In the peroxo complex K3[Cr(O2)4] each O2 unit is a peroxide ligand (O2^2-). The oxidation number of chromium is

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In the peroxo complex $K X_{3} [Cr (O X_{2}) X_{4}]$ each $O X_{2}$ unit is a peroxide ligand ( $O X_{2} X^{2 -}$ ). The oxidation number of chromium is

The anion is

[Cr (O X_{2}) X_{4}] X^{3 -}

. So

x + 4 (- 2) = - 3

gives

x = + 5

. Treating the eight O atoms as

- 2

each would wrongly give

x = + 13