  The electronic states of isostructural single-component molecular conductors
[M(tmdt)2] (M= Ni, Au, and Cu) are theoretically studied. By considering
fragments of molecular orbitals as basis functions, we construct a multiorbital
model common for the three materials. The tight-binding parameters are
estimated from results of first-principles band calculations, leading to a
systematic view of their electronic structures. We find that the interplay
between a p\pi-type orbital (L) on each of the two ligands and a pd\sigma-type
orbital (M\sigma) centered on the metal site plays a crucial role: their energy
difference controls the electronic states near the Fermi energy. For the
magnetic materials (M= Au and Cu), we take into account Coulomb interactions on
different orbitals, i.e., we consider the multiorbital Hubbard model. Its
ground-state properties are calculated within mean-field approximation where
various types of magnetic structures with different orbital natures are found.
An explanation for the experimental results in [Cu(tmdt)2] is provided: The
quasi-degeneracy of the two types of orbitals leads to a dual state where
localized M$\sigma$ spins appear, and L sites show a nonmagnetic state owing to
dimerization. On the other hand, [Au(tmdt)2] locates in the subtle region in
terms of the degree of orbital mixing. We propose possible scenarios for its
puzzling antiferromagnetic phase transition, involving the M\sigma orbital in
contrast to previous discussions mostly concentrating on the L sector.
