Why Cd Outcompetes Zn at Sulfur: Transferable Orbital Rules from Hydration to Thiolate Binding

Cadmium's persistent preference over zinc at sulfur affects biology, sensing, and separations, yet a general predictive rationale from aqueous speciation to thiolate binding has been lacking. We integrate continuum-solvation thermodynamics with Natural Bond Orbital (NBO) analysis to derive five transferable rules (Box 1) that quantify how hydration energetics, chalcogen π-delocalization, and cooperative LP(S)→σ*(M-S) and σ→σ* interactions stabilize Cd-thiolate motifs relative to Zn analogues. The framework predicts (i) stronger Cd-S binding free energies, (ii) IR red-shifts of M-S modes, (iii) near-UV S→Cd charge-transfer bands, and (iv) EXAFS Cd-S metrics consistent with six-coordinate hydration giving way to thiolate capture. Benchmarking small Cd/Zn pairs confirms the expected ΔΔG_bind and second-order E2 trends; cross-solvent checks retain rank order. Cast as a ligand-only screen, the rules classify thiolate vs thiol microstates and anticipate Cd > Zn outcomes without TDDFT, using NBO charges, E2 channels, Wiberg indices, and M-S distances. This work provides a compact, empirically usable basis for forecasting Cd capture and Zn discrimination in bioinorganic models and for guiding selective chelator design.