Chapter 10 Answers to Problem Questions

10.1 Variable temperature ⁵⁷Fe Mössbauer spectra for [Fe(phen)₂(NCS)₂] (phen = 1,10-phenanthroline) are shown in Figure 10.18. Use the isomer shift and quadrupole splitting data in the 300 K and 77 K spectra and the magnetic susceptibility data in Figure 8.6 to explain the variable temperature behaviour.

The ⁵⁷Fe Mössbauer spectra in Figure 10.1P clearly indicate that there is a significant change in the iron environment at temperatures around 185 K in [Fe(phen)₂(NCS)₂]. The magnetic data for this compound in Figure 8.6 show that at room temperature it is high-spin and at low temperature it is low-spin. [Fe(phen)₂(NCS)₂] contains Fe(II), and is a classic example of a d⁶ spin-crossover complex. The isomer shift (δ) and quadrupole splitting (<ΔE_Q) parameters for the high-spin and low-spin isomers are shown on the spectra in Figure 10.1P.

 

Figure 10.1P Variable temperature ⁵⁷Fe Mössbauer spectra of [Fe(phen)₂(NCS)₂] (phen = 1,10-phenanthroline). (Adapted from P. Gütlich, C. P. Kohler, H. Köppen, E. Meissner, E. W. Müller, H. Spiering, Trends in Mössbauer Spectroscopy, University of Mainz, 1983 and I. Dézsi, B. Molnar, T. Tarnozci, K. Tompa, J. Inorg. Nucl. Chem., 29, 2486 (1967))

The isomer shift changes from 0.72 mm s⁻¹ for the high-spin isomer to 0.46 mm s⁻¹ for the low-spin isomer, as expected from Figure 10.6, and are consistent with an increase in covalency in the low-spin complex. High-spin octahedral Fe(II) has a ⁵T_2g ground term (see Section 5.13.1 for determining molecular ground terms), and the unequal distribution of electron density causes an electric field gradient (EFG) which results in a quadrupole splitting (<ΔE_Q). (Although [Fe(phen)₂(NCS)₂] is not strictly octahedral it is common practice to use the octahedral terms.) The observed large <ΔE_Q value of 2.70 mm s⁻¹ is very characteristic of high-spin Fe(II). Low-spin Fe(II) has a ¹A_1g ground term, and as the electron density is now cubic, there is no EFG, and hence no quadrupole splitting caused by electronic effects. The observed <ΔE_Q value of 0.48 mm s⁻¹ for the low-spin state is largely due to a geometric contribution to the EFG from the phen and NCS⁻ ligands.

The rapid change from high-spin to low-spin at ca. 185 K is not what is expected from a Boltzmann distribution between two states, and is indicative of cooperative behaviour, which is common in Fe(II) spin-crossover complexes. In Fe(III) spin-crossover complexes it is more usual to observe a much more gradual change in spin state, often to the extent that it is not possible to go from pure high-spin to pure low-spin over accessible temperature ranges.

 

10.2 Figure 10.20 shows pairs of ⁵⁷Fe Mössbauer spectra for iron(II) and iron(III) octahedral spin-crossover complexes recorded at different temperatures. Determine the isomer shift and quadrupole splitting for each spectrum, and use these to assign the spectra to the iron oxidation state and spin state.

⁵⁷Fe Mössbauer spectroscopy is a very powerful way of identifying the oxidation and spin states of iron complexes.

 

Figure 10.2P ⁵⁷Fe Mössbauer spectra of iron(II) and iron(III) octahedral spin-crossover complexes.

 

The isomer shift is expected to move to more negative values as the iron oxidation state increases from Fe(II) to Fe(III). However, apart from the 0.85 mm s⁻¹ value for the 383 K data in Figure 10.2P, the other values are reasonably similar, so whilst this indicates that the left hand spectra are probably from Fe(II) complexes, the δ values alone are not always sufficiently diagnostic. Octahedral high-spin Fe(II) has a ⁵T_2g ground term, low-spin Fe(II) has a ¹A_1g ground term, high-spin Fe(III) has a ⁶A_1g ground term and low-spin Fe(III) has a ²T_2g ground term. Therefore, electronically induced EFGs are predicted for high-spin Fe(II) and low-spin Fe(III). As significant <ΔE_Q values are observed at high-temperature in the left-hand spectra and at low-temperature for the right-hand spectra, this confirms that the left-hand spectra are from an Fe(II) complex, and the right-hand spectra from an Fe(III) complex.

The spectra in Figure 10.2P show the high-spin and low-spin spectra for Fe(II) and Fe(III) complexes. Figure 10.1P showed the spectra in the region of the spin-crossover transition for Fe(II). Whilst variable temperature ⁵⁷Fe Mössbauer spectra have been recorded for Fe(III) spin-crossover complexes, for temperatures (or pressures) where a mix of high-spin and low-spin isomers are expected, it is more common to observe a single quadrupole doublet that is an average of the two spin states, rather than a superposition of the two spectral features as in Fe(II). This is because the rate of exchange between the high-spin and low-spin isomers is faster than the relaxation rate of 150 ns of the ⁵⁷Fe I = ³/₂ Mössbauer excited state, so the iron nucleus effectively sees an average of the two states on the Mössbauer timescale. This is especially true for the classic iron(III) dithiocarbamate ([Fe(S₂CNR₂)₃]) spin-crossover complexes. Slow cross-over is much less common, but is found in some complexes with a N₄O₂ donor set. In addition, in Fe(III) spin-crossover complexes the spin transition usually extends over a much wider temperature range (i.e. obeys Boltzmann statistics) compared to the abrupt changes often or usually observed for Fe(II) complexes. Therefore, care must be used when interpreting the ⁵⁷Fe Mössbauer spectra of Fe(III) complexes when both high-spin and low-spin isomers are expected at any given temperature.

10.3. The ⁵⁷Fe Mössbauer spectra and structures of some iron carbonyls are shown in Figure 10.21. Assign the spectra to the correct structure.

The isomer shift, δ, for each spectrum is close to zero, and does not vary much between the three carbonyl compounds as all of the compounds have iron in a zero oxidation state. However, there are marked differences in the quadrupole splitting, <ΔE_Q, observed in the three spectra, and this is related to the different EFGs at the iron nucleus. In this case the greatest contribution to the EFG comes from structural/geometric effects, rather than electronic ones.

 

 

Figure 10.3P ⁵⁷Fe Mössbauer spectra and structure of iron carbonyls. (Data from R. Greatrex and N. N. Greenwood, Disc. Farad. Soc., 47, 126 (1969))

In [Fe(CO)₅], the iron is in a relatively low symmetry trigonal bipyramidal environment and the electric field has axial symmetry as it is different in the z direction, compared to that in the x and y directions, which will be the same. Therefore, there will be an appreciable EFG, and hence ΔE_Q, observed in the spectrum of [Fe(CO)₅]. In [Fe₂(CO)₉] there are two equivalent iron atoms so there will be one multiplet. As each is in an approximately octahedral configuration, the EFG and <ΔE_Q will be smaller than in [Fe(CO)₅]. In [Fe₃(CO)₁₂], there are two different iron environments, so two multiplets are expected, one from the iron at the top of the triangle, and one from the two iron atoms at the bottom of the triangle, and as these are in a 1:2 ratio, this will be reflected in the peak intensities. The iron at the top of the triangle is in a more symmetric, almost octahedral environment, with no bridging carbonyl ligands, whereas the two iron atoms at the bottom of the triangle have a mix of both terminal and bridging carbonyl ligands. On this basis there is expected to be a widely spaced quadrupole doublet with twice the intensity of a narrowly spaced quadrupole doublet. Spectrum (a) with a large <ΔE_Q is assigned to [Fe(CO)₅], spectrum (b) with a small <ΔE_Q is assigned to [Fe₂(CO)₉], and spectrum(c) with two multiplets is assigned to [Fe₃(CO)₁₂].

⁵⁷Fe Mössbauer experiments were the first to identify the structure of [Fe₃(CO)₁₂].

10.4 2-Acetylpyridinethiosemicarbazone can act as a tridentate ligand either in its neutral thione state (HATP), or after deprotonation of the thiol tautomer to give ATP⁻,Figure 10.22. This ligand can form an iron(II) complex, [Fe(HAPT)₂]Cl₂, and an iron(III) complex, [Fe(APT)(HAPT)]Cl₂. Use the isomer shift and quadrupole splitting data in the ⁵⁷Fe Mössbauer spectra in Figure 10.22 to identify the oxidation states and spin states and hence assign the spectra to the [Fe(HAPT)₂]Cl₂ and [Fe(APT)(HAPT)]Cl₂ complexes.

For ⁵⁷Fe Mössbauer spectra the isomer shift is expected to be at lower values for Fe(III) than Fe(II). On this basis, the spectrum in Figure 10.4P(a) with δ = 0.065 mm s⁻¹ belongs to Fe(III) and spectrum (b) with δ = 0.264 mm s⁻¹ belongs to Fe(II).

Figure 10.4P Structures of HATP and ATP⁻ ligands and ⁵⁷Fe Mössbauer spectra of (a) [Fe(APT)(HAPT)]Cl₂ and (b) [Fe(HAPT)₂]Cl₂ (data from R. H. U. Borges, A. Abras and H. Beraldo, J. Braz. Chem. Soc., 8, 33-38 (1997))

The quadrupole splitting, <ΔE_Q, can be used to identify the spin state using the same methodology as in Problem 10.3 as these complexes are also six coordinate, therefore the structural EFG is likely to be small, and the EFG to be dominated by electronic effects. Octahedral high-spin Fe(II) has a ⁵T_2g ground term, low-spin Fe(II) has a ¹A_1g ground term, high-spin Fe(III) has a ⁶A_1g ground term and low-spin Fe(III) has a ²T_2g ground term. Therefore, electronically induced EFGs and hence appreciable <ΔE_Q values are predicted for high-spin Fe(II) and low-spin Fe(III), with small EFGs and <ΔE_Q values for low-spin Fe(II) and high-spin Fe(III). The <ΔE_Q value in spectrum (a) is 2.430 mm s⁻¹ and that in spectrum (b) is 0.537 mm s⁻¹. Using this information, in conjunction with the isomer shift data, we can assign spectrum (a) to the low-spin Fe(III) complex [Fe(APT)(HAPT)]Cl₂ and spectrum (b) to the low-spin Fe(II) complex [Fe(HAPT)₂]Cl₂. (This is a good example where a large quadrupole doublet does not automatically imply high-spin Fe(II).)

10.5 Ferredoxins are iron-sulfur proteins that mediate electron transfer in a variety of metabolic reactions. Electron transfer is accompanied by iron redox chemistry in the Fe₂S₂ core. The ⁵⁷Fe Mössbauer spectra of the oxidised and reduced forms of Scenedesmus ferredoxin are shown in Figure 10.23. Use these data to account for the electronic and magnetic behaviour of ferredoxin in its oxidised and reduced form.

Figure 10.5P shows the structure of the Fe₂S₂ core and the ⁵⁷Fe Mössbauer spectra of the oxidised and reduced forms of Scenedesmus ferredoxin (similar spectra were obtained from ferredoxins extracted from spinach and Euglena.) (Data from C. E. Johnson, J. Appl. Phys., 1971, 42, 1325).

In the oxidised version, there is a simple quadrupole doublet (δ = +0.20 mm s⁻¹, ΔE_Q = 0.60 mm s⁻¹), but on reduction this is replaced by a pair of quadrupole doublets. In the oxidised form, both iron atoms in the Fe₂S₂ core are tetrahedral high-spin iron(III), therefore each Fe is expected to have a ⁶A ground term giving rise to a small or negligible valence EFG and hence small ΔE_Q. As the iron atoms are strongly antiferromagnetically coupled by both exchange and superexchange mechanisms through the sulfur atoms, the ground state is in fact a spin singlet. In the reduced form, there is a central quadrupole doublet at +0.22 mm s⁻¹, with ΔE_Q = 0.59 mm s⁻¹, both of which are very similar to those in the spectrum of the oxidised form. In addition, there is a second quadrupole doublet at +0.56 mm s⁻¹, with ΔE_Q = 2.75 mm s⁻¹. The increase in isomer shift is consistent with this quadrupole doublet belonging to iron(II). The large quadrupole splitting is consistent with a large EFG caused by the non-cubic electron density in tetrahedral high-spin iron(II) which has a ⁵E ground term. Therefore, in the reduced form there is one tetrahedral high-spin iron(III) with a ⁶A term giving rise to a small ΔE_Q and a tetrahedral high-spin iron(II) with a ⁵E term, giving rise to a large ΔE_Q, but as these are also antiferromagnetically coupled, overall it is a spin doublet system.

 

 

Figure 10.5P Structure of Fe₂S₂ core of ferredoxins and ⁵⁷Fe Mössbauer spectra of Scenedesmus ferredoxin at 195 K (data from C. E. Johnson, J. Appl. Phys., 42, 1325 (1971) and K. K. Rao, R. Cammack, D. O. Hall and C. E. Johnson., Biochem. J., 122, 257 (1971))

Therefore, the ⁵⁷Fe Mössbauer spectra show convincingly the difference between the oxidised form of ferredoxin with two iron(III) centres and the reduced form with one iron(III) and one iron(II) centre.

10.6. The ³⁵Cl NQR spectral frequencies (MHz) of some lanthanide trichlorides are summarised in Table 10.5. Interpret these data as far as possible. (E. H. Carlson and H. S. Adams J. Chem. Phys.  51, 388 (1969))

NQR spectra are only observed when the quadrupolar nucleus (³⁵Cl in this case) is in a non-cubic environment, giving rise to an electric field gradient (EFG). The larger the EFG the higher the NQR frequency that is observed. For many solid state materials with cubic structures, such as NaCl, no ³⁵Cl NQR spectrum is observable, unless the structure is distorted by external strain, and NQR can be used to measure the extent of strain in some functional materials. Therefore, the fact that the NQR resonances in Table 10.6P are observed for the lanthanide trichlorides indicates that at least some of the ³⁵Cl atoms are situated on non-cubic sites, giving rise to an EFG at the chlorine.

 

 

 

LaCl3	CeCl3	PrCl3	NdCl3	SmCl3	GdCl3	DyCl3	HoCl3	ErCl3	YbCl3
4.167	4.387	4.567	4.729	5.033	5.315	4.203	4.277 4.334	4.424 4.445	4.738 4.788

Table 10.6P ³⁵Cl NQR data (MHz) for some lanthanide trichlorides. (E. H. Carlson and H. S. Adams J. Chem. Phys.  51, 388 (1969))

The NQR data for the lanthanide trichlorides in Table 10.6P are plotted in Figure 10.6P.

 

Figure 10.6P Plot of ³⁵Cl NQR resonant frequencies for a selection of lanthanide trichlorides.

 

From this plot it is clear that there are two distinct sets of data, those from La to Gd, and a second set from Dy to Yb, both of which show a consistent increase in the ³⁵Cl NQR frequency as the atomic number increases. The origin of this is that there are two different structural types for the lanthanide trichlorides. La to Gd trichlorides are isomorphous with UCl₃, while those from Dy to Yb are isomorphous with AlCl₃. The UCl₃ structure is hexagonal and the nine coordinate uranium has a tricapped trigonal prismatic configuration and the nearest neighbours to the chloride ions are three metal ions at ca. 2.9 Å. The AlCl₃ structure is a distorted cubic close packing of the chloride ions, with two chlorine environments, with the metal in one third of the octahedral holes. The ions are arranged in sandwiches as Cl-M-Cl:Cl-M-Cl. The increase in ³⁵Cl NQR frequency as one goes across the series is related to an increase in the EFG due to the “lanthanide contraction” where the ionic radius decreases as the atomic number increases because the f orbitals are poor at shielding the increasing nuclear charge. The two data points for each of HoCl₃, ErCl₃ and YbCl₃ are because of two distinct chlorine environments in the AlCl₃ structure.

10.7 The features at 31.7 MHz and 34.25 MHz due to ⁶⁵Cu and ⁶³Cu, respectively, in the NQR spectra of CuGeO₃ were both split by about 250 kHz. What does this say about the copper environment(s) in this material? (A. A. Gippius, E. N. Morozova, D. F. Khozeev, A. N. Vasil'ev, M. Baenitz, G. Dhalenne, A. Revcolevschi, J. Phys.: Condens. Matter 12, L71 (2000))

The splitting of 250 kHz in the spectrum shown in Figure 10.7P indicates that there are two slightly different copper environments in CuGeO₃. These data were obtained from single crystal samples, earlier data from polycrystalline data had not observed this splitting, but it is consistent with other X-ray and EPR data.

 

Figure 10.7P ^63,65Cu NQR spectrum of CuGeO₃ single crystal at 4.2 K. (Data from A. A. Gippius, E. N. Morozova, D. F. Khozeev, A. N. Vasil'ev, M. Baenitz, G. Dhalenne, A. Revcolevschi, J. Phys.: Condens. Matter 12, L71 (2000))

10.8 The ³⁵Cl NQR frequency increased from 11.785 MHZ for [TiCl₂Cp₂] to 11.930 MHz for [TiCl₂Cp₂*]. (Cp = C₅H₅⁻ and Cp* = C₅Me₅⁻) Explain this observation. (Data from J. Kubišta, M. Civiš, P. Španěl and S. Civiš, Analyst 137 1338 (2012))

An increase in the NQR frequency is associated with an increase in the EFG at the ³⁵Cl nucleus for [TiCl₂Cp₂*] compared to [TiCl₂Cp₂], and a reduction in the ionicity of the Ti-Cl bond according to the Townes Dailey formalism. This has been interpreted as being due to weaker T-Cl π bonding in [TiCl₂Cp₂*₂] due to stronger bonding of the Cp* ligand to the Ti, compared to the Cp ligand, as Cp* is a better π donor than Cp.