Q. The stability of complexes of ${Cu}^{2 +}\ ,\ {Ni}^{2 +},\ {Co}^{2 +}\ and\ {Fe}^{2 +}\$varies in the order
A) ${Cu}^{2 +}\ > \ {Ni}^{2 +}\ > {Co}^{2 +}\ > {Fe}^{2 +}$
B) ${Cu}^{2 +}\ > {Fe}^{2 +}\ > {Ni}^{2 +} > \ {Co}^{2 +}$
C) ${Ni}^{2 +}\ > {Co}^{2 +}\ > {Fe}^{2 +} > \ {Cu}^{2 +}$
D) ${Cu}^{2 +} < \ {Ni}^{2 +} < {Co}^{2 +} < \ {Fe}^{2 +}$
Explanation
(d)
When cations have same charge but number of $d$-electrons are different then the stability (or CFSE) decreases with increase in the number of $d$-electrons. Therefore, the correct order is
$Fe^{2 +} > Co^{2 +} > Ni^{2 + \ } > Cu^{2 + \ }$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.