Ligand Preferences of Divalent Main-Group and Transition Metal Ions

The coordination preferences of divalent metal cations toward neutral and anionic ligands were examined by positive-ion electrospray ionization (ESI) mass spectrometry, using aqueous MCl2 solutions (M = Mg, Ca, Cu, Zn) subjected to sequential ligand (L) perturbation with acetone, acetonitrile, N,N-dimethylformamide (DMF), and acetic acid. Under identical ESI conditions, Mg²⁺, Ca²⁺, and Cu²⁺ converge toward similar ion populations dominated by charge-compensated chloride/acetate and DMF-containing adducts, indicating substantial ligand exchange during droplet desolvation and ion transfer. Zn²⁺ behaves differently: It retains acetone- and acetonitrile-containing complexes and forms a broader set of mixed-ligand and polynuclear species. Quantum-chemical calculations show that acetate and DMF are the strongest ligands in the simplified M-Cl-L⁺ model, but they also demonstrate that the experimental Zn²⁺ selectivity cannot be reduced to maximum binding energy alone. Instead, Zn²⁺ appears to occupy a favorable regime where a combination of intermediate hardness, closed-shell d¹⁰ electronic structure is the proposed root cause of sufficient yet not irreversible ligand binding to allow coordination motifs to survive the ESI process. As much as these are proposed root-causes. The results identify Zn²⁺ as an exceptional divalent cation for retaining ligand memory under ESI and define limits on the use of ESI-MS as a direct probe of solution-phase metal-ligand preferences.