Subject Chemistry

Paper No and Title Paper 7: Inorganic Chemistry-II (Metal-Ligand Bonding, Electronic Spectra and Magnetic Properties of Transition Metal Complexes) Module No and Title 12, Electronic spectra of coordination complexes IV

Module Tag CHE_P7_M12

CHEMISTRY Paper 7: Inorganic Chemistry-II (Metal-Ligand Bonding, Electronic Spectra and Magnetic Properties of Transition Metal Complexes) Module 12, Electronic spectra of coordination complexes IV

TABLE OF CONTENTS

1. Learning outcomes

2. Introduction

3. Emission spectrum

4. Absorption spectrum

5. Electronic spectra of transition metal complexes

6. Selection rules and their breakdown

6.1 Selection rules

6.2 Breakdown of the selection rules

7. Summary

CHEMISTRY Paper 7: Inorganic Chemistry-II (Metal-Ligand Bonding, Electronic Spectra and Magnetic Properties of Transition Metal Complexes) Module 12, Electronic spectra of coordination complexes IV

1. Learning Outcomes

After studying this module, you shall be able to

 Know various selection rules for the transitions  Learn about various factors involved in the breakdown of the selection rules  Identify various aspects of the absorption and emission spectrum  Analyze the transition probability of the complexes

2. Introduction

Spectrum in general for various species (compounds, complexes and elements) can be correlated in the following two ways:

(a) Absorption spectrum: This type of spectrum is obtained when absorption of a radiation occurs by the molecule, atom or any absorbing species, which leads to transition of the electron(s) from the lower to the higher level.

Figure 1. Absorption and emission phenomenon taking place

CHEMISTRY Paper 7: Inorganic Chemistry-II (Metal-Ligand Bonding, Electronic Spectra and Magnetic Properties of Transition Metal Complexes) Module 12, Electronic spectra of coordination complexes IV

(b) Emission spectrum: This is just reverse of the absorption spectrum in which transition from the higher to the lower level occurs due to the emission of radiation by electrons to the radiation field. In case no radiation is observed then the transition from higher to lower level is termed as non-radiate decay.

3. Emission spectrum

Emission spectra are of three types namely,

(1) Continuous spectrum

(2) Band spectrum

(3) Line spectrum

Continuous spectra: The continuous spectrum for a substance is obtained when the molecules of the substance are thermally excited by providing large amount of heat. For example, when solids such as iron, aluminum, carbon is heated upto the extent that they start blazing. It is the spectrum of electromagnetic waves that involves a wide range (continuous range) of wavelengths. In the visible portion of electromagnetic spectrum, a continuous spectrum would look like a rainbow.

Figure 2. The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption

Band spectrum: When the thermal excitation given to the molecules of a substance is not enough to lead to a continuous spectrum, we get a band type of spectrum. In this case, we get spectrum in the form of huge number of bands, different in color. Basically the spectrum consists of a large number of closely spaced lines which appear to give a look of different colored bands. The spectrum is sharp at one end which fades away at the other end. This sharp end is called the head whereas the faded region is the tail (Figure 3).

CHEMISTRY Paper 7: Inorganic Chemistry-II (Metal-Ligand Bonding, Electronic Spectra and Magnetic Properties of Transition Metal Complexes) Module 12, Electronic spectra of coordination complexes IV

Figure 3. Band emission spectrum of the nitrogen gas

Line spectrum: Line spectrum is given by the elements in their vapor form. It consists of lines at different intervals of the spectrum against dark background. The intensity of lines is mostly different from each other. The molecular and electronic arrangement of the elements leads to this type of line spectrum. Hence, two elements can not have same type of line spectrum (Figure 4).

Figure 4. Thin or low-density cloud of gas emits light only at specific wavelengths that depend on its composition and temperature, producing a spectrum with bright emission lines

CHEMISTRY Paper 7: Inorganic Chemistry-II (Metal-Ligand Bonding, Electronic Spectra and Magnetic Properties of Transition Metal Complexes) Module 12, Electronic spectra of coordination complexes IV

4. Absorption spectrum

In order to have the absorption spectrum, sample is placed between a light source and the spectrum recorder which measures the amount, intensity and frequency of the light absorbed by the sample (Figure 5). The part of the spectrum absorbed by the sample hence correlates to the absorption spectrum of the sample. Electronic absorption spectrum for the transition metal complexes are of two types namely d-d transition and charge transfer transition spectrum. As the name suggest, in case of d-d spectrum the transitions occur within the d orbitals and in case of charge transfer spectrum, the transitions take place from ligand to the metal or vice-versa .

Figure 5. Basic absorption spectrometer

5. Electronic spectra of transition metal complexes

There are various principles governing the transitions occurring between various electronic levels in transition metal complexes. Some of these principles have been discussed below.

(a) Frank-Codon principle: The time taken by the electronic transition is quite small that is in the order of 10-15 sec and thus the molecule or the atom undergoing transition does not have enough time to appreciably change its position or else configuration. Because of this it can be said that the vibrational kinetic energy of the species undergoing transition will be same in the excited state and ground state. Also, an alternate statement can be said that since the movement of the nuclei during the course of transition is negligible, the internuclear distances do not alter and the most probable transitions are those which occur vertically.

CHEMISTRY Paper 7: Inorganic Chemistry-II (Metal-Ligand Bonding, Electronic Spectra and Magnetic Properties of Transition Metal Complexes) Module 12, Electronic spectra of coordination complexes IV

Figure 6. Frank-Codon principle

(b) Electronic transitions between transition states: Quite often, transitions arise from the ground vibrational level of the ground electronic state to several other vibrational levels of potential excited electronic states. These transitions give rise to vibrational fine structure in the foremost peak of the electronic transition. Since all the molecules are present in the ground vibrational level, nearly all transitions that give rise to a peak in the absorption spectrum will arise from the ground electronic state. If the different excited vibrational levels are represented as υ1, υ2, etc., and the ground state as υ0, the fine structure in the main peak of the spectrum is assigned to υ0 → υ0 , υ0 → υ1, υ0 → υ2 etc., vibrational states. The υ0 → υ0 transition is the lowest energy (longest wave length) transition.

Figure 7. Basic absorption spectrum which gets vibrational fine structure when resolved CHEMISTRY Paper 7: Inorganic Chemistry-II (Metal-Ligand Bonding, Electronic Spectra and Magnetic Properties of Transition Metal Complexes) Module 12, Electronic spectra of coordination complexes IV

(c) Transition probability: For absorption spectrum to appear light must be absorbed by the molecule. Light will be absorbed if it can interact with the molecule by its oscillating electric and magnetic fields. For this to take place the dipole moment of the molecule must change during the time of transition. If the change in dipole moment is quite large, number of photons will be absorbed which further lead to intense bands of large area. Otherwise, small change in the dipole moment will lead to less absorption of photons and hence lesser area and weak absorption bands with low intensity.

Figure 8. The vibrations that lead to change in dipole moment, taken from

CHEMISTRY Paper 7: Inorganic Chemistry-II (Metal-Ligand Bonding, Electronic Spectra and Magnetic Properties of Transition Metal Complexes) Module 12, Electronic spectra of coordination complexes IV

6. Selection rules and their breakdown

6.1 Selection rules

There are various selection rules that govern the feasibility of a transition for transition metal complexes. Some of the most important selection rules have been listed below.

(1) Laporte selection rule: This rule states that for a molecule having centre of symmetry, transitions within the same sub-shell are forbidden. As per this rule the p-p or d-d transitions are forbidden. Mathematically, the rule can be stated in the form of an equation.

For any transition to take place, change in the value of total orbital angular momentum between the final and initial stage should be

ΔL = ±1 (ΔL = Lf – Li)

Where ΔL is equal to Lf (total angular momentum of the final state) subtracted by Li (total angular momentum of the initial state) .

The transitions occurring between the states of same parity are disallowed which means the d orbitals which are gerade in terms of parity or symmetric with respect to the centre of inversion cannot have transitions from one d orbital to the other. However, p orbitals are ungerade that are asymmetric with respect to the centre of inversion and thus, transition can take place between d and p orbitals. In very simple words d-d transitions as well as p-p transitions are not allowed, whereas d to p and p to d are allowed.

(2) In case of states with same spin multiplicity the transitions cannot occur. The spin multiplicity 1 1 is given by the value (2S+1). Thus transitions between T1 and A1 are allowed in terms of spin 3 4 multiplicity whereas between T1 and A2 are not. The transitions which are feasible are called spin allowed whereas those are not allowed, are termed as spin forbidden. i.e.

S = 0

In case these rules are completely followed by the transition metal complexes, many of the transition must have not been observed. But, actually these transitions occur which suggest that we can have cases where these rules are broken down to actually display the forbidden transitions.

6.2 Breakdown of the selection rules

There are various phenomena that lead to the breakdown of the solution rules mentioned above. These can be explained as follows.

CHEMISTRY Paper 7: Inorganic Chemistry-II (Metal-Ligand Bonding, Electronic Spectra and Magnetic Properties of Transition Metal Complexes) Module 12, Electronic spectra of coordination complexes IV

(a) During the course of vibrations in a transition metal complex which is flexible in terms of movement about the bonds, the orbitals present can temporarily loose their center of symmetry. Since the parity is lost for the moment, it can be said that vibronic coupling results in the loss of their (orbitals) symmetric identity due to which the transitions from d to d orbitals are partially allowed. Although these transitions are observed but their molar extinction coefficient is extremely low, in the order of 10 - 50 L mol-1 cm-1. These transitions are actually responsible for various colors of the transition metal complexes.

(b) Tetrahedral complexes have no centre of symmetry and thus they are not gerade in symmetric terms. Thus the d levels in them are e and t2 with no ‘g’ as subscript. In case of a metal-σ bonding, the hybridizations sp3 as well as sd3 both are sustainable and thus the p and d orbitals present are not pure but mixed. Because of this p-d mixing the Laporte selection rule is relaxed. Hence tetrahedral complexes are often more intensely colored as compared to their octahedral counterpart (figure 7). This is actually a consequence of laporte relaxation.

Figure 9. Comparison of the tetrahedral and octahedral complex of Co2+ ion

(c) Spin-orbit coupling in case of certain complexes leads to relaxation of the spin selection rule. For metal ions, mostly 4d and 5d spin orbit coupling is quite strong and thus in these complexes the spin multiplicity rule is relaxed to a good extent that results in the occurrence of the forbidden transition.

CHEMISTRY Paper 7: Inorganic Chemistry-II (Metal-Ligand Bonding, Electronic Spectra and Magnetic Properties of Transition Metal Complexes) Module 12, Electronic spectra of coordination complexes IV

5. Summary

 Absorption spectrum is obtained when absorption of radiation occurs by the absorbing species, which leads to transition of the electrons from the lower to higher level.  Emission spectrum is obtained by transition from higher to the lower level that occurs due to emission of the radiation by electrons to the radiation field.  Emission spectra are of three types namely,

(1) Continuous spectra

(2) Band spectra

(3) Line spectra

 Frank-Codon principle states that movement of the nuclei during the course of transition is negligible thus, it can be said that internuclear distances do not alter during the course of transition and the most probable transitions are those which occur vertically.  Transitions arise from the ground vibrational level of the ground electronic state to several different vibrational levels of potential excited electronic states. These transitions give rise to vibrational fine structure in the foremost peak of the electronic transition.  Light will be absorbed if it can interact with the molecule by its oscillating electric and magnetic fields. For this to take place the dipole moment of the molecule must change during the time of transition. If the change in dipole moment is quite large, more number of photons will be absorbed which further lead to intense bands of large area.  Laporte selection rule states that for the molecules having centre of symmetry, the transitions within the same sub-shell are forbidden.  In case of states with same spin multiplicity, the transitions cannot occur. The spin multiplicity is given by (2S+1).  During the course of vibrations in a transition metal complex which is flexible in terms of movement about the bonds, the orbitals present can temporarily loose their center of symmetry. Since the parity is lost for the moment, the vibronic coupling results in the loss of their (orbitals) symmetric identity due to which the transitions from d to d orbitals become allowed.  Tetrahedral complexes are often more intensely colored as compared to their octahedral counterpart due to d-p mixing.  For metal ions, mostly 4d and 5d spin orbit coupling is quite strong and thus in their complexes the spin multiplicity rule is relaxed to a good extent that results in the occurrence of the forbidden transition.

CHEMISTRY Paper 7: Inorganic Chemistry-II (Metal-Ligand Bonding, Electronic Spectra and Magnetic Properties of Transition Metal Complexes) Module 12, Electronic spectra of coordination complexes IV