NONAROMATIC PLANAR SI-12 RING-SYSTEM OF APPROXIMATE D-6H SYMMETRY IN CA7MG7.5+ -DELTA-SI14/

Ca7Mg7.5+/-deltaSi14 is the correct stoichiometry of the so called Ca7 Mg7.5+/-deltaSi14 of Gladishevskii et al.([1]) Ca7Mg7.5+/-deltaSi14 is an approximate or unusual Zintl phase with metallic conductivity that is expected to be highly anisotropic. The crystal structure (P6/mmm ( no. 191), a = 12.696(2), c = 4.4025(5) Angstrom, Z = 1) contains plana r hexasilyl-hexasilacyclohexene anions (Si-12) Of D-6h symmetry, with a formal charge of - (21 +/- 2 delta), besides isolated Si-4(4-) anion s. Quantum mechanical calculations based on the Extended Huckel and LM TO-ASA methods establish metallic properties due to a pi - pi* intera ction of the ecliptically stacked Si-12 units. The planar ring system does not show aromaticity, but contains approximately 1.75 double bond s that are equally delocalized.