We study the isotopologue-selective binding of dihydrogen at the undercoordinated boron site of B₁₂X₁₁⁻(X = H, F, Cl, Br, I, CN) usingab initioquantum chemistry. With a Gibbs free energy of H₂attachment reaching up to 80 kJ mol⁻¹(ΔGat 300 K for X = CN), these sites are even more attractive than most undercoordinated metal centers studied so far. We thus believe that they can serve as an edge case close to the upper limit of isotopologue-selective H₂adsorption sites. Differences of the zero-point energy of attachment average 5.0 kJ mol⁻¹between D₂and H₂and 2.7 kJ mol⁻¹between HD and H₂, resulting in hypothetical isotopologue selectivities as high as 2.0 and 1.5, respectively, even at 300 K. Interestingly, even though attachment energies vary substantially according to the chemical nature of X, isotopologue selectivities remain very similar. We find that the H-H activation is so strong that it likely results in the instantaneous heterolytic dissociation of H₂in all cases (except, possibly, for X = H), highlighting the extremely electrophilic nature of B₁₂X₁₁⁻despite its negative charge. Unfortunately, this high reactivity also makes B₁₂X₁₁⁻unsuitable for practical application in the field of dihydrogen isotopologue separation. Thus, this example stresses the two-edged nature of strong H₂affinity, yielding a higher isotopologue selectivity on the one hand but risking dissociation on the other, and helps define a window of adsorption energies into which a material for selective adsorption near room temperature should ideally fall.