   Chapter 21, Problem 57E

Chapter
Section
Textbook Problem

# Rank the following complex ions in order of increasing wavelength of light absorbed. Co ( H 2 O ) 6 3 + ,  Co ( CN ) 6 3 − ,  CoI 6 3 − ,  Co ( en ) 3 3 +

Interpretation Introduction

Interpretation: The given complex ions are to be ranked in order of increasing wavelength of the light absorbed.

Concept introduction: The capacity to absorb light by complex ion is directly proportional to crystal field splitting energy and energy absorbed is inversely proportional to the wavelength.

To rank: The complex ions Co(H2O)63+,Co(CN)63,CoI63 and Co(en)33+ in order of increasing wavelength of the light absorbed.

Explanation

Explanation

The spectrochemical series (crystal field splitting energy) of strong to weak ligand is,

CN>NO2>en>NH3>H2O>OH>F>Cl>SCN>Br

Hence, order of increasing crystal field splitting energy for complex ions Co(H2O)63+,Co(CN)63,CoI63 and Co(en)33+ is,

CoI63<Co(H2O)63+<Co(en)33+<Co(CN)63

The energy absorbed by light is given by the expression,

E=hcλ

Where,

• E is the energy absorbed.
• h is the Planck’s constant.
• c is the velocity of light.

It is clear from the above equation that, energy absorbed is inversely proportional to the wavelength. It means that higher the energy, lower will be the wavelength

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