Question

The IUPAC name of the complex $\left[ Co{{(N{{H}_{3}})}_{5}}(N{{O}_{2}}) \right]C{{l}_{2}}$is:
[A] nitrito-N-pentaammine cobalt (III) chloride.
[B] nitrito-N-pentaammine cobalt (II) chloride.
[C] pentaammine nitrito-N-cobalt (II) chloride.
[D] pentaammine nitrito-N-cobalt (III) chloride.

Answer 1

Hint: We can get the correct answer by naming the ligands followed by the metal atom and then the chloride present outside the coordination sphere. The ligands should be written in alphabetical order as there are two ligands here followed by the metal centre and its oxidation number and then ass chloride at the end.

Complete answer:
While writing the IUPAC names of a complex, there are certain rules that we need to follow.
Firstly we have to know that if we have water as a ligand in the complex, while naming it according to IUPAC we write it as aqua and not water. Similarly, there are such names for every ligand for example,\[N{{O}_{2}}^{-}\] is nitrito and for $N{{H}_{3}}$, we use the name ammine.
According to IUPAC, the order of writing the name of the complex should be-
Ligand name + Metal name + ate (if the complex is anionic)/ no specific ending is required if it is cationic.

There are several sub-rules which we have to follow to write the ligand and the metal name-
- Generally, for the anions which ends with “-ide”, “-ate”, “-ite” the anionic ligand ends with “-o”, “-ite” and “-ito” respectively.
-The name of the ligand should be written in alphabetical order i.e. if we have a chloride and an amine in the complex we have to write the ammine first followed by chloride.
-For more than one ligand, we use prefixes like di, tri, tetra but we also use bis, tris and tetra in certain cases where the above prefixes are already used. For example if we have two ethylene diamine ligands, we write it as bis-(ethylenediamine).
- We have to write the name of the cations before the anion always. If we have a cation outside the coordination sphere and the complex is anionic, we write the cation first followed by the anion.
-We should not leave any space between the ligand name and the metal name. If the ligand is ambidentate, we mention the atom to which the metal is bonded. For example, if we have a complex having\[N{{O}_{2}}^{-}\] as the ligand and cobalt as the metal centre, we can write it as (–N-Cobalt).
-If the complex is anionic, we have to add “-ate” in the end followed by the oxidation number of the metal ion. If the complex is cationic, there is no such specific rule.
-While writing any species outside the coordination sphere, we should leave a space.

By following the above rules, we can write the IUPAC name of the complex $\left[ Co{{(N{{H}_{3}})}_{5}}(N{{O}_{2}}) \right]C{{l}_{2}}$ as
pentammine (for 5 amine ligands) + nitrito (for 1\[N{{O}_{2}}^{-}\]ligand) + (-N-cobalt (III)) (the oxidation state on cobalt is 3 as amine is neutral, both \[N{{O}_{2}}^{-}\] and chlorine have -1 charge each) + chloride (for the chlorine present outside the coordination sphere).
Therefore, the name will be pentaammine nitrito-N-cobalt(III) chloride.

So, the correct answer is “Option D”.

Note: A metal atom associated with other ligands which is a group of neutral or anionic species is a complex. Coordination compounds are generally neutral but they have at least one ion present as a complex.