  Motivated by Mo$_3$S$_7$(dmit)$_3$, we investigate the Hubbard model on the
triangular necklace lattice at two-thirds filling. We show, using second order
perturbation theory, that in the molecular limit, the ground state and the low
energy excitations of this model are identical to those of the spin-one
Heisenberg chain. The latter model is known to be in the symmetry protected
topological Haldane phase. Away from this limit we show, on the basis of
symmetry arguments and density matrix renormalization group (DMRG)
calculations, that the low-energy physics of the Hubbard model on the
triangular necklace lattice at two-thirds filling is captured by the
ferromagnetic Hubbard-Kondo lattice chain at half filling. This is consistent
with and strengthens previous claims that both the half-filled ferromagnetic
Kondo lattice model and the two-thirds filled Hubbard model on the triangular
necklace lattice are also in the Haldane phase. A connection between Hund's
rules and Nagaoka's theorem is also discussed.
