Question

Which among the following is the diamagnetic species :
A. ${\left[ {{\text{Co}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_6}} \right]^{2 + }}$
B. ${\left[ {{\text{Ni}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_6}} \right]^{2 + }}$
C. ${\left[ {{\text{Co}}{{\left( {{\text{N}}{{\text{H}}_3}} \right)}_6}} \right]^{3 + }}$
D. ${\left[ {{\text{Ni}}{{\left( {{\text{N}}{{\text{H}}_3}} \right)}_6}} \right]^{3 + }}$

Answer 1

Hint: Diamagnetism is a quantum mechanic effect that is found in all material, but for substance to be termed ‘’diamagnetic’’ it must be the only contribution to the matter’s magnetic effect.
To find a diamagnetic substance try to examine the number of atoms of each element in a given compound. If the substance has unpaired electrons then it is paramagnetic and if the electrons are paired with one another then the substance is diamagnetic.

Complete step by step answer: In chemistry, diamagnetic indicates that a substance contains no unpaired electrons and is not attracted to a magnetic field.
So, we will take each of the given options one by one and examine their structural formula and the nature and number of the electrons of each element in the compound.
-${\left[ {{\text{Co}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_6}} \right]^{2 + }} \Rightarrow $There are three unpaired electrons and calculated value of magnetic moment is $3.87{\text{BM}}$ which is quite different from the experiment value of $4.40{\text{BM}}$. It is Paramagnetic.
-${\left[ {{\text{Ni}}{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_6}} \right]^{2 + }} \Rightarrow $${\text{N}}{{\text{i}}^{2 + }}(3{{\text{d}}^8})$ion contains two unpaired electrons irrespective of the strength of the ligand field strength. Hence, the magnetic moment is $2.83{\text{BM}}$.It is Paramagnetic.
-${\left[ {{\text{Co}}{{\left( {{\text{N}}{{\text{H}}_3}} \right)}_6}} \right]^{3 + }} \Rightarrow $${\text{C}}{{\text{o}}^{3 + }}$ion has six electrons in d- subshell(one lone pair and four bond pairs), as ${\text{N}}{{\text{H}}_3}$ is a strong field ligand all six electrons get paired, makes this complex diamagnetic.
-${\left[ {{\text{Ni}}{{\left( {{\text{N}}{{\text{H}}_3}} \right)}_6}} \right]^{3 + }} \Rightarrow $${\text{Ni}}$has an electronic configuration of $4{{\text{s}}^2},3{{\text{d}}^8}$ therefore, ${\text{N}}{{\text{i}}^{3 + }}$ has an electronic configuration of $4{{\text{s}}^0},3{{\text{d}}^7}$. Electrons always removed the $4{\text{s}}$atomic orbital in the $3{\text{d}}$orbital. It has one unpaired electron. It is Paramagnetic.
Hence, the correct answer is C.

Additional Information: The crystal field theory (CFT) is an electrostatic model which considers the metal-ligand bond to be ionic arising purely from electrostatic interactions between the metal ion and the ligand. CFT describes the strength of metal ligand bond and thus with the help of strength of the bond, the energy of the system is altered which in turn changes the magnetic and other properties of the compound.

Note: Firstly calculate the total number of electrons in the given compound then write their electronic configurations and check for unpaired electrons in the outermost subshell of each element. Also if the number of electrons comes to be odd, then the substance is paramagnetic and if the number is even then it is diamagnetic with an exception of numbers $10$ or $16$.