Question

The EAN of cobalt in the complex ion ${\left[ {Co{{\left( {en} \right)}_2}C{l_2}} \right]^ + }$ is:
A. 27
B. 36
C. 33
D. 35

Answer 1

Hint: EAN stands for effective atomic number which means when in a metal complex metal surrounded by some ligands then the total no. of electrons to the metal atom is equal to atomic number of noble gas which is present in the same period where that metal is also present.

Formula used:
To calculate EAN following formula is used:
EAN = (Atomic number of metal – oxidation state of metal) + 2 (no. of ligands in case of monodentate ligand)

Complete step by step solution:
To find out the EAN, we should know about the basic concept of EAN. This rule is used for most of the metal complexes with the carbon monoxide. With the help of this rule, we can easily find out the number of ligands in compounds. This rule is also called the 18-electron rule.
The question given compound is ${\left[ {Co{{\left( {en} \right)}_2}C{l_2}} \right]^ + }$.
- Here metal is cobalt $\left( {Co} \right)$ whose atomic number is 27.
- Oxidation state of metal is calculated as:
$x + o + \left( { - 2} \right) = + 1$
So, the oxidation state of cobalt is 3 i.e. $x = 1 + 2 = 3$.

In the given coordination sphere ethylene diamine $\left( {en} \right)$ is a neutral bidentate ligand & chlorine $\left( {C{l^ - }} \right)$ is a negative monodentate ligand.
- No. of electrons donate by $\left( {en} \right)$ is 8 i.e. $4 \times 2$ and according to the formula no. of electrons donate by $\left( {C{l^ - }} \right)$ is 4 i.e. $\left( {2 \times 2} \right)$.
- Now, we put all these values in the given formula then we get EAN for metal; so
$EAN = \left( {27 - 3} \right) + 4 \times 2 + 2 \times 2 = 36$ .
Effective atomic number of cobalt metal in ${\left[ {Co{{\left( {en} \right)}_2}C{l_2}} \right]^ + }$ is 36 which is equal to the atomic number of krypton $\left( {Kr} \right)$ .

Note:
EAN is also known as 18 electron rule under different calculation conditions, so sometimes you may get confused during calculating no. of electrons for the metal present in the metal complex. As in 18 electron rule only valence shell electrons of metal are taken into consideration during calculation.