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.