Question

Which one of the following is an example of octahedral complex?
A.$\left[ {Fe{F_6}^{3 - }} \right]$
B. ${\left[ {Zn{{\left( {N{H_3}} \right)}_4}} \right]^{2 + }}$
C.${\left[ {Ni{{\left( {CN} \right)}_4}} \right]^{2 - }}$
D.${\left[ {Cu{{\left( {N{H_3}} \right)}_4}} \right]^{2 + }}$

Answer 1

Octahedral molecular geometry describes the shape of the compounds wherein six atoms or groups of atoms or ligands are symmetrically arranged around a central atom. The octahedron has eight faces, hence the prefix $octa -$.

Complete answer:
The term octahedral is used by the chemists to focus on the geometry of the bonds to the central atom and not considering differences among the ligands themselves
In $\left[ {Fe{F_6}^{3 - }} \right]$ complex ion, the oxidation state of central iron metal is $+ {\mathbf{3}}$. The atomic number of iron metal is 26 and that of ion (III)ion is 23. The valence shell electronic configuration of Fe(III) ion is [Ar] d5. The coordination number of the central metal Fe(III)ion is 6 and the arrangement of $\left[ {Fe{F_6}^{3 - }} \right]$complex ion is octahedral.
Now ligand ${{\mathbf{F}}^ - }$ -, which is a weak field ligand and under the influence of the octahedral crystal field, the five degenerate d-orbitals of Fe(III) ion are splitted up into two sets of energetically different orbitals. These two sets of orbitals are energetically lower ${t_2}g$ orbital and energetically higher eg orbital. Since ligand ${{\mathbf{F}}^ - }$, is a weak field ligand, hence the complex is high spin one.
So, under the influence of the octahedral crystal field, the possible electronic arrangement of Fe(III) ion is ${t_2}{g_3}$, $e{g_2}$.
From the octahedral crystal field splitting of $\left[ {Fe{F_6}^{3 - }} \right]$ complex ion, it is evident that the Fe(III) ion have five unpaired electrons in its outer 3d-orbital.
So, six ligands are required for an octahedral complex. There are 6 F- molecules in $\left[ {Fe{F_6}^{3 - }} \right]$.

Therefore, the correct answer is option (A).

Note:
The concept of octahedral coordination geometry was developed by Alfred Werner to explain the stoichiometries and isomerism in coordination compounds. His insight allows chemists to rationalize the number of isomers of coordination compounds. Octahedral transition-metal complexes containing amines and simple anions are often referred to as Werner-type complexes. The term "octahedral" is used somewhat loosely by chemists, focusing on the geometry of the bonds to the central atom and not considering differences among the ligands themselves.