Question

Complete the following statement by selecting the correct alternative from the choices given:
In the complexes ${\left[ {{\text{Fe}}\left( {{\text{CN}}} \right)} \right]^{ - {\text{3}}}}$ and ${\left[ {{\text{Pt}}\left( {{\text{en}}} \right){{\left( {{{\text{H}}_2}{\text{O}}} \right)}_2}\left( {{\text{N}}{{\text{O}}_2}} \right)\left( {{\text{Cl}}} \right)} \right]^{ + 2}}$the respective oxidation numbers of central metal atoms are:
A. ${\text{ + 3}}\,\,{\text{and}}\,\,{\text{ + }}\,{\text{4}}\,$
B. ${\text{ + 6}}\,\,{\text{and}}\,\,{\text{ + }}\,{\text{4}}\,$
C. ${\text{ + 6}}\,\,{\text{and}}\,\,{\text{ + }}\,3\,$
D. ${\text{ + }}\,{\text{3}}\,\,{\text{and}}\,\,{\text{ + }}\,3\,$

Answer 1

To determine the oxidation number of the central metal we will assume that the oxidation number of the metal is x. Then take the sum of oxidation numbers of all ligands and metal. Put them equal to the charge of the complex and determine the value of x. The value of x is the oxidation number of the metal.

Complete answer:
Metal combines with ligands to form a complex. During the formation of the complex metal gets oxidized so metal gets a positive charge. The charge of the metal is known as the oxidation number of the metal. The positive charge of the metal cancels out by anionic ligand. The remaining charge (may be positive or negative) is represented as the charge of the complex.
The oxidation number of the central metal in the${\left[ {{\text{Fe}}\left( {{\text{CN}}} \right)} \right]^{ - {\text{3}}}}$complex as follows:
The central metal in ${\left[ {{\text{Fe}}\left( {{\text{CN}}} \right)} \right]^{ - {\text{3}}}}$complex is iron.
The charge of the coordination entity is$- 3$ . The oxidation number of cyano ligand is $- 1$.
$x + \,\left( { - 1 \times 6} \right) = - 3$
$x = - 3 + 6$
$x = + 3$
So, the oxidation number of iron metal in ${\left[ {{\text{Fe}}\left( {{\text{CN}}} \right)} \right]^{ - {\text{3}}}}$complex as $+ 3$.
The oxidation number of central metal in ${\left[ {{\text{Pt}}\left( {{\text{en}}} \right){{\left( {{{\text{H}}_2}{\text{O}}} \right)}_2}\left( {{\text{N}}{{\text{O}}_2}} \right)\left( {{\text{Cl}}} \right)} \right]^{ + 2}}$complex as follows:
The central metal in ${\left[ {{\text{Pt}}\left( {{\text{en}}} \right){{\left( {{{\text{H}}_2}{\text{O}}} \right)}_2}\left( {{\text{N}}{{\text{O}}_2}} \right)\left( {{\text{Cl}}} \right)} \right]^{ + 2}}$complex is platinum.
The charge of the coordination entity is$+ 2$ . The oxidation number of chloro and nitrito ligand is $- 1$. Water and ethylenediamine are neutral ligands.
$x + \,\left( {0 \times 1} \right) + \left( {0 \times 1} \right) + \left( { - 1 \times 1} \right)\left( { - 1 \times 1} \right) = + 2$
$x = + 2 + 2$
$x = + 4$
So, the oxidation number of platinum metal in ${\left[ {{\text{Pt}}\left( {{\text{en}}} \right){{\left( {{{\text{H}}_2}{\text{O}}} \right)}_2}\left( {{\text{N}}{{\text{O}}_2}} \right)\left( {{\text{Cl}}} \right)} \right]^{ + 2}}$complex as$+ 4$.

**Therefore, option (A) ${\text{ + 3}}\,\,{\text{and}}\,\,{\text{ + }}\,{\text{4}}\,$is correct.

Note:**
The sum of the oxidation number of the metal and ligands is equal to the charge of the complex. If no charge is given on the complex then put the sum of the oxidation number of the metal and ligands, equal to the zero. The metal has a positive charge whereas the ligands can be neutral, anionic or neutral. Only the anionic ligands cancel the positive charge of the metal. The anionic ligand can be identified by their names. The name of the anionic ligand has the suffix ‘O’, ‘ato’, or ‘ito’.