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In the complex ion $\ce{[Na(H2O)6]+}$, the sodium cation forms 6 coordinate bonds with water ligands. Typically this octahedral form is associated with $\mathrm{sp^3d^2}$ hybridization as far as I know, but in the case of sodium the $d$ orbitals aren't readily available, so how can this be described as $\mathrm{sp^3d^2}$ hybridization?

I would guess that its hybridization is $\mathrm{s^2p^4}$?

On further thought I think the d orbitals probably are accessible and that it is indeed $\mathrm{sp^3d^2}$ but this has a high hybridisation energy and so we dont see $\ce{[Na(H2O)6]+}$ in large amounts instead we see the ion solvated by partial charges and electrostatic rather than dative covalent interactions

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    $\begingroup$ Hybridisation is the wrong concept here and for most other coordination complexes. I am sorry I cannot answer right now, molecular orbital theory will give the answer though. I am confident it won't take long here. $\endgroup$ – Martin - マーチン May 16 '15 at 15:55
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    $\begingroup$ $s^2p^4$ is definitely wrong. The 2s (or 4s) and 2p (or 4p) orbitals are waaay to far removed in energy to be involved in hybridisation. As Martin said though, hybridisation is a poor model for this type of bonding interaction. $\endgroup$ – bon Apr 20 '16 at 9:11
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Solvation is the process by which a species is dissolved in a solvent. The most classic example of this is when a metal ion is dissolved in water. The electronegative oxygens in the water molecules are attracted electrostatically to the positive charge on the metal ion. A solvation shell of water molecules result.

According to this article, which written about the hydration of alkali metal ions:

In spite of many conducted studies the knowledge of the structures and bonding properties of the hydrated alkali metal ions in aqueous solution is scarce and deviating.

Also according to another letter, which reports research done on the stability of the different possible forms of the ion, there is not a consensus on only one form. This letter discusses three possible forms of the ion all in the form of $n_1 +n_2$, where $n_1$ is the number of molecules in the first shell of hydration and $n_2$ is the number of molecules in the second shell of hydration. The forms discussed are:

shells of hydration

demonstrating that the water molecules do not necessarily have to be coordinately bonded to the metal itself, but in fact that the second shell of hydration could include water molecules interacting with the water molecules in the first shell of hydration.

In short the hydration of sodium (and the other alakali metals) is not well understood yet, though it is under study.

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