Question:
$\text{A paramagnetic ion among the following, forms an outer orbital complex is:}$ $1.\ [\text{Ni(NH}_3\text{)}_6]^{2+}$ $2.\ [\text{Zn(NH}_3\text{)}_6]^{2+}$ $3.\ [\text{Cr(NH}_3\text{)}_6]^{3+}$ $4.\ [\text{Co(NH}_3\text{)}_6]^{3+}$
Solution:
$\text{HINT: In outer complex, 4d orbital participate in hybridization and in inner complex 3d participate in hybridization.}$ $\text{Explanation:}$ $\text{An orbital possessing unpaired d-orbital has the ability to exhibit paramagnetic behaviour. Since NH}_3\text{ is a weak field ligand, so it cannot force the electrons to pair up.}$ $(1)\ \text{In}\ [\text{Ni(NH}_3\text{)}_6]^{2+},\ \text{Ni is in +2 state, so its electronic configuration will be}$ $[\text{Ar}]3d^84s^0.$ $\text{Since there are two unpaired electrons in this complex and it has}$ $sp^3d^2\ \text{hybridisation.}$ $\text{So, this is an outer orbital complex which has the paramagnetic character.}$ $(2)\ \text{In}\ [\text{Zn(NH}_3\text{)}_6]^{2+},\ \text{Zn is in +2 state, so its electronic configuration will be}$ $[\text{Ar}]3d^{10}4s^0.$ $[\text{Zn(NH}_3\text{)}_6]^{2+}\text{has}\ sp^3d^2\ \text{hybridisation, it is an outer orbital complex.}$ $\text{However, due to the absence of unpaired d-orbitals, it is diamagnetic in nature.}$ $(3)\ \text{In}\ [\text{Cr(NH}_3\text{)}_6]^{3+},\ \text{Cr is in +2 state, so its electronic configuration will be}$ $[\text{Ar}]3d^34s^0$ $\text{In this complex, the (n-1)d, i.e., 3d orbital is involved in the hybridization.}$ $\text{Therefore, this complex is an inner orbital complex.}$ $\text{Due to the presence of three unpaired electrons, it is paramagnetic in nature.}$ $(4)\ \text{In}\ [\text{Co(NH}_3\text{)}_6]^{3+},\ \text{Co is in +3 state, so its electronic configuration will be}$ $[\text{Ar}]3d^64s^0.\ \text{In this complex, NH}_3\text{ is a strong field ligand and pairing of electron takes place in d orbital.}$ $[\text{Co(NH}_3\text{)}_6]^{3+}\text{ is an inner d-orbital orbital having }d^2sp^3\text{ hybridisation.}$ $\text{Due to the absence of unpaired d-orbitals, it is diamagnetic in nature.}$ $\text{Hence, option first is the correct answer.}$