Superconductivity in layered metal borides originates from strong coupling between $\sigma$-bonding electrons and in-plane phonon modes. Herein, we demonstrate that layer-resolved $\sigma$-orbital occupancy governs the superconducting behavior of two-dimensional C ($M=\mathrm{Mg}$, Al, Ca, Sc, Y, and In). The inequivalent chemical environments of the threefold-coordinated ${\mathrm{B}}_1$ layer and sixfold-coordinated ${\mathrm{B}}_2$ layer induce different degrees of charge transfer, resulting in layer-dependent $\sigma$-orbital filling. Partially filled ${\mathrm{B}}_1 \sigma$ orbitals provide the fundamental electron-phonon coupling across the designed structures, whereas filling states of ${\mathrm{B}}_2 \sigma$ orbitals dictates the variation of $T_{\mathrm{c}}$. When ${\mathrm{B}}_2 \sigma$ orbitals remain partially filled, both boron layers contribute cooperatively to electron-phonon coupling, yielding $T_{\mathrm{c}}$ values above 30 K in $\mathrm{Ca}{\mathrm{B}}_3, \mathrm{In}{\mathrm{B}}_3$, and $\mathrm{Mg}{\mathrm{B}}_3$. In contrast, saturation of the ${\mathrm{B}}_2 \sigma$ orbital drives charge redistribution toward ${\mathrm{B}}_1$ layers, suppressing ${\mathrm{B}}_2$ contribution and lowering $T_{\mathrm{c}}$ below 15 K in $\mathrm{Y}{\mathrm{B}}_3, \mathrm{Sc}{\mathrm{B}}_3$, and $\mathrm{Al}{\mathrm{B}}_3$. These results identify orbital-selective $\sigma$-band occupancy, mediated by layer-specific charge transfer, as a microscopic principle for tuning superconductivity in boride materials.

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• Charge
• Density of states
• Electrical properties
• Electronic structure
• Fermi surface
• First-principles calculations
• Superconductivity
• Superconductors