Question

Statement − 1: EAN of \[Cr\] in its complexes is always 36.
Statement − 2: Oxidation state of \[Cr\] in \[Cr{\left( {CO} \right)_6}\] is zero.
A. Statement − 1 is true, Statement − 2 is true and Statement − 2 is the correct explanation for Statement − 1.
B. Statement − 1 is true, Statement − 2 is true and Statement − 2 is NOT the correct explanation for Statement − 1.
C. Statement − 1 is true, Statement − 2 is false.
D. Statement − 1 is false, Statement − 2 is true.

Answer 1

Hint: EAN i.e. Effective atomic number refers to the total number of electrons that surrounds the nucleus of an atom of metal in a metal complex. It comprises the electrons of the atom of metal as well as bonding electrons from the surrounding electron-donating molecules and atoms. On the other hand, oxidation number refers to the charge left on the central atom especially when all the bonding pairs of electrons get broken, with the charge allocated to the most electronegative atom.

Complete answer:
EAN value of any element depends upon the structure of the complex, it is not necessary that it is always 36. EAN value of \[Cr\]may or may not be equal to 36 depending upon the complex structure. Thus, Statement – 1 cannot be true.
Now, let us find out the oxidation state of \[Cr\] in \[Cr{\left( {CO} \right)_6}\].
Let us assume ‘x’ as the oxidation state of \[Cr\] in \[Cr{\left( {CO} \right)_6}\].
The overall charge of the complex is 0 so the sum of the oxidation states of each element in it should be 0.
Therefore:
\[
  x + 6\left( 0 \right) = 0 \\
\Rightarrow x = 0
\]
This indicates that the oxidation state of \[Cr\] in \[Cr{\left( {CO} \right)_6}\] is 0. Thus, statement – 2 is true.

Hence, the correct answer is Option D i.e. Statement − 1 is false, Statement − 2 is true.

Note:
EAN value can be calculated as: \[EAN = \left( {Atomic{\text{ }}number{\text{ }}of{\text{ }}Metal} \right) - \left( {Oxidation{\text{ }}state{\text{ }}of{\text{ }}metal} \right) + \left( {Number{\text{ }}of{\text{ }}electrons{\text{ }}donated{\text{ }}by{\text{ }}ligands} \right)\]
For instance: EAN value of \[Cr\] in \[Cr{\left( {CO} \right)_6}\] can be calculated as: $24 - 0 + \left( {2 \times 6} \right) = 36$.