of Ge-20 was not localised on the potential surface, instead an opened structure derived from an octagermaprismane (IV B) can be predict ed. Notable kinetic stabilisation in the (SiR)(20) systems is suggested by the HOMO-LUMO differences.