# How to find the oxidation number of Co and Cr in [Co(NH3)6] [Cr(C2O4)3]?

In the question I actually got I had to name the above compound, and I know how to name it but I don't know how to find the oxidation number of the above compound(which has to be mentioned in the IUPAC name). I actually know how to find the oxidation number in complexes but here both the ions are complex ions which makes it confusing. If someone needs the name its Hexaamminecobalt(?) trioxalatochromate(?)

• So, what you propose to be the name? Nov 21 '20 at 16:11

$$\ce{Co}$$ is known to have two possible oxidation numbers in solution : +II and +III. Both make complexes with 6 $$\ce{NH3}$$. If $$\ce{Co}$$ is at +II, the charge of the complex is $$2$$+, as $$\ce{NH3}$$ is not charged. If $$\ce{Co}$$ is at +III, the charge of the complex is $$3$$+ for the same reason.
For chromium, it is similar, as $$\ce{Cr}$$ may have oxidation number +II and +III. But the ligand oxalate is charged $$-2$$. The complex of $$\ce{Cr}$$ with three oxalate ions must compensate the charges of the cobalt complex (+$$2$$ or +$$3$$). So the charge of the chromium complex has to be -$$2$$ or -$$3$$. In other words, the complex ion is either $$\ce{[Cr(C2O4)3]^{2-}}$$ or $$\ce{[Cr(C2O4)3]^{3-}}$$. Let's compare the two possibilities, and the oxidation numbers of $$\ce{Cr}$$ in these two structures.
To get a complex $$\ce{[Cr(C2O4)3]^{2-}}$$, $$\ce{Cr}$$ oxidation number should be equal to $$x$$, with $$x + 3(-2) = -2$$. So $$x$$ = $$+4$$ or +IV. And unfortunately, this oxidation number IV does not exist for $$\ce{Cr}$$ in solution. This structure has to be abandoned,
On the other hand, to get a complex $$\ce{[Cr(C2O4)3]^{3−}, Cr}$$ must have an the oxidation number $$y$$ given by : $$y +3(-2) = +3$$. So $$y$$ = $$+3$$ = +III, and this is possible and even welcome. It corresponds to one of the requirements developed previously. As a final result, the complex ion $$\ce{[Cr(C2O4)3]^{3-}}$$ has a charge $$3-$$ that can be coupled with $$\ce{Co(NH3)6^{3+}}$$.
As a consequence, both Co and Cr are at oxidation number +III in the original structure $$\ce{[Co(NH3)6][Cr(C2O4)3]}$$