Find the number of inner $-$ orbital octahedral complexes $\left({\mathrm{d}}^2{\mathrm{sp}}^3\mathrm{hybridization}\right)$ of the following.

$$\begin{gathered} [\mathrm{Co}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6{]}^{3+},[\mathrm{Fe}{\left({\mathrm{NH}}_3\right)}_6{]}^{2+}, {\left[{\mathrm{CoF}}_6\right]}^{3-}, {\left[{\mathrm{NiF}}_6\right]}^{2-},\left[\mathrm{Pt}{\left({\mathrm{NH}}_3\right)}_4\right]{\mathrm{Cl}}_2, \\ [\mathrm{Cr}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6{]}^{3+}, [\mathrm{Mn}{\left(\mathrm{CN}\right)}_6{]}^{4-},[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6{]}^{3-},{\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\right]}^{2+} \end{gathered}$$

Solution:

Water is a moderately weak ligand and in case of Co³⁺, water acts as strong ligand. So, pairing of 3d-electrons takes place and ${\left[\mathrm{Co}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{3+}$complex becomes an inner-orbital complex.

In ${\left[{\mathrm{NiF}}_6\right]}^{2-},$ Ni is in +4 oxidation state. Although Fluoride ion is a weak ligand but due to high oxidation state of Ni, pairing of electrons takes place. Therefore, it is an inner orbital complex.

In $\left[\mathrm{Pt}{\left({\mathrm{NH}}_3\right)}_4\right]{\mathrm{Cl}}_2,$ Pt is present in +2 oxidation state. Due to presence of NH₃, pairing of electrons takes place and becomes an inner orbital complex.

Water is a moderately weak ligand and in case of ${\left[\mathrm{Cr}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{3+}$, water acts as strong ligand. So, pairing of 3d-electrons takes place and complex becomes an inner-orbital complex.

${\left[\mathrm{Mn}{\left(\mathrm{CN}\right)}_6\right]}^{4-}, {\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]}^{3-}, {\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\right]}^{2+}$ are also inner-orbital complexes because of the presence of strong ligands.

Complex that are inner orbital complexes:

$$\begin{gathered} [\mathrm{Co}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6{]}^{3+} , {\left[{\mathrm{NiF}}_6\right]}^{2-} , [\mathrm{Pt}{\left({\mathrm{NH}}_3\right)}_4]{\mathrm{Cl}}_2 ,[\mathrm{Cr}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6{]}^{3+} , [\mathrm{Mn}{\left(\mathrm{CN}\right)}_6{]}^{4-} , \\ [\mathrm{Fe}{\left(\mathrm{CN}\right)}_6{]}^{3-} , [\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6{]}^{2+} \end{gathered}$$