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I realise that there is a similar question here Difference between sodium ion and a transition metal ion dissolving in water? and it seems to answer my question, however I was reading about how aluminion ions form complexes in water, and aluminium is not a transition metal. So how can aluminium form complexes like transition metal ions, but sodium does not?

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Although less common than transition metal complexes, sodium does form complexes with some ligands, particularly oxygen based ligands.

Aqua complexes are formed in aqueous solution, the most common being $\ce{[Na(H2O)6]+}$.

Sodium forms many complexes with crown ethers, cryptands and other related ligands. For example, 15-crown-5:

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A sodium ion has no outer electrons, and only has a charge of one. The complexes it forms are mostly defined by the structure of the ligand(s) and electrostatics.

Simply speaking, a pure aluminium 3+ ion would be too highly charged to be allowed in condensed matter. The hughe electrostatic field would just rip off electrons from any unsuspecting neighbour. Complexes of Al3+ ions are partially covalent, involving ligand electrons "donated" into the outer (formally unoccupied) orbitals, making directional bonds to the ligand or to the eg. oxygen atoms in alumina.

Complexes of transition metals work the same, only they have more outer orbitals, which also have some electrons, so there are even more possibilities to form complex complexes. :-)

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