Subject Chemistry Paper No and Title Module No and Title Module Tag Paper 7: Inorganic Chemistry-II (Metal-Ligand Bonding, Electronic Spectra and Magnetic Properties of Transition Metal Complexes) 16. Electronic spectra of coordination complexes VIII CHE_P7_M16
TABLE OF CONTENTS 1. Learning outcomes 2. Introduction 3. Interpretation of the Tanabe-Sugano diagrams 3.1 d 4 configuration 3.2 d 5 configuration 3.3 d 6 configuration 3.4 d 7 configuration 3.5 d 8 configuration 3.6 d 9 configuration 4. Summary
1. Learning Outcomes After studying this module, you shall be able to Know the importance of the Tanabe-Sugano diagrams Learn the interpretation of the Tanabe-Sugano diagrams Identify the parameters involved in calculation of energy for the transitions Analyze the various type of transitions feasible in the complexes Learn the distortions which lead to changes in the absorption spectrum 2. Introduction Tanabe-Sugano diagrams are a special class of correlation diagrams in which the lowest energy free ion term is plotted as X-axis and all the other terms are hence plotted relative to this lowest energy term. In these plots energy of different groups of degenerate electronic states are plotted as a function of ligand field strength represented as Δo. The parameter B is the Racah parameter which is the repulsion term which arises due to repulsion between the terms of same symmetry. The ordinate is represented as E/B and abscissa is given as Δ o /B. The diagrams are qualitative as well as quantitative in nature. In cases where both high as well as low spin complexation is possible that is d 4, d 5, d 6 and d 7 electronic configurations a vertical line is drawn in the middle of the diagram to separate the high spin complexes from the low spin one. 3. Interpretation of the Tanabe-Sugano diagrams 3.1 d 4 configuration When we talk about d 4 configuration, both the low and high spin cases are possible in the presence of either weak field leading to high spin or strong field leading to low spin complexes. This possibility is further reflected in the Tanabe-Sugano diagrams in the form of discontinuity which is quite apparent in them. Both the cases are represented below (figure 1).
Figure 1. Calculation of spin multiplicity in weak and strong field for d 4 configuration High spin, weak field d 4 configuration has four unpaired electrons of parallel spin. In this case the spin multiplicity comes out to be 5. In other case the low spin, strong field d 4 configuration leads to two unpaired electrons with spin multiplicity of 3. The Tanabe Sugano diagram of a d 4 configuration is somewhat complex, since now we have a vertical line in between that separates the weak field and strong field cases. On moving from the weak field to the strong field the ground state terms also changes. The diagram for the d 4 case is represented below (figure 2). Figure 2. Tanabe-Sugano diagram and absorption spectrum for complexes with d 4 configuration
from the Tanabe-Sugano diagram, it can be observed that on the left of the vertical line is weak field, up till the value of 27 for Δo/B. The free ion term for a d 4 configuration is 5 D. in case of weak field the term 5 D splits with the ground state term as 5 E g, having the anticipated spin multiplicity of 5. Now as we move from weak towards the strong field limit, it can be seen that the ground state changes to 3 T 1g, which actually correlates with the 3 H term in the free ion term case. The spin multiplicity of this term is 3 as was likely to be. Hence the vertical line in between can be thought of as a divider which separates but relates the two type of fields namely weak and strong. At the dividing line the ground state changes from 5 E g to 3 T 1g. Thus the spin multiplicity also changes from 5 to 3 which suggest the change in number of the electrons in both the electronic configurations. Above diagram also illustrates the absorption spectrum for the complex [Cr(H 2 O) 6 ] 2+, which shows an absorption band at 14,100 cm -1 (680 nm). Since water is a weak field ligand, it can be said that for this complex left side of the Tanabe-Sugano diagram has to be considered. Thus the origin of band can be said to be from a single spin allowed transition from 5 E g to 5 T 2g as can be seen in the figure 2. This single transition band gives the energy of transition which refers to the value of Δ o in this case. Hence in order to find the value of splitting parameter, we simply have to find the energy of absorption for the corresponding transition band. Sometimes a single band in case of few complexes of d 4 configurations is split up into two due to the Jahn- Teller distortion. 3.2 d 5 configuration In case of d 5 configuration, we can again have change in the filling up of electrons according to weak or strong field. Along with the spin multiplicity also changes in both the cases. In case of weak field there are a total of five unpaired electrons and consequently spin multiplicity is 6. In case of strong field there is only one unpaired electron which leads to spin multiplicity of 2. Tanabe-Sugano diagram for a d 5 case can be shown below along with the absorption spectrum (figure 3).
Figure 3: Tanabe-Sugano diagram and absorption spectrum for complexes with d 5 configuration From the Tanabe-Sugano diagram it can be noted that free ion term 6 S split up in the presence of ligand field giving 6 A 1g term of the same spin multiplicity as the ground state. Now in case of strong field, the ground state term changes to 2 T 2g which corresponds to 2 I of the free ion terms. The vertical line separates the two states namely high spin and low spin states. From the absorption spectrum of complex [Mn(H 2 O) 6 ] 2+, some important observations can be made namely, (a) The bands are very less in terms of intensity and value of molar extinction coefficient. (b) The spectrum is complicated due to large number of overlapping bands The reason for these two observations lies in the Tanabe-Sugano diagram of the complex. Before analysis of the diagram, some generalizations about the complex can be made. The complex here we are refereeing to is [Mn(H 2 O) 6 ] 2+, where Mn 2+ ion is in d 5 configuration of electrons with H 2 O as the ligand which is a weak field ligand that certainly directs the formation of high spin complexes. Now if we are talking about a weak field ligand we have to look at the left side of Tanabe-Sugano diagram. By looking at the diagram it can be noted that the ground state term is 6 A 1g. From this ground state there can be no spin allowed transitions possible since there are no excited states of same spin multiplicity. Hence the bands observed are very less in intensity since originating from forbidden transitions. Due to this the color of the complex is pale pink which weakly absorbs and thus low value of absorption coefficient ε. Also since there are several excited states, hence several spin forbidden transitions are possible leading to a complex overlapping spectrum. In case we are talking about the strong field ligands there are again several transitions possible but this time they are spin allowed (Tanabe-Sugano diagram on the right)
leading to again a complicated spectrum that is difficult to interpret. In both the cases determining the value of Δo is quite difficult by the normal theoretical methods. 3.3 d 6 configuration The d 6 configuration is again having two sets of electronic filling depending upon the field applied. In case of weak field there are 4 unpaired electrons leading to spin multiplicity of 5. When the field applied is strong, the number of unpaired electrons is zero leading to a symmetric field with the spin multiplicity value of 1. The Tanabe-Sugano diagram and absorption spectrum of the [Fe(H 2 O) 6 ] 2+ complex is shown in figure 4). Figure 4. Tanabe-Sugano diagram and absorption spectrum for complexes with d 6 configuration The free ion term 5 D dissociates in presence of ligand field where ground state term is 5 T 2g. in case of strong field the ground state term changes to 1 A 1g which correlates with the 1 I term of the free ion states. In case of the complex [Fe(H 2 O) 6 ] 2+, the ligand is H 2 O which is a weak field ligand, thus looking at the left part of the Tanabe-Sugano diagram it can be noted that only a single spin allowed transition is possible from 5 T 2g to 5 E g state. This transition is shown in the absorption spectrum of the complex but looking closely at the spectrum, it can be noted that a single peak is split up into two. This is actually a result of Jahn-Teller distortion which leads to splitting of the bands. The value of Δ o is given by the energy of this absorption band.
3.4 d 7 configuration For a d 7 configuration the weak field leads to spin multiplicity of 4 and in strong field it alters to 2. The Tanabe Sugano diagram of the complex along with the absorption spectrum is shown below (figure 5). Figure 5: Tanabe-Sugano diagram and absorption spectrum for complexes with d 7 configuration In this case the free ion term of ground state is 4 F which splits into three states of same spin multiplicity namely 4 T 1g, 4 T 2g and 4 A 2g. The next higher term of same spin multiplicity is 4 P which transforms as 4 A 2g in the presence of the weak ligand field. Under ordinary condition the most populated state is the ground state 4 T 2g (F). Excitation leads to three spin allowed absorption bands namely υ 1, υ 2 and υ 3 as given in the diagram. These transitions are as follows
The Tanabe Sugano diagram for this case is quite similar to d 2 case and the method to determine the value of Δo is also similar. 3.5 d 8 configuration For the d 8 configuration the electronic arrangement in weak field as well as strong field is t 2g 6 e g 2 as the ground electronic state. In the excited state the electron from the lower t 2g level is shifted to the e g level, thus now the configuration changes to t 2g 5 e g 3. The ground state has F as the lowest energy term which in the presence of the ligand field gets split up into A 2g, T 2g and T 1g. the difference between two lowest energy terms corresponds to the value of Δ o. Therefore in order to find the value of Δ o we simply have to determine the energy difference of between states A 2g and T 2g. The Tanabe-Sugano diagram for this configuration can be represented as follows (figure 6). Examples of this electronic configuration include [Ni(H 2 O) 6 ] 2+. Figure 6. Tanabe-Sugano diagram and absorption spectrum for complexes with d 8 configuration
3.6 d 9 configuration For d 9 electronic configuration the number of electrons is nine and this can also be interpreted as having a one hole and thus the Tanabe Sugano diagram is quite simple to be drawn and interpret. The configuration can be represented as t 2g 6 e g 3. Now the excitation of electron from the lower t 2g level to the upper e g level forms the excited state. The spin multiplicity of the ground and the excited state is same and hence transition from the ground to excited state is spin allowed transition. The examples of the d 9 configuration include [Cu(H 2 O) 6 ] 2+ etc. The Tanabe-Sugano diagram along with the absorption spectrum can be represented as follows (figure 7 ). From the figure it can be seen that only one transition is possible from the ground 2 E g state to the excited 2 T 2g state, thus only one transition band should be observed corresponding to the value of Δ o. But in actual practice, a single absorption band is split up into two as can be seen from the absorption spectrum (figure 7). This actually occurs because of the Jahn-Teller distortion which leads to splitting of the single band into two. Also, it can be seen that no Racah parameter is involved in the diagram since only single excited state is possible. Figure 7. Tanabe-Sugano diagram and absorption spectrum for complexes with d 9 configuration
4. Summary In case of d 4, d 5, d 6 and d 7 configurations, both the low and high spin case are possible in the presence of either weak field leading to high spin or strong field leading to low spin complexes. In cases where both high spin as well as low spin complexation is possible a vertical line is drawn in the middle of the Tanabe-Sugano diagram in order to separate the high spin complexes from the low spin one. The free ion term for a d 4 configuration is 5 D. In case of weak field 5 D term splits with the ground state term as 5 E g, having the anticipated spin multiplicity of 5. In the strong field limit the ground state changes to 3 T 1 g, which actually correlates with the 3 H term in the free ion term case. In case of d 4 configuration single transition band gives the energy of transition which refers to the value of Δo From the absorption spectrum of complex [Mn(H 2 O) 6 ] 2+, some important observations can be made namely, (a) The bands are very less in terms of intensity and value of the molar extinction coefficient. (b) The spectrum is complicated due to large number of overlapping bands In the case of complex [Fe(H 2 O) 6 ] 2+, the ligand is H 2 O which is a weak field ligand, thus looking at the left part of the Tanabe-Sugano diagram it can be noted that only a single spin allowed transition is possible from 5 T 2g to 5 E g state. In d 7 case the free ion term of ground state is 4 F which split into three states of same spin multiplicity namely 4 T 1g, 4 T 2g and 4 A 2g. The next higher term of same spin multiplicity is 4 P which transforms as 4 A 2g in the presence of the weak ligand field. For d 8 configuration the electronic arrangement in the weak as well as strong field is t 2g 6 e g 2 as the ground electronic state. The ground state has F as the lowest energy term which in presence of the ligand field gets split up into A 2g, T 2g and T 1g. the difference between the two lowest energy terms corresponds to the value of Δo. For d 9 electronic configuration the number of electrons is nine and this can also be interpreted as having a one hole and thus the Tanabe Sugano diagram is quite simple to be drawn and interpret. The configuration can be represented as t 2g 6 e g 3. Only one transition is possible in d 9 case from the ground 2 E g to the excited 2 T 2g state, thus only one transition band should be observed corresponding to the value of Δo. But in actual practice, a single absorption band is split up into two as can be seen from the absorption spectrum.