Question

Octahedral ${\mathbf{\left[}\mathbf{Ni}{\left({\mathrm{NH}}_3\right)}_{\mathbf{6}}\mathbf{\right]}}^{\mathbf{2}\mathbf{+}}$is paramagnetic, whereas planar${\mathbf{\left[}\mathbf{Pt}{\left({\mathrm{NH}}_3\right)}_{\mathbf{6}}\mathbf{\right]}}^{\mathbf{2}\mathbf{+}}$ is diamagnetic, even though both metal ions are d⁸species. Explain.

Short answer

${\left[\mathrm{Ni}{\left({\mathrm{NH}}_3\right)}_6\right]}^{2+}$ - when the complex ion is formed, three lower energy orbitals are filled with all paired electrons, while higher energy orbitals remain with two unpaired electrons, thus, this complex ion is paramagnetic.

${\left[\mathrm{Pt}{\left({\mathrm{NH}}_3\right)}_6\right]}^{2+}$- when the complex ion is formed, two lower energy orbitals and two higher energy orbitals are filled with paired electrons, while one orbital stay empty, thus, this complex ion is diamagnetic.

Step-by-step solution

Given Information

Given Compounds are${\left[\mathrm{Ni}{\left({\mathrm{NH}}_3\right)}_6\right]}^{2+}\mathrm{and}{\left[\mathrm{Pt}{\left({\mathrm{NH}}_3\right)}_6\right]}^{2+}$

Diamagnetic and Paramagnetic

Unpaired electrons do not exist in diamagnetic complexes, but they do exist in paramagnetic complex central metal atoms.

Explanation

${\left[\mathrm{Ni}{\left({\mathrm{NH}}_3\right)}_6\right]}^{2+}$is octahedral complex. These six ${\mathrm{NH}}_3$molecules donate their lone pairs to form six metal-ligand bonds. This complex ion has three lower energy orbitals and two higher energy orbitals. When a complex ion is formed, three lower energy orbitals are filled with all paired electrons, while higher energy orbitals remain with two unpaired electrons, thus, this complex ion is paramagnetic.

${\left[\mathrm{Pt}{\left({\mathrm{NH}}_3\right)}_6\right]}^{2+}$is a square planar complex. These four ${\mathrm{NH}}_3$molecules donate their lone pairs to form four metal-ligand bonds. In this complex ion, two lower energy orbitals and three higher energy orbitals are separated. When a complex ion is created, two lower energy orbitals and two higher energy orbitals are filled with paired electrons, while one orbital stay empty, thus, this complex ion is diamagnetic.