[byebyebadman]

Just gone over why tansition metal complex ions are acidic, and its seems its due to the presence of the charge on the metal ion. The charge makes the proton on the ligands of the complex more positive therefore easier to remove/react. However the explainations I have read say this happens because the charge on the metal ion pulls the ligand closer to the metal ion, and therefore the proton of the ligand is further away from the ligand it is bonded to, therefore slightly more positive. If it is further away from the partially negative ligand it is bonded to why does this make the proton more positive itself? By the way this is mainly relating to aqua complex ions.

[EierVonSatan]

A highly charge density cation (like Al³⁺) is pulling all the electrons in the water molecule closer to itself. This means the already polar O-H bond is becoming even more polarised i.e. the oxygen has an even greater share of the two electrons in the bond than normal. So in a sense it's becoming more ionic/less covalent - leaving the proton more exposed