  The occurrence of planar hexacoordination is very rare in cluster chemistry,
particularly for main group elements. We report here a class of planar
hexacoordinate silicon (phSi) in the global minimum isomer of SiE3M3+ (E = N,
P, As, Sb; M = Ca, Sr, Ba). Three Si-E multiple bonds between phSi and E
centers is a key structural and electronic prerequisite for the observation of
their prefect planarity and high stability. Especially, the electrostatic
interactions between phSi and three M centers become less repulsive with
decrease in electronegativity of E. Eventually, a sizable electrostatic
attractive interaction exists in between phSi and M centers in SiSb3M3+,
leading to a true unprecedented phSi bonding motif which features three Si-Sb
multiple bonds and three Si-M ionic bonds.
