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

A major feature of the chemistry of multiply charged solvated metal ions is dissociative charge transfer. It happens because the second ionization potential (IP) of a metal atom usually exceeds the first IP of a solvent molecule. This raises the issue of whether there is a minimum number of ligands below which the species would charge-separate spontaneously. To elucidate this, doubly charged aqueous cations of most common divalent metals (group 2 elements Mn, Fe, Co, Ni, Zn, Cd, and Cu) have been generated using electrospray and examined by collision-induced dissociation in a triple-quadrupole mass spectrometer. We have clearly observed the monoaqua complexes for all aforementioned doubly charged metal ions, except Be for which the smallest complex found is the dihydrate. We have also systematically revisited the matter of critical size--the maximum number of ligands at which dissociative charge transfer is competitive with simple ligand loss.