What is the IUPAC name of the complex [Co(NH3)2Cl(ONO)(py)2]NO3? (py = pyridine)
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Diamminechloridonitrito-O-dipyridinecobalt(III) nitrate
The complex ion \( \ce{[Co(NH3)2Cl(ONO)(py)2]^+} \) is the cation, so it is named first and the nitrate counter ion last. Inside it, the ligands are named in alphabetical order before the metal.
Oxidation state of Co: NH3 and py are neutral, Cl− and ONO− carry −1 each, and the cation carries +1: x + 2(0) + (−1) + (−1) + 2(0) = +1, so x = +3.
Ligand names: NH3 = ammine; Cl− = chlorido; the nitrite ion bound through oxygen (–ONO) = nitrito-O; py = pyridine.
Alphabetical order (multiplying prefixes ignored): ammine, chlorido, nitrito-O, pyridine.
Prefixes: two NH3 = diammine; two pyridine = dipyridine (the name pyridine has no numerical prefix, so “di” is used, not “bis”).
Metal: in a complex cation the metal keeps the element name: cobalt(III).
Why the other options are incorrect
Option (1) Diamminechloridodipyridinenitrito-N-cobaltate(III) nitrate — Incorrect. “Cobaltate” is used only for a complex anion; “nitrito-N” would mean binding through N, but ONO binds through O; and pyridine is placed before nitrito, out of alphabetical order.
Option (2) Diamminechloridonitrito-O-bipyridinecobalt(III) nitrate — Incorrect. “Bi” is not one of the prefixes (di, tri, tetra …) that count ligands; two pyridine ligands are “dipyridine”.
Option (4) Chloridodiamminenitrito-O-bis(pyridine)cobalt(III) nitrate — Incorrect. The ligands are out of order: ammine comes before chlorido alphabetically (the prefix “di” is not counted).
NCERT (2026-27 edition):
5.3.2 Naming of Mononuclear Coordination Compounds – “(ii) The ligands are named in an alphabetical order before the name of the central atom/ion. (This procedure is reversed from writing formula).”
“(iv) Prefixes mono, di, tri, etc., are used to indicate the number of the individual ligands in the coordination entity. When the names of the ligands include a numerical prefix, then the terms, bis, tris, tetrakis are used, the ligand to which they refer being placed in parentheses.”
“(vi) If the complex ion is a cation, the metal is named same as the element. For example, Co in a complex cation is called cobalt and Pt is called platinum.”
5.4.3 Linkage Isomerism – “… Jørgensen discovered such behaviour in the complex [Co(NH3)5(NO2)]Cl2, which is obtained as the red form, in which the nitrite ligand is bound through oxygen (–ONO), and as the yellow form, in which the nitrite ligand is bound through nitrogen (–NO2).”