the sites connected by the largest transfer integral along the B bonds form dimers. See also Fig. 1 of ref. .[]{data-label="fig3"}](fig3.eps){width="6cm"}

Systematic view of electronic structures {#subsec2-3}
----------------------------------------

The orbital energies $\Delta_{\textrm{M}\sigma}^0$ and $\Delta_{\textrm{M}\pi}^0$ together with the transfer integrals determining the band structures, and the electron numbers, 4 ($M$ = Ni) or 5 ($M$ = Au, Cu) in the four orbitals, lead us to a systematic view of this family, as shown in Fig. \[fig4\].

The M$\pi$ level is low in energy but mixes with the L bands particularly in \[Ni(tmdt)$_2$\] and \[Cu(tmdt)$_2$\]; $\bar{n}_{\textrm{M}\pi}$ deviates from 2, as seen in Table \[table1\]. However, as far as the main characteristics of the electronic states near $\epsilon_{\rm F}$ are concerned, the M$\pi$ orbital does not play an important role. Then, $\Delta_{\textrm{M}\sigma}^0$ is the essential difference among the three members, controlling the orbital mixing between the M$\sigma$ and L orbitals. It becomes monotonically small as $M$ = Ni $\rightarrow$ Au (room-$T$) $\rightarrow$ Au (low-$T$) $\rightarrow$ Cu. This gives rise to a crucial difference in the magnetic states of the Au and Cu systems, as will be shown in § 3.

The M$\sigma$ level in \[Ni(tmdt)$_2$\] is high, and approximately 2 electrons enters the 