Chapter 22.7, Problem 22.7E

Interpretation Introduction

Interpretation:

${\left[Fe{\left(H_{2}O\right)}_6\right]}^{3+}$ is pale purple in color. ${\left[Fe{\left(CN\right)}_6\right]}^{3-}$ is ruby-red in color. Approximate wavelength of maximum absorption has to  be given for each complex.  Explanation has to be given whether the color shift is in expected direction.

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-orbital 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.

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

The ligand that causes larger crystal field splitting is strong ligand and the ligand that causes smaller splitting is termed as weak ligand.