Oxidation Number in Coordination Complexes

Steps:

• The [Co(NH3)6]3+ complex has an overall charge of +3.
• Each NH3 ligand is neutral, contributing 0 to the charge.
• Six NH3 ligands together contribute 0 charge.
• Thus, cobalt's oxidation number must be +3 to balance the +3 charge.

Why C is correct:

• Per oxidation number rules, the metal's charge equals the complex charge minus ligand contributions; here, +3 - 0 = +3 for Co.

Why the others are wrong:

• A (+1): Yields total +1 charge, not +3.
• B (+2): Yields total +2 charge, not +3.
• D (+6): Yields total +6 charge, not +3.

Final answer: C