Is there a minimum size for aqueous doubly charged metal cations?

Area of Science:

• Inorganic chemistry
• Physical chemistry
• Mass spectrometry

Background:

• Multiply charged solvated metal ions exhibit dissociative charge transfer.
• This phenomenon occurs when a metal atom's second ionization potential exceeds the solvent molecule's first ionization potential.
• The study investigates the critical number of ligands influencing spontaneous charge separation.

Purpose of the Study:

• To determine the minimum number of ligands required for spontaneous charge separation in doubly charged aqueous metal cations.
• To investigate the critical size for dissociative charge transfer versus simple ligand loss.

Main Methods:

• Electrospray ionization was used to generate doubly charged aqueous cations of divalent metals (Mn, Fe, Co, Ni, Zn, Cd, Cu).
• Collision-induced dissociation in a triple-quadrupole mass spectrometer was employed to analyze these ions.
• The study systematically examined the size-dependent behavior of these metal-ligand complexes.

Main Results:

• Monoaqua complexes were observed for most divalent metal ions studied (Mn, Fe, Co, Ni, Zn, Cd, Cu).
• Beryllium was an exception, with the smallest observed complex being the dihydrate.
• The study revisited and provided data on the critical size where dissociative charge transfer competes with ligand loss.

Conclusions:

• The findings provide insights into the stability of multiply charged solvated metal ions.
• The research clarifies the threshold for spontaneous charge separation based on ligand number.
• This work contributes to understanding the competition between different dissociation pathways in metal-ligand systems.