Question

Write down the IUPAC names of the following complexes:
${{\left( N{{H}_{4}} \right)}_{2}}\left[ Cr(N{{H}_{3}})\left( NCS \right)\left( S \right) \right]$

Answer 1

Here the primary valency of the complex is satisfied by two $N{{H}_{4}}^{+}$ ions.
The charge for $NCS{{~}^{-}}$ ion is -1 and for ${{S}^{2-}}$ is -2.

Complete Solution :
So here, a complex is given and we have to write the IUPAC name of the complex.
We know that in a coordination complex, there will be a counterion and the complex ion. The complex ion consists of central metal ions and ligands.
There are some rules framed by IUPAC for naming the complexes. So let’s memorize the rules,
The given below are the rules for naming the coordination compounds-

1. The positive entity of the coordination complex will be named first, then comes the name of the negative part.
2. The ligands should be named in the alphabetical order prior to the central metal ion.
3. Nomenclature of the anionic ligands will end with –o, and for neutral ligands the names remain as such, but there are exceptions for water which is named aqua, for ammonia, it is named as ammine etc. For positive ligands the names end with –ium. For example, $N{{O}_{2}}^{+}$-Nitronium, $N{{O}^{+}}$- Nitrosonium etc
4. In a polynuclear complex in which there are two metal atoms and the bridging ligands. The bridging ligands are represented by a prefix-$\mu$ before the name. The bridging ligands are indicated in the formulae of the complex by separating it from the complex using hyphen.
5. The oxidation state of the central metal atom will be represented in roman numerals in parentheses. But for the anionic complexes, where the central metal atom shows the negative oxidation state, the oxidation state of the central metal atom is represented using Arabic numerals.
6. Prefixes are used while naming ligands to show the number of same ligands present. The prefixes like mono, bi, tri etc. are used. If the ligand names contain numerical prefix then we use as bis, tris for representing the numbers etc.
   Example-bis (ethylene diamine)
7. If the complex ion is a cation i.e. with a positive charge then the metal will be named as such. But if the complex ion is an anion, then the name of the metal ends with the suffix –ate.
8. Points of attachment for the ambidentate ligands-Monodentate ligands with more than one atom that could coordinate with the central metal atom are called the ambidentate ligands. And they are represented either using different names like thiocyanato and isocyanato or by giving the symbol of the donor atom, which is attached with the central atom after the ligand using a hyphen.

- Now keeping these rules in mind let’s try to write the IUPAC name.
First find the counter ion, here ion is ammonium ion.
Then find the central metal ion, here chromium is the central metal ion.
Now move on to find the oxidation state of Cr.
Here the oxidation state of Cr is 3+, it exists as $C{{r}^{3+}}$
$N{{H}_{3}}$ is a neutral ligand and named as ammine, $NC{{S}^{-}}$ is thiocyanato or called as isocyanato, since N is donor atom of the ligand and ${{S}^{2-}}$ as sulfido, since they are anionic ligands.
Therefore the final name of the complex is,
Ammonium triammine isocyanato disulfide chromate (III).

Note: The names of the ligands and their charges should be known for calculating the oxidation state of central metal ions and for the nomenclature.
- The coordination complex consists of primary and secondary valencies.
- Primary valency represents the oxidation state of the central metal atom or it may be said as the number of counter ions or anions that satisfy the charge of the complex. Primary valence is ionisable valency.
- Secondary valency is non-ionisable and is satisfied by the anions, neutral molecules and sometimes by the cations and it is a specific number for each central atom or ion. The secondary valency is the coordination number of the central metal atom or ion.