Is Lewis Acidity in Metal Dications Triggered by Solvent Shell Fluctuations?

Using experimental data collected on the stabilities of gas phase clusters consisting of metal dications in association with water molecules, a model is proposed to account for Lewis acidity. It is suggested that acidity is driven be fluctuations in the numbers of water molecules surrounding a metal ion, and that these fluctuations reduce numbers in the coordination shells to below those need to stabilise a +2 charge on the metal. The timescale on which these fluctuations are calculated to occur is approximately 6 orders of magnitude longer than any of the measure mean residence times of water molecules surrounding a central metal ion. This time difference reflects both the rarity of the event require to drive acidity and the extreme nature of some of the fluctuations.