[saqibjahtemuri]

I have gone through a chemical called hexaflorosilicic acid. This in aqueous form give SiF6 2- ion and the formation of this ion is quite strange, as Si is filling its d orbital with electrons from F- ion.
First, is the bonding between F- and Si +4 coordinate, or its just ionic bonding.
If ionic why it can have -2 charge, but why is Si+4 not completing its all orbitals of 3 shells ?
If coordinate covalent please explain how ?
Secondly, I have read that Si hybridization is d2sp3 here. How can this be possible ?
How can Si fill 3d orbital before going to 4s ?

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[Radu]

  humm, have you thought about Si⁴⁺  capacity of polarization? It is huge, so can we talk about ionic bonds or covalent ones?  And why d²sp³? It is an Oh simmetry, involving the lowest unoccupied orbitals(3d) in its hybridization. Silicium hasn't got inner shells with l=2. ( a 2d set of orbitals). SiF₄ is a Lewis acid, that is why it reacts with F^-, because fluorine is highly elctron-withdrawing.

   P.S.: coordinate means mostly covalent

[saqibjahtemuri]

Sorry to say but things are still not clear to me. Till SiF4, its ok, as Si +4 ion forming ionic bond with F- ions, by sharing a lone pair (2e-). But according to electronic confiq rules now its octet is complete and if it wants more e- they should fill 4s2 orbital, not 3d10.
And if they have started filling 3d orbital why havnt they completed filling all 5 degenerated orbitalsor atleast 1 e- in each orbital  ?

[Radu]

  You don't need to account for the lack or  the formation of the octet in this compound: when SiF₄ is formed, even though the Lewis acidity is relatively low( due to retrodative 2p :rarrow:3d bonds), fluoride is a hard nucleophile and manages to overcome the octet stabilization and attacks with his lone pair via LUMO of silicium, which is one 3d. And 3d orbitals aren't degenerated anymore, can you tell why? ( the same reason as at Crystal field theory).
     Anyway, this compund shows best the way the energies compense and overcome metastable states, to reach a maximum stability.

   P.S: can you tell why  SiCl_6^2- doesn't form by dissolving, say, H₄SiO₄ in concentrated HCl?