Chapter 22, Problem 22.36QP

Interpretation Introduction

Interpretation:

Of the complexes – ${\left[M{F}_6\right]}^{3-},{\left[M{\left(CN\right)}_6\right]}^{3-},{\left[M{F}_6\right]}^{4-},{\left[M{\left(CN\right)}_6\right]}^{4-}$ the complex in which the d-electrons are distributed as given in the problem has to be identified.

Concept Introduction:

The term Crystal field splitting refers to segregation of d-orbitals in to higher energy orbitals and lower energy orbitals when ligands approaches metal ion to co-ordinate.  Crystal field splitting is said to be larger when the energy gap between higher energy d-orbitals and lower energy d-orbitals is larger.  Crystal field splitting is said to be smaller when the energy gap between higher energy d-orbitals and lower energy d-orbitals is smaller.

Spectrochemical series refers to the arrangement of ligands with respect to crystal field splitting they cause in the d-orbitals of a metal ion.  The ligand that causes larger splitting pattern is referred to as stronger ligand.

The ligands are arranged in increasing order of crystal field splitting they produce.

$\begin{array}{l}{I}^{-}<B{r}^{-}<S^{2-}<SC{N}^{-}<C{l}^{-}<N{O}_3{}^{-}<N_3{}^{-}<F^{-}<O{H}^{-}<C_2{O}_4{}^{2-}<H_{2}O<NC{S}^{-}\\ <C{H}_{3}CN<py<N{H}_3<en<bipy<phen<N{O}_2{}^{-}<PP{h}_3<C{N}^{-}<CO\end{array}$

Pairing energy refers to the energy required to pair the electrons - that is energy required to make two electrons to be paired in same orbital with opposite spins.

If the pairing energy of a complex is high it means the electrons are difficult to be paired and so the complex will be high spin complex with unpaired electrons.  If the pairing energy of a complex is low it means the electrons are readily paired and so the complex will be low spin complex with paired electrons.

When ligands approach the metal ion the degeneracy in d-orbitals of the metal ion is destroyed and they split into two different energy levels. In case of tetrahedral complex, the $d_{x^2-y^2}and{d}_{z^2}$ orbitals occupy lower energy level and the $d_{xy},d_{xz},d_{yz}$ orbitals occupy higher energy level.  The energy gap between these two set of orbitals is termed as crystal field splitting energy.