  We discuss the ground state of the spin-orbital model for spin-one ions with
partially filled $t_{2g}$ levels on a honeycomb lattice. We find that the
orbital degrees of freedom induce a spontaneous dimerization of spins and drive
them into nonmagnetic manifold spanned by hard-core dimer (spin-singlet)
coverings of the lattice. The cooperative ``dimer Jahn-Teller'' effect is
introduced through a magnetoelastic coupling and is shown to lift the
orientational degeneracy of dimers leading to a peculiar valence bond crystal
pattern. The present theory provides a theoretical explanation of nonmagnetic
dimerized superstructure experimentally seen in Li$_2$RuO$_3$ compound at low
temperatures.
