Framework chirality and optical activity of organometallic cluster compounds

The contributions by the author's research group towards the preparation and the investigation of physical and chemical properties of optically active clusters are reviewed. Such clusters with EM₃ tetrahedrane frameworks can be prepared by stepwise synthesis or by metal exchange. Enantiomer separation is possible by means of optically active phosphine ligands via the intermediate formation of diastereomeric substitution derivatives or by means of chromatography on triacetyl cellulose. The optical properties of the diastereoisomers and the enantiomers (molar rotations, rotatory dispersion) are extreme. The optical activity of the clusters containing light transition elements is lost in the presence of donor ligands (CO, phosphines, catalysis substrates) due to the opening of metal---metal bonds. Reactions in the ligand sphere, however, can be performed with up to 100% diastereoselectivity.