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$\ce {[Al(H2O)6]^3+}$ is a well-known complex ion formed by the $\ce {Al^3+}$. I have always had the understanding that only the aqua complexes of transition metal ions, such as $\ce {[Fe(H2O)6]^3+}$, would legitimately possess dative covalent bonding between the water ligands and the central metal ion. I had always thought that the primary reason for this is that ions such as $\ce {Mg^2+}$ and $\ce {Al^3+}$ lack vacant low-lying orbitals for them to be able to accept electrons donated from the ligands.

However, after further thinking on the issue, it seems quite ridiculous to say that there are no low-lying vacant orbitals on $\ce {Al^3+}$. For example, $\ce {AlCl3}$ is a fairly good Lewis acid and is able to accept dative covalent bonds from various Lewis bases. If the $\ce {3p}$ orbital in the $\ce {Al}$ atom in $\ce {AlCl3}$ is of sufficiently low energy to accept electron donation, then clearly, the $\ce {3p}$ orbital in $\ce {Al^3+}$ would certainly be energetically accessible as well since the large magnitude of positive charge on the ion would lower the oribtal energies greatly.

The answer in this post posits that the interaction between $\ce {Na+}$ and water ligands is mostly of the electrostatic nature while there is much more covalent involvement in the complexes of the transition metal ions. However, no reasons were given as to why this was so. Perhaps, someone with expert knowledge in this area of chemical bonding would like to enlighten me on this issue.

I have also read up some literature on the aqua complexes of alkali metal ions, which have been studied in great detail due to their biochemical significance. Talekar (1975) informs that the bonding between alkali metal ions and water ligands is approximately $\ce {20}$% covalent and $\ce {80}$% ionic and he further adds that the covalent component is due to donation of electrons into vacant orbitals on the alkali metal ions. However, the computational methods used at the time to compute these data may have been quite limited and thus, it may be unreliable.

Reference

Talekar, S. V. The nature of metal-ligand bonding in the primary hydration shell and in ionophores with alkali metal ions. Biochimi. Biophys. Acta 1975, 375, 157-164.

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  • $\begingroup$ So what would you thought the bonding was? You start with imaginary Al 3+ and 6 H2O - s and 3 p orbitals of Al and 6 lone pair and they somehow mix - you can write some 3-c-4-e bonds or whatever. $\endgroup$ – Mithoron Jul 17 at 15:26
  • $\begingroup$ @Mithoron I had always thought that the bonding in this aluminium complex was mostly ionic, between the ion and the water molecules. However, I have seen some sources describing a high degree of covalency in the bonds. $\endgroup$ – Tan Yong Boon Jul 17 at 22:56

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