Question

The formula of the complex, tris – (ethylenediamine) cobalt (III) sulphate is:
A. $\left[ {{\text{Co}}{{\left( {{\text{en}}} \right)}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}} \right]$
B. $\left[ {{\text{Co}}{{\left( {{\text{en}}} \right)}_3}{\text{S}}{{\text{O}}_{\text{4}}}} \right]$
C. ${\left[ {{\text{Co}}{{\left( {{\text{en}}} \right)}_3}} \right]_2}{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)_3}$
D. \[{\left[ {{\text{Co}}{{\left( {{\text{en}}} \right)}_3}} \right]_2}{\text{S}}{{\text{O}}_{\text{4}}}\]

Answer 1

Hint: The given compound is a coordination compound.
According to the IUPAC rules, the formula of the cation which may be simple or complex is written first followed by the formula of the anion.

Complete step by step answer:
According to the IUPAC rules, the formula of the entire coordination entity which may be charged or neutral is written in square brackets.
Within the coordination sphere, the symbol of the metal atom is written at first. It is then followed by the symbols or the formulas of the ligands.
To determine the number of cations or anions, the charge carried by each type of ion must be known. This is done as:
- The simple ion whether cation or anion behaves like a free ion and carries usual charge as one radical.
- The charge on the complex ion is calculated on the basis that it is equal to the charge on the central metal atom plus the total charge carried by the ligands.
- The total positive charge must be equal to the total negative charge since the complex as a whole is electrically neutral. The cations and anions are then multiplied by constants till the charges become equal. These constants will give the number of each type of ions.
Now, the given compound is tris – (ethylenediamine) cobalt (III) sulphate.
The simple ion is the anion sulphate or ${\text{S}}{{\text{O}}_{\text{4}}}^{{\text{2 - }}}$ .
For the complex ion, writing first the symbol for metal atom and then the ligands we have:
Complex ion or cation ${\text{ = }}{\left[ {{\text{Co}}{{\left( {{\text{en}}} \right)}_3}} \right]^{\text{x}}}$ where x is the charge on the complex ion.
x is equal to the oxidation state of cobalt plus the charge on the ethylenediamine ligands. Since cobalt is in 32 oxidation state and ethylenediamine or en is a neutral ligand and hence carries no charge, we have:
${\text{x}} = + 3 + 3 \times 0 = + 3$
Now, charge on one anion is -2 and charge on the cation is +3. So, 3 anions will neutralize the charge of 2 cations.
So, the formula of tris – (ethylenediamine) cobalt (III) sulphate will be ${\left[ {{\text{Co}}{{\left( {{\text{en}}} \right)}_3}} \right]_2}{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)_3}$ .

Note:
Within the coordination sphere, the symbols of the ligands which follow the symbol of the metal atom are arranged alphabetically according to their names. This alphabetical arrangement is irrespective of the charge present on the ligands.
For example, for the coordination compound tetrammineaquachloridocobalt (III) chloride, the formula is written as $\left[ {{\text{Co}}{{\left( {{\text{N}}{{\text{H}}_{\text{3}}}} \right)}_{\text{4}}}\left( {{{\text{H}}_{\text{2}}}{\text{O}}} \right){\text{Cl}}} \right]{\text{C}}{{\text{l}}_2}$ where the three ligands present are ammine, aqua and chloro.