  Group-5 transition metal ditellurides $M$Te$_2$ ($M$ = V, Nb, Ta) are unique
CdI$_2$-type layered materials that exhibit peculiar quasi-one-dimensional
intralayer superstructures, known as ribbon-chains and butterfly-like clusters
of $M$ atoms. In this review article, we attempt to systematically understand
their electronic band structures based on our recent angle-resolved
photoemission spectroscopy (ARPES) studies and first-principles calculations.
We underscore the role of the localized molecular-like orbital bonds that form
the anomalous flat bands in the momentum space, and demonstrate how they
influence the Fermi surface anisotropy, phase stabilities of crystal
structures, and nontrivial topological properties in some cases. We also
elaborate the future prospects for novel quantum phenomena that can be realized
in these materials.
