  The interplay of $sp^2$- and $sp^3$-type bonding defines silicon allotropes
in two- and three-dimensional forms. A novel two-dimensional phase bearing
structural resembleance to a single MoS$_2$ layer is found to possess a lower
total energy than low-buckled silicene and to be stable in terms of its phonon
dispersion relations. A new set of cigar-shaped, nematic orbitals originating
from the Si $sp^2$ orbitals realizes bonding with a 6-fold coordination of the
inner Si atoms of the layer. The identification of these nematic orbitals
advocates diverse Si bonding configurations different from those of C atoms.
