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Wikipedia has the following reaction:

$\ce{Mg2Si + 4 HCl → 2 MgCl2 + SiH4}$

This seems to be a double displacement (metathesis) reaction, where $\ce{Si}$ has oxidation state $-4$. However, I thought that silane had silicon in the $+4$ oxidation state? For example, in this reaction: $\ce{3 SiO2 + 6 H2 + 4 Al → 3 SiH4 + 2 Al2O3}$, silicon remains in the $+4$ oxidation state.

Why is there the discrepancy between the oxidation state of silicon in silane?

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    $\begingroup$ If one reacted SiO2 with two reducing agents, why you'd expected it to not get reduced? $\endgroup$
    – Mithoron
    Dec 28, 2019 at 23:31
  • $\begingroup$ The answer seems to be in a couple of paragraphs further away. Also, have a look at Negative oxidation states of Si. $\endgroup$
    – andselisk
    Dec 28, 2019 at 23:37
  • $\begingroup$ Electronegativities: Mg < Al < Si < H < O. I would identify both reactions as redox, In the first, silicon and hydrogen change oxidation states. In the second, aluminum and hydrogen change oxidation states while silicon and oxygen don't. $\endgroup$
    – Karsten
    Dec 28, 2019 at 23:41
  • $\begingroup$ @KarstenTheis No, in the second reaction, silicon, aluminium and hydrogen all experience a change in oxidation state. Only oxygen does not experience a change in oxidation state. $\endgroup$ Dec 29, 2019 at 0:28
  • $\begingroup$ @Mithoron are you saying that Si in SiH4 goes from +4 to -4? $\endgroup$ Dec 29, 2019 at 0:38

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Magnesium silicide is a somewhat strange beast, whose electronic structure has features that cannot be accounted for by just bonding between magnesium and silicon.

Basically, magnesium silicide has some properties of a salt and some properties of an intermetallic compound. As an intermetallic compound we can expect it to reduce hydrogen, just as an elemental metal would if it were as electropositive as magnesium or even silicon (which displaces hydrogen from steam when hot).

So hydrogen could reduced by magnesium silicide, and may thereby reach the -1 oxidation state (or zero, as magnesium silicide can also form elemental hydrogen from acid or water). With hydrogen thus reduced to the -1 state in silane, silicon goes from -4 to +4 which represents the oxidation of the silicide. The magnesium, already +2 in the silicide, remains there.

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