Question

(i) What are the postulates of Werner's theory?
(ii) Write the IUPAC name of \[{K_3}[Fe{(CN)_6}]\], \[[Co{(N{H_3})_6}]C{l_3}\].

Answer 1

Hint: i) Werner's theory is responsible for the formation of structures of various complex compounds. It defines the geometrical arrangements of the different coordinated groups around the central cation. It also explains the cause behind the optical and geometrical isomerism of these complex compounds.
ii) The IUPAC naming of the coordination compound will provide a method to describe the name of the chemical or coordination compound in a systematic way. The given compounds are also named with reference to the rules set up by IUPAC.

Complete Step by step answer:
The important postulates of Werner's theory are:
- In a complex coordination compound, there are two types of valencies that are shown by the central metal or the metal ion. They are the primary and secondary valencies.
- The oxidation state is given by the primary valency and the coordination number is given by the secondary valency.
- The coordination number is fixed for every metal atom as the secondary valences are fixed on the complex compound.
- The metal atom makes sure that the primary and secondary valencies are satisfied. The primary valency is satisfied by the negative ion. And on the other hand, secondary valency is satisfied by the negative ion or the neutral molecule.
- The fixed position in space is pointed by the secondary valency. And due to this, the coordinate compound has a definite geometry. For example, let us consider the case of a metal ion having four secondary valencies. These are arranged around the central metal ion in either tetrahedral or square planar arrangement. When the secondary valency of the central metal is six, it will have an octahedral arrangement.
- The primary valency, on the other hand, is found to be non-directional.

ii) The rules of complex nomenclature are given below:
Rule 1: When we are naming a compound, we must always first name the cation before the anion.
Rule 2: When we find that there are multiple types of ligands present in any complex compound, we must name the ligands in alphabetic order after the name of central metal atom/ion.
We end the name of the anionic ligands with ‘o’.
And for the neutral ligands we use their common name.
Rule 3: When the names of the ligands have a numerical prefix, then we have to use the terms like bis, tris, tetrakis.
Rule 4: After naming the ligand in the alphabetic order, we have to name the central metal atom/ion.
If the complex ion is a cation, then we name the metal the same as the element.
If the complex is an anion, then the name of metal ends with the suffix -ate for its Latin name.
Rule 5: We give the oxidation state of the metal in the complex as a roman numeral in parentheses after writing the name of the metal.
Rule 6: The name of the neutral complex molecule is similar to that of the complex cation.

Let us now see how to give the IUPAC name to the given complex compounds.
- \[{K_3}[Fe{(CN)_6}]\]
In the given chemical compound, the cation present is potassium. Its IUPAC name would start from potassium. As the ligand that is present in the given compound is the -CN group, we have to name it cyano which we get from the name cyanide. Since there are six molecules of cyanide ions in the chemical compound therefore, we have to write as hexacyano. Now, the central atom is iron and has a +3-oxidation state. Therefore, the IUPAC name of the compound becomes potassium hexacyanoferrate (III).

- \[[Co{(N{H_3})_6}]C{l_3}\]
In this chemical compound, there is no main cation present rather a main anion is present. The ligand present in the compound is \[N{H_3}\] group, which is named as ammine which we get from the name of the group, amine. And since there are 6 molecules of amine, we write hexammine. Now, the central atom is cobalt which has a +3-oxidation state. Therefore, the name becomes hexaamminecobalt (III). Now the main anion present is the chloride ion. Hence, the IUPAC name of the compound becomes hexaamminecobalt (III) chloride.

Note: There are some limitations of the Werner's theory:
- It did not explain the directional properties of bonds in coordination compounds.
- It does not explain the colour, the magnetic and optical properties shown by the coordination compounds.
- It did not explain the inability of other elements to form coordination compounds.

ii) In the above given compound, students tend to get confused with \[{K_3}[Fe{(CN)_6}]\] and \[{K_4}[Fe{(CN)_6}]\]. We have to note that the IUPAC name of the first compound is Potassium hexacyanoferrate (III) and that of latter is potassium hexacyanoferrate (II). There is the difference in oxidation states.