**17. Manganese**

The atomic number of manganese is 25 and its outermost electronic configuration is [Ar] 3d54s2 . It exhibits several oxidation states, +2, +3, +4, +6 and +7, of which the most stable are +2 +4 and +7. The ionic radii of Mn2+ and Mn4+ are 0.80 and 0.54 A.U. respectively. Twenty three isotopes and isomers are known. A number of minerals of manganese exists in nature (~ 300 minerals) giving rise to an overall abundance of 0.106%. Twelve of the important among them are economically exploited and the most important of these are pyrolusite (MnO2), manganite (Mn2O3.H2O), hausmannite (Mn3O4) rhodochrosite (MnCO3) and manganese(ocean) nodules. Much of the (85-90%) manganese is consumed in the manufacture of ferromanganese alloys. The other uses are: manganese coins, dry cell and alkaline batteries and glass. It is an essential trace element for all forms of life.

Octahedral complexes of Mn(III) are prone to Jahn-Teller distortion. It is of interest, therefore, to compare the structures of Cr(acac)3 with Mn(acac)3 since the former is a regular octahedron while the latter is prone to dynamic Jahn-Teller distortion.

#### **17.1. EPR spectra of manganese compounds**

1. Manganese(II): Manganese(II), being a d5 ion, is very sensitive to distortions in the presence of magnetic field. Mn(II) has a total spin, S = 5/2. The six spin states labeled as ±5/2>, ±3/2> and ±1/2> are known as the three Kramers' doublets; in the absence of external magnetic field ,they are separated by 4D and 2D respectively, where D is the zero-field splitting parameter. These three doublets split into six energy levels by the application of an external magnetic field. Transitions between these six energy levels give rise to five resonance lines. Each of these resonance lines, in turn, splits into a sextet due to the interaction of the electron spin with the nuclear spin of 55Mn, which is 5/2. Thus one expects a 30- line pattern. However, depending on the relative magnitudes of D and A (hyperfine coupling constant of manganese), these 30 lines appear as a separate bunch of 30 lines or 6 lines (if D = 0). The separation between the extreme set of resonance lines is approximately equal to 8D (first order). If D is very small compared to hyperfine coupling constant (A), the 30 lines are so closely packed that one could see only six lines corresponding -1/2 to +1/2 transition. If D = 0, the system is perfectly octahedral. Deviation from axial symmetry leads to a term known as E in the spin- Hamiltonian. The value of E can be easily calculated from single crystal measurements. A non-zero value of E results in making the spectrum unsymmetrical about the central sextet.

Further, the following parameters have been calculated from the powder spectrum using the Spin- Hamiltonian of the form:

$$H = \beta \mathbf{1} B\_{\mathbf{3}\_{\tilde{\mathbf{3}}}} \mathbf{S} + D \left( \mathbf{S}\_{\mathbf{z}}^2 - \frac{1}{3} \mathbf{S} \left( \mathbf{S} + \mathbf{1} \right) \right) + \mathbf{S} A \mathbf{1} \tag{23}$$

Here the first term represents the electron-Zeeman interaction, the second term represents the zero field contribution and the third term represents the nuclear-Zeeman interaction. The extra set of resonances within the main sextet is due to the forbidden transitions. From the forbidden doublet lines, the Zero field splitting parameter, D is calculated using the formula,

$$
\Delta H^\circ = \left(\frac{2D^2}{H\_m}\right) \left[\frac{1 + 16\left(H\_m - 8Am\right)^2}{9H\_iH\_m - 64Am}\right] \tag{24}
$$

$$H\_m = H\_o - Am - \frac{\left[I\left(I+1\right) - m^2\right]A^2}{2H\_o} \\ \text{or } H\_m = H\_o - Am - \left(35 - 4m^2\right)\left(\frac{A^2}{8H\_o}\right) \tag{25}$$

where Hm is the magnetic field corresponding to m m in HF line; H0 is the resonance magnetic field and m is the nuclear spin magnetic quantum number.

Percentage of covalency of Mn-ligand bond can be calculated in two ways using (i) Matumura's plot and (ii) electro negativities, Xp and Xq using the equation,

$$\mathcal{C} = \frac{1}{m} \left[ 1 - 0.16 \left( X\_p - X\_q \right) - 0.035 \left( X\_p - X\_q \right)^2 \right] \tag{26}$$

Here n is the number of ligands around Mn(II) ion; Xp = XMn = 1.6 for Mn(II) and Xq = Xligand .

Also hyperfine constant is related to the covalency by,

$$A\_{\rm iso} = \left(2.04C - 104.5\right) \times 10^{-4} cm^{-1} \tag{27}$$

Further, the g value for the hyperfine splitting is indicative of the nature of bonding. If the g value shows a negative shift with respect to the free electron g value (2.0023), the bonding is ionic and conversely, if the shift is positive, then the bonding is said to be more covalent in nature.

#### **17.2. Typical examples**

28 Advanced Aspects of Spectroscopy

**17. Manganese** 

2. Tetravalent chromium (d2):

cm-1overlaps with the bands at 16130 and 23065 cm-1.

Absorption spectra of Cr4+ in forsterite and garnet show the absorption band at 9460 cm-1 which is the typical of Cr4+ ions. It is attributed to the 3A2g 3T2g transition. The absorption band at 19590 cm-1 is also attributed to 3A2g 3T1g transition. The absorption band at 19590

The atomic number of manganese is 25 and its outermost electronic configuration is [Ar] 3d54s2 . It exhibits several oxidation states, +2, +3, +4, +6 and +7, of which the most stable are +2 +4 and +7. The ionic radii of Mn2+ and Mn4+ are 0.80 and 0.54 A.U. respectively. Twenty three isotopes and isomers are known. A number of minerals of manganese exists in nature (~ 300 minerals) giving rise to an overall abundance of 0.106%. Twelve of the important among them are economically exploited and the most important of these are pyrolusite (MnO2), manganite (Mn2O3.H2O), hausmannite (Mn3O4) rhodochrosite (MnCO3) and manganese(ocean) nodules. Much of the (85-90%) manganese is consumed in the manufacture of ferromanganese alloys. The other uses are: manganese coins, dry cell and

Octahedral complexes of Mn(III) are prone to Jahn-Teller distortion. It is of interest, therefore, to compare the structures of Cr(acac)3 with Mn(acac)3 since the former is a regular

1. Manganese(II): Manganese(II), being a d5 ion, is very sensitive to distortions in the presence of magnetic field. Mn(II) has a total spin, S = 5/2. The six spin states labeled as ±5/2>, ±3/2> and ±1/2> are known as the three Kramers' doublets; in the absence of external magnetic field ,they are separated by 4D and 2D respectively, where D is the zero-field splitting parameter. These three doublets split into six energy levels by the application of an external magnetic field. Transitions between these six energy levels give rise to five resonance lines. Each of these resonance lines, in turn, splits into a sextet due to the interaction of the electron spin with the nuclear spin of 55Mn, which is 5/2. Thus one expects a 30- line pattern. However, depending on the relative magnitudes of D and A (hyperfine coupling constant of manganese), these 30 lines appear as a separate bunch of 30 lines or 6 lines (if D = 0). The separation between the extreme set of resonance lines is approximately equal to 8D (first order). If D is very small compared to hyperfine coupling constant (A), the 30 lines are so closely packed that one could see only six lines corresponding -1/2 to +1/2 transition. If D = 0, the system is perfectly octahedral. Deviation from axial symmetry leads to a term known as E in the spin- Hamiltonian. The value of E can be easily calculated from single crystal measurements. A non-zero value of E results in making the spectrum unsymmetrical

alkaline batteries and glass. It is an essential trace element for all forms of life.

octahedron while the latter is prone to dynamic Jahn-Teller distortion.

**17.1. EPR spectra of manganese compounds** 

about the central sextet.

1. Manganese(II): The EPR spectrum of clinohumite contains a strong sextet at the centre corresponding to the electron spin transition +1/2> to -1/2>. In general, the powder spectrum is characterized by a sextet, corresponding to this transition. The other four transitions corresponding to ±5/2> ↔ ±3/2> and ±3/2>↔ ±1/2> are not seen due to their high anisotropy in D. However, in a few cases only, all the transitions are seen. Moreover, the low field transitions are more intense than the high field transitions. In addition, if E ≠ 0, the EPR spectrum will not be symmetrical about the central sextet. In clinohumite, the spectrum indicates the presence of at least three types of Mn(II) impurities in the mineral.

The extra set of resonances within the main sextet is due to the forbidden transitions. From the powder spectrum of the mineral, the following parameters are calculated:

Site I: g = 2.000(1), A = 9.15(2) mT; and D = 43.8(1) mT.

Site II: g = 2.003(2), A = 9.23(2) mT; and D = 44.1(1) mT.

Site III: g = 2.007(1), A = 9.40(2) mT; and D = 44.1(1) mT.

This large value of D indicates a considerable amount of distortion around the central metal ion. Since EPR is highly sensitive to Mn(II) impurity, three such sites are noticed. These two sites have close spin- Hamiltonian parameters. A close look at the EPR spectrum indicates a non-zero value of E, which is very difficult to estimate from the powder spectrum.

2. Pelecypod shell EPR spectrum of powdered sample obtained at room temperature indicates the presence of Mn(II) and Fe(III) impurities. The spectrum contains a strong sextet at the centre of the spectrum corresponding to the electron spin transition +1/2> to -1/2>. Also, the powder spectrum indicates the presence of, at least, three types of Mn(II) impurities in the *pelecypod shell* which is noticed at the sixth hyperfine resonance line. The third Mn(II) site is of very low intensity. The extra set of resonances within the main sextet is due to the forbidden transitions. The variations of intensity are also due to the zero field splitting parameter. From the powder spectrum of the compound, the following parameters are calculated using the spin- Hamiltonian of the form:

$$H = \beta B \text{gS} + D \left[ S\_Z^2 - S^{\binom{S}{2} - 1} \bigvee\_{\mathcal{B}}^{} + SAI \right] \tag{28}$$

where the symbols have their usual meaning.

Site I: g = 2.002(1), A = 9.33(2) mT; and D = 43.8(1) mT

Site II: g = 1.990(2), A = 9.41(2) mT; and D = 44.1(1) mT

Site III: g = 1.987(1), A = 9.49(2) mT; and D = 44.1(1) mT

This large value of D indicates a considerable amount of distortion around the central metal ion. A close look at the EPR spectrum indicates a non-zero value for E.

The hyperfine constant 'A' value provides a qualitative measure of the ionic nature of bonding of Mn(II) ion. The percentage of covalency of Mn-ligand bond is calculated using 'A' (9.33 mT) value obtained from the EPR spectrum and with Matumura's plot. It corresponds to an ionicity of 94%. Also, the approximate value of hyperfine constant (A) is calculated by using the equation (27).

The value obtained is 92x 10-4 cm-1. This calculated value agrees well with the observed hyperfine constant (93.3 x 10-4 cm-1) indicating ionic character of Mn-O bond in the shell under study.

Using the covalency, the number of ligands around Mn(II) ion is estimated using the equation (26)

$$\mathbf{C} = \frac{1}{n} \left[ \mathbf{1} - 0.16 \left( \mathbf{X}\_p - \mathbf{X}\_q \right) - 0.035 \left( \mathbf{X}\_p - \mathbf{X}\_q \right)^2 \right]$$

Where XP and Xq are the electronagativities of metal and ligand. Assuming Xp = XMn = 1.4 and Xq = XO = 3.5, the number of ligands (n) obtained are 18. This suggests that Mn(II) may be surrounded by eighteen oxygens of six <sup>2</sup> *CO*<sup>3</sup> ions.


#### **17.3. Optical absorption studies**

30 Advanced Aspects of Spectroscopy

powder spectrum.

high anisotropy in D. However, in a few cases only, all the transitions are seen. Moreover, the low field transitions are more intense than the high field transitions. In addition, if E ≠ 0, the EPR spectrum will not be symmetrical about the central sextet. In clinohumite, the spectrum indicates the presence of at least three types of Mn(II) impurities in the mineral.

The extra set of resonances within the main sextet is due to the forbidden transitions. From

This large value of D indicates a considerable amount of distortion around the central metal ion. Since EPR is highly sensitive to Mn(II) impurity, three such sites are noticed. These two sites have close spin- Hamiltonian parameters. A close look at the EPR spectrum indicates a non-zero value of E, which is very difficult to estimate from the

2. Pelecypod shell EPR spectrum of powdered sample obtained at room temperature indicates the presence of Mn(II) and Fe(III) impurities. The spectrum contains a strong sextet at the centre of the spectrum corresponding to the electron spin transition +1/2> to -1/2>. Also, the powder spectrum indicates the presence of, at least, three types of Mn(II) impurities in the *pelecypod shell* which is noticed at the sixth hyperfine resonance line. The third Mn(II) site is of very low intensity. The extra set of resonances within the main sextet is due to the forbidden transitions. The variations of intensity are also due to the zero field splitting parameter. From the powder spectrum of the compound, the

following parameters are calculated using the spin- Hamiltonian of the form:

ion. A close look at the EPR spectrum indicates a non-zero value for E.

where the symbols have their usual meaning.

calculated by using the equation (27).

Site I: g = 2.002(1), A = 9.33(2) mT; and D = 43.8(1) mT Site II: g = 1.990(2), A = 9.41(2) mT; and D = 44.1(1) mT Site III: g = 1.987(1), A = 9.49(2) mT; and D = 44.1(1) mT

*<sup>S</sup> H BgS D S S SAI*

This large value of D indicates a considerable amount of distortion around the central metal

The hyperfine constant 'A' value provides a qualitative measure of the ionic nature of bonding of Mn(II) ion. The percentage of covalency of Mn-ligand bond is calculated using 'A' (9.33 mT) value obtained from the EPR spectrum and with Matumura's plot. It corresponds to an ionicity of 94%. Also, the approximate value of hyperfine constant (A) is

<sup>2</sup> 1 3 *<sup>Z</sup>*

(28)

 

the powder spectrum of the mineral, the following parameters are calculated:

Site I: g = 2.000(1), A = 9.15(2) mT; and D = 43.8(1) mT. Site II: g = 2.003(2), A = 9.23(2) mT; and D = 44.1(1) mT.

Site III: g = 2.007(1), A = 9.40(2) mT; and D = 44.1(1) mT.

1. Manganese(II): The free ion levels of Mn2+ are 6S, 4G, 4P. 4D and 4F in the order of increasing energy. The energy levels for Mn2+ ion in an octahedral environment are 6A1g(S), 4T1g(G), 4T2g(G), 4Eg(G), 4T1g(G) 4A1g, 4T2g(G), 4Eg(D), 4T1g(P) respectively with increasing order of energy. The 4Eg(G), 4A1g and 4Eg(D) levels are less affected when compared to other levels by crystal field. Hence, sharp levels are expected relatively in the absorption spectrum which is the criterion for assignment of levels of Mn(II) ion. Since all the excited states of Mn(II) ion will be either be quartets or doublets, the optical absorption spectra of Mn(II) ions will have only spin forbidden transitions. Therefore, the intensity of transitions is weak.

Energy level diagram of Mn(II) is extremely complex. Exact solutions for the excited state energy levels in terms of Dq, B and C may be obtained from T-S matrices. These matrices are very large (up to 10 x10) and ordinary calculations are not feasible. For this reason, the T-S diagrams given in many places in the literature are not sufficiently complete to allow the assignment of all the observed bands. Therefore a set of computer programmes is written to solve the T-S secular equations for any selected values of B, C and Dq. With the computer program, it is only necessary to obtain values of B and C and the complete scheme for any Dq can be quickly calculated. Fortunately B and C can be obtained analytically, if a sufficiently complete spectrum is obtained using the transitions given below:

$$\begin{aligned} \,^4A\_{1\_{\mathcal{S}}\prime} \,^4E\_{\mathcal{S}} \begin{pmatrix} G \end{pmatrix} \rightarrow \,^6A\_{1\_{\mathcal{S}}} = 10B + 5C = \nu\_1 \\\\ \,^4E\_{\mathcal{S}} \begin{pmatrix} D \end{pmatrix} \rightarrow \,^6A\_{1\_{\mathcal{S}}} = 17B + 5C = \nu\_2 \end{aligned}$$

If ν1 and ν2 are correctly observed and identified in the spectrum, B and C can be calculated. Identification is particularly easy in these cases because of the sharpness of the bands of these levels and are independent of Dq.

2. Manganese(III): This ion has four 3d electrons. The ground state electronic configuration is 3 1 <sup>2</sup>*g g t e* . It gives a single spin-allowed transition 5Eg→ 5T2g corresponding to one electron transition. This should appear around 20000 cm-1. Mn3+ cation is subject to Jahn-Teller distortion. The distortion decreases the symmetry of the coordination site from octahedral to tetragonal (D4h) or by further lowering the symmetry to rhombic (C2v). Under the tetragonal distortion, the t2g orbital splits into eg and b2g orbitals whereas the eg orbital splits into a1g and b1g orbitals. Hence in a tetragonal site, three absorption bands are observed instead of one. Further distortion splits the eg orbital into singly degenerate a1g and b1g orbitals. Thus four bands are observed for rhombic symmetry (C2v).

The transitions in the tetragonal field are described by the following equations:

$$\mathrm{^2B\_{1g}} \rightarrow \mathrm{^2A\_{1g}} \cdot \left[ 6Dq - 2Ds - 6Dt - \left( 6Dq + 2Ds - Dt \right) \right] = 4Ds + 5Dt \tag{29}$$

$$\mathbf{B}^2 \mathcal{B}\_{\mathbf{1}\_{\mathcal{S}}} \to \,^2 \mathcal{B}\_{\mathbf{2}\_{\mathcal{S}}} : \left[ -4Dq - 2Ds - Dt - \left( 6Dq + 2Ds - Dt \right) \right] = 10Dq \tag{30}$$

$$\mathrm{P}^{2}\mathrm{B}\_{\mathrm{lg}} \rightarrow \,^{2}\mathrm{E}\_{\mathrm{g}} : \left[ -4Dq - Ds + 4Dt - \left( 6Dq + 2Ds - Dt \right) \right] = 10Dq + 3Ds \tag{31}$$

In the above equations, Dq is octahedral crystal field and Ds and Dt are tetragonal field parameters. The same sign of Dq and Dt indicates an axial elongation and opposite sign indicates an axial compression.

The optical absorption bands observed for Mn(III) in octahedral coordination with rhombic distortion (C2h) in montmorillonite are given in Table -14.


**Table 14.** Assignment of bands for Mn(III) in montmorillonite
