# Negative oxidation states of Si

According to List of oxidation states of the elements, silicon has a possible oxidation state of $-4$.

Now, I've been looking everywhere for a compound that contains Si(-IV), but I cannot find any definitive references. Perhaps one of the synthetic carbonyls of silicon has this OS? I've seen references to $\ce{[Mo(CO)_4]^{-4}}$ and $\ce{[W(CO)_4]^{-4}}$ in my research, but silicon never shows up.

I am also uncertain as to how these carbonyls come together, and whether or not they actually complex in the same way for metalloids as for the transition metals.

• Only some of d-block elements form carbonyl complexes, and well how you even found these complexes and not simple salt-like compounds of silicon? – Mithoron Sep 12 '15 at 12:59

I am not sure if Silane ($\ce{SiH_4}$) can really be considered.

But there are silicides which silicon forms with strongly electropositive metals. In these compounds, silicon has a negative oxidation state.

For magnesium silicide - $\ce{Mg_2Si}$, the oxidation state of silicon would be -4.

• For silane, there seems to be some ambivalence as to whether silicon takes on a +4 or a -4 OS. It's hard to tell since silicon is a metalloid, and hydrogen changes its oxidation state between metal and non-metal hydrides. – Hernandez Sep 12 '15 at 7:53
• Silane is definitely no good. See, it hydrolyzes to $\ce{SiO_2}$ and $\ce H_2$, hence it is Si that must have been with plus. – Ivan Neretin Sep 12 '15 at 7:54
• Silicides are good, you are right. – Ivan Neretin Sep 12 '15 at 7:57

Silane can not be considered as one of these. This is mainly because the electronegativities of silicon and hydrogen are 1.8 and 2.1 respectively.

$$\ce{Si^{-4}}$$ compounds include compounds of the form $$\ce{SiM2}$$ where $$\ce{M}$$ is a group 2 metal as $$-4 + 2\cdot 2 = 0$$. Examples of these include magnesium silicide $$(\ce{Mg2Si})$$, dicalcium silicide $$(\ce{Ca2Si})$$ and distrontium silicide $$(\ce{Sr2Si})$$!