What are the oxidation numbers of the central metal atoms in [Co(NH3)…

The correct answer is Option (3) → +3 and +2

The oxidation number of the central atom is the charge it would carry if all the ligands were removed along with their shared electron pairs. So the metal’s oxidation number plus the charges of all the ligands must equal the charge of the coordination entity.

• \( \ce{[Co(NH3)5(CO3)]Cl} \): one Cl⁻ is outside the bracket, so the entity is \( \ce{[Co(NH3)5(CO3)]^+} \). NH₃ is neutral; carbonate, \( \ce{CO3^2-} \), carries −2.

      \( \displaystyle x + 5(0) + (-2) = +1 \quad \Rightarrow \quad x = +3 \)

• \( \ce{[Pt(NH3)2Cl(NO2)]} \): a neutral entity. NH₃ is neutral; Cl⁻ and \( \ce{NO2-} \) carry −1 each.

      \( \displaystyle y + 2(0) + (-1) + (-1) = 0 \quad \Rightarrow \quad y = +2 \)

Their IUPAC names show the same values: pentaamminecarbonatocobalt(III) chloride and diamminechloridonitrito-N-platinum(II).

NCERT (2026-27 edition):
5.2 (g) Oxidation number of central atom – “The oxidation number of the central atom in a complex is defined as the charge it would carry if all the ligands are removed along with the electron pairs that are shared with the central atom. The oxidation number is represented by a Roman numeral in parenthesis following the name of the coordination entity.”

Example 5.3 – “Write the IUPAC names of the following coordination compounds:” … “(a) diamminechloridonitrito-N-platinum(II)” … “(d) pentaamminecarbonatocobalt(III) chloride”