Aquated cations as Bronsted acids

In the aqueous chemistry of cations, hydrolysis refers to the reversible loss of from an aqua species. The term hydrolysis is, however, also used in a wider context, e.g. the reaction  

PCI3 + 3H2O H3PO3 + 3HC1 is a hydrolysis process. 

Aquated cations can act as Bronsted acids by loss of H+ from a coordinated water molecule (equation 6.32). 

The position of the equilibrium (and thus, the strength of the acid) depends on the degree to which the O—H bonds are polarized, and this is affected by the charge density of the cation (equation 6.33). 

When H2O coordinates to M , charge is withdrawn towards the metal centre, leaving the H atoms more (structure 6.8) than in bulk water. Small cations such as 

Suggest why the coordination number of Li in its first hydration sphere in aqueous solution is 4, whereas that for Na is 6. 

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