Select the correct statements about oxidation states in the following species.

AMn in $MnO X_{4} X^{-}$ is in the highest oxidation state ( $+ 7$ ) shown by 3d metals
BCr in $CrO X_{5}$ (one oxo and two peroxo groups) is in the $+ 6$ state
CNi in $Ni (CO) X_{4}$ is in the $+ 4$ state
DCu in $[Cu (NH X_{3}) X_{4}] X^{2 +}$ is in the $+ 2$ state

Answer: A, B, D

(a) Correct:

x - 8 = - 1

gives

+ 7

, equal to the number of valence electrons of Mn. (b) Correct: the two peroxo groups contribute four O atoms at

- 1

each and the oxo group one O at

- 2

, so

x + 4 (- 1) + (- 2) = 0

gives

+ 6

. (c) Wrong: CO is neutral, so Ni is

0

. (d) Correct:

x + 0 = + 2