The IUPAC name for the complex, [Co(NH3)2Cl(ONO)(Py)2] NO3 is:

The correct answer is option 3. Diamminechloridonitrito–O–dipyridine cobalt (III) nitrate.

The IUPAC name for the complex, \([Co(NH_3)_2Cl(ONO)(Py)_2] NO_3\) is Diamminechloridonitrito–O–dipyridine cobalt(III) nitrate.

The name is broken down as follows:

• Diammine - This indicates that there are two ammonia ligands (NH₃) coordinated to the cobalt center.
• chlorido - This indicates that there is one chloride ligand (Cl^-) coordinated to the cobalt center.
• nitrito–O– - This indicates that there is one nitrite ligand (ONO^-) coordinated to the cobalt center, with the oxygen atom bonded to the cobalt center.
• dipyridine - This indicates that there are two pyridine ligands (Py) coordinated to the cobalt center.
• cobalt(III) - This indicates that the oxidation state of cobalt in the complex is +3.
• nitrate - This indicates that the complex is also coordinated to a nitrate ion (NO₃^-).

The complete name is therefore: (3) Diamminechloridonitrito–O–dipyridine cobalt(III) nitrate.

The other options are incorrect because they do not correctly represent the structure of the complex or the oxidation state of cobalt.