What else other than zinc can reduce Cr(III) to Cr(II)?

Question

I successfully completed the synthesis of bis(chromium(II)hydrate)tetraacetate (aka chromous acetate) using zinc as the reducing agent. For my lab report, I was asked to suggest at least two other reagents which may used in place of elemental zinc in the synthesis of Cr2(H2O)2(u-OAc)4.

Would magnesium metal and manganese metal work since both their standard reduction potentials are more negative than Zn(s)? And why does zinc make the more practical choice?

I couldnt find ANY literature showing this synthesis using any other reducing agent other than zinc...

Answer 1

Wikipedia describes the reduction of $\ce{CrCl3}$ to $\ce{CrCl2}$ with zinc metal, $\ce{LiAlH4}$$^{*}$, and $\ce{H2}$. You could use any of these methods and stir with $\ce{NaOAc}$ to form $\ce{NaCl}$ and $\ce{Cr2(OAc)4}$:

$$\ce{2CrCl2 + 4NaOAc + 2H2O -> Cr2(OAc)4(H2O)2 + 4NaCl}$$

I'm not sure if magnesium or manganese would work well, but I would assume they would do the reduction. The problem is that you want something that will reduce $\ce{Cr^{3+}}$ to $\ce{Cr^{2+}}$ but not any further. Using something with a more negative reduction potential could cause overreduction. These are good ideas, and I would certainly give credit to my students for proposing them, they just aren't mentioned specifically anywhere I'm seeing.

$^*$You would need to isolate $\ce{CrCl2}$ in the case of $\ce{LiAlH4}$ reduction (or kill off any excess LAH, at least) because you don't want to reduce the acetate added after the reduction.