An electron concentration parameter, expressed as the number of itinerant electrons per number of atoms N in a structural unit cell of an alloy, e/uc, is a useful parameter that can be used in interpreting the origin of a band structure pseudogap often evident at the Fermi level for structurally complex metallic alloy phases. It can be expressed in terms of the interference condition e/uc=$π/3[|G|_{c}^2]^{3/2}$, where $|G|_{c}^2$ is the square of the critical reciprocal lattice vector associated with the specific set of lattice planes interfering with electrons at the Fermi level. This parameter is similar to the well-known Hume-Rothery electron concentration parameter e/a, which represents the number of electrons per atom and is linked with e/uc through the relation e/uc = N(e/a). We have demonstrated that certain complex metallic alloy structures appear to be stable at or near certain values of e/a. We show that the e/a=1.60 rule holds for the sub-group of gamma-brasses with space group I4̅3m and Pearson symbol cI52, the e/a=4.34 rule for skutterudite compounds with Im3̅ and cI32, the e/a=2.74 rule for $Al_6TM$ (TM = Mn, Tc, Re, Fe, and Ru) compounds with Cmcm and oC28, the e/a=1.62 rule for the sub-group of the Heusler compounds with Fm3̅m and cF16, and the e/a=2.09 rule for the sub-group of Zintl compounds $MX_1$ (M = Li and Na, $X_1$ = Al, Ga, In, and Tl) with Fd3̅m and cF32. The e/a rule holds in sub-groups of isostructural compounds, regardless of the degree of orbital hybridizations and the polarity involved.
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