Question

Atomic number of Cobalt is 27. The EAN of cobalt in \[{\text{N}}{{\text{a}}_3}{\text{[Co(N}}{{\text{O}}_2}{)_4}{\text{C}}{{\text{l}}_2}]\] is:
A. 35
B. 24
C. 36
D. 34

Answer 1

Hint: The EAN or effective atomic number of the central metal ion in a complex can be calculated by the formula: \[{\text{EAN}}{\text{ = Z - (O}}{\text{.N}}{\text{.) + 2 }} \times {\text{ C}}{\text{.N}}{\text{.}}\] Where, O.N. is oxidation number, C.N. is coordination number and Z is atomic number of the metal.

Complete step by step answer:
Total number of electrons associated with metal ions including the electrons donated by the ligands in a complex is called its effective atomic number.
Atomic number of Co (Z) = 27
Because of the , the entire complex has -3 charge.
Let ‘x’ be the oxidation state of cobalt. The charge on will be \[( - 1) \times 4 = ( - 4)\].
And from there we have \[( - 1) \times 2 = ( - 2)\].
So, the oxidation state becomes –
 \[{\text{x + ( - 4) + ( - 2) = - 3}}\]
\[{\text{x + ( - 6) = - 3}}\]
\[{\text{x = - 3 + 6}}\]
\[{\text{x = + 3}}\]
Therefore, O.N. = +3
And the coordination number of Co in the complex will be 6.
The effective atomic number of a central metal ion in a complex is calculated by the
formula –
\[{\text{EAN}}{\text{ = Z - (O}}{\text{.N}}{\text{.) + 2 }} \times {\text{ C}}{\text{.N}}{\text{.}}\]
\[{\text{EAN = 27 - ( + 3) + 2 }} \times {\text{ 6}}\]
           \[{\text{ = 27 - ( + 3) + 12}}\]
          \[{\text{ = 27 + 9}}\]
          \[{\text{ = 36}}\]
So, the effective atomic number is 36. Hence, option C is correct.

Additional information:
Effective atomic number is defined as the total number of electrons surrounding the nucleus of a metal atom in a metal complex. It is composed of the metal atoms electrons and the bonding electrons from the surrounding electron-donating atoms and molecules.
The Coordination number of an atom refers to the total number of atoms, ion, or molecules bonded to the atom.
Oxidation number of an atom is defined as the charge that appears on an atom while forming ionic bonds with other atoms.

Note: According to the EAN rule, a stable complex species is formed, if the EAN of the central metal atom or ion is equal to the atomic number of a noble gas which follows the central metal atom in the same period of the periodic table.