12 Electronic and Magnetic

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1 12 Electronic and Magnetic Properties of the Actinides 12.1 Introduction By the end of this chapter you should be able to: understand that the Russell Saunders coupling scheme is not a good approximation here; recognize that the electronic spectra of compounds in the + and +4 states are dominated by f f transitions; know that transitions are more sensitive to ligand than with the 4f metals; appreciate that interpretation of spectroscopic and magnetic properties is more difficult than for the lanthanides. In general, it is more difficult to interpret the spectra and magnetic behaviour of actinide compounds than those of lanthanide compounds, so this chapter will provide a qualitative and somewhat superficial discussion of the phenomena rather than a qualitative one, concentrating largely upon uranium compounds. The reasons for this is that spin orbit coupling plays a more important part in actinide chemistry as the 5f orbitals and their electrons are not so core-like as the 4f, particularly in the early part of the actinide series. This means that the Russell Saunders (RS) coupling scheme, which treats spin orbit coupling as being much weaker than interelectronic repulsion terms, is not applicable in most cases. Neither, however, can one usually apply the other extreme, the j j coupling scheme, which relies on spin orbit coupling being strong compared with electrostatic repulsion. Thus the intermediate coupling scheme (intermediate between RS and jj) is used. It will be recalled that the f f electronic transitions in the spectra of lanthanide complexes are relatively weak in comparison with those of transition metal complexes. However, in the case of the actinides, the 5f orbitals are larger than lanthanide 4f orbitals, so that they interact more with ligand orbitals, causing much higher extinction coefficients and also, because covalency is greater, to create greater nephelauxetic effects in actinide spectra. This means that there is more variation in both position and intensity of absorption bands than in lanthanide compounds. The forbidden electronic dipole transitions are allowed in the presence of an asymmetric ligand field, which can arise by either a permanent distortion or by temporary coupling with an asymmetric metal ligand vibration (vibronic coupling). Apart from the f f transitions, there are two more types of absorption bands to note in actinide spectra. In general, the parity-allowed 5f 6d transitions occur above cm 1, since the 6d levels are considerably above 5f for most actinides; these are more intense (and broader) than the f f Lanthanide and Actinide Chemistry S. Cotton C 2006 John Wiley & Sons, Ltd. ISBN:

2 202 Electronic and Magnetic Properties of the Actinides transitions. In the case of the free U + ion, the 5f 2 6d 1 level is over cm 1 above the 5f ground state, while in U + (aq) the charge-transfer transitions start around cm 1 ; solvation thus has a very significant effect upon the relative energies of the 5f and 6d electrons. Metal ligand charge-transfer transitions have their maxima out in the ultraviolet, but, as with transition metals, the tail of these broad and often very intense absorption bands runs into the visible region of the spectrum, and is responsible for the red, brown, or yellow colours often noted for actinide complexes with polarizable ligands like Br or I Absorption Spectra Uranium(VI) UO 2 2+ f 0 The ground state of the uranyl ion has a closed-shell electron configuration. There is a characteristic absorption cm 1 (400 nm) which frequently gives uranyl compounds a yellow colour (though other colours like orange and red are not infrequent). This absorption band often exhibits fine structure due to progressions in symmetric O =U =O vibrations in the excited state, sometimes very well resolved, sometimes not (Figures 12.1 and 12.2). It should also be remarked that uranyl complexes tend to emit a bright green fluorescence under UV irradiation, from the first excited state. This is used by geologists both to identify and to assay uranium-bearing minerals in deposits of uranium ores Molar absorptivity Wavelength (nm) Figure 12.1 The absorption spectrum of (1) [UO 2 (OAc) 4 ] 2 in liquid Et 4 NOAc.H 2 O, showing the lack of vibronic structure, due to hydrogen bonding; (2) [UO 2 (OAc) ] in MeCN solution, showing the progression due to the O =U =O stretching vibration (from J.L. Ryan and W.E. Keder, Adv. Chem. Ser., 1967, 71, 5 and reproduced by permission of the American Chemical Society).

3 Absorption Spectra Absorbance Wavelenght (nm) Uranium(V) f 1 Figure 12.2 Absorption spectra of THF solutions of 1 [UO 2 Cl{η -CH(Ph 2 PNSiMe ) 2 }(thf)] and and 2 [UO 2 Cl{η - N(Ph 2 PNSiMe ) 2 }(thf)] (reproduced with permission of the Royal Society of Chemistry from M.J. Sarsfield, H. Steele, M. Helliwell, and S.J. Teat, Dalton Trans., 2004, 44). The 2 F ground state is split into two levels, 2 F 7/2 and 2 F 5/2,byspin orbit coupling in the free ion. These are split further under the influence of a crystal field; the effect on the energy levels of increasing the crystal field up to the strong field limit is shown in Figure 12.. Four transitions are thus expected in the electronic spectrum and generally, in practice, four groups of lines are seen, between the near-ir and the visible, bearing out this prediction. The ground state is a Kramers doublet (Ɣ 7 ), so U V compounds are EPR active. Doublet Γ 6 Quartet Γ 8 2 F 7/2 t 1u (Γ 4 ) Doublet Γ 7 Energy 2 F 5/2 Quartet Γ 8 Doublet Γ 7 t 2u (Γ 5 ) a 2u (Γ 2 ) Increasing crystal field Figure 12. The effect of increasing crystal field upon the energy of an electron in an f 1 system such as U v (reproduced with permission from S.A. Cotton. Lanthanides and Actinides, Macmillan, 1991, p. 109).

4 204 Electronic and Magnetic Properties of the Actinides Uranium(IV) f 2 The ground state arising from the f 2 configuration is H 4 (Figure 12.4) and the effect of a crystal field is to split both that and excited states further. A large number of electronic transitions are thus expected, and this is borne out in practice (Figures 12.5 and 12.6). It will be noted that the transitions are often broader than those found in the spectra of lanthanide complexes and indeed the later actinides, see Section The 5f energy levels are more sensitive to coordination number than are the corresponding levels in the lanthanides; since there are bigger crystal-field effects, one sees pronounced differences between the spectra of 6-coordinate [UCl 6 ] 2 and of U 4+ (aq) (Figure 12.5), leading to the conclusion that the uranium(iv) aqua ion was not six coordinate (most recent EX- AFS results suggest a value of 9 or 10, see Table 11.1). Figure 12.6 displays another example of the difference in spectra between similar complexes of different coordination number. 1 S 1 S 0 P P 2 P 1 P 0 1 I 1 I 6 1 D 1 D 2 Figure 12.4 A qualitative energy-level diagram for the U 4+ ion, showing successively the effects of electrostatic repulsion, spin orbit coupling, and crystal-field splitting (the latter shown only for the ground state). Overlap between levels is neglected. Adapted from M. Hirose et al., Inorg. Chim. Acta, 1988, 150, L9, and reproduced by permission of the Editor. f 2 1 G F H Electrostatic repulsion Spin orbit coupling 1 G 4 F 4 F F 2 H 6 H 5 H 4 O h T 2a E g T 1g A 2g Crystal field E D 4h E g B 2g A 2g B 1g A 2g E g A 1g

5 Absorption Spectra 205 B Absorption (arbitrary) A Wavelength (nm) Figure 12.5 The absorption spectra of octahedral [UCl 6 ] 2 (A) and 9 10-coordinate U 4+ (aq) (B) (redrawn from D.M. Gruen and R.L. Macbeth, J. Inorg. Nucl. Chem., 1959, 9, 297 and reproduced by permission of Elsevier Science Publishers). Figure 12.6 Solid-state absorption spectra of octahedral [UCl 4 (Bu t 2 SO) 2] (A) and 8-coordinate [U(Me 2 SO) 2 ]I 4 (B) (redrawn from J.G.H. DuPreez and B. Zeelie, Inorg. Chim. Acta, 1989, 161, 187 and reproduced by permission of Elsevier Science Publishers).

6 206 Electronic and Magnetic Properties of the Actinides Detailed investigations have been made of the octahedral [UCl 6 ] 2 ion. Its spectrum is largely vibronic in nature, with electronic transitions accompanied by vibrations of the complex ion (odd-parity modes the T 1u asymmetric stretch and T 1u and T 2u deformations). Here, as in other U IV cases, overlap of bands from different states occurs because of the similarity in crystal-field and spin orbit coupling effects. Its spectrum can be altered by destroying the centre of symmetry (e.g., by hydrogen bonding), which enables pure electronic transitions to be observed, and alters band patterns in multiplets Spectra of the Later Actinides Because of the relatively short half-lives of many later actinides, purity of samples and correct identification of lines can be a matter of uncertainty, but Figure 12.7 shows how this Figure 12.7 The absorption spectrum of the hexagonal form of BkCl as a function of time. The changes in the spectrum are due to the formation of CfCl. Note the sharp lanthanide-like transitions characteristic of the later actinide (+) state. (from J.R. Peterson et al., Inorg. Chem., 1986, 25, 779 reproduced by permission of the American Chemical Society).

7 Magnetic Properties 207 can be turned to advantage. It shows spectra obtained over a period of time from a sample of 249 BkCl beginning 11 days after synthesis. Now, 249 Bk is a β-emitter with a half-life of 20 days, and the spectrum obtained over a 976 day period (three half-lives, during which time the berkelium decays to some 12.5 % of its original amount) shows the loss of the characteristic absorptions due to the 249 BkCl and their replacement by a spectrum due to 249 CfCl. These spectra are very reminiscent of the sharp, line-like absorptions obtained from the lanthanides. This reflects the fact that chemically the heavy actinides are lanthanide-like, suggesting that with increasing atomic number the 5f orbitals are now more core-like and thus less readily influenced by environment. After the emission of a β-particle from the Bk nucleus the californium ion regains an electron to maintain the (+ ) oxidation state: Bk Cf 4+ + e Cf 4+ + e Cf + It may also be noted that crystal type is retained; X-ray diffraction confirms that the CfCl retains the hexagonal structure of the original BkCl rather than adopting the orthorhombic modification. 12. Magnetic Properties Uranium(VI) compounds are expected to be diamagnetic, with their 1 S 0 (f 0 ) ground state. However, compounds like UF 6 and uranyl complexes in fact exhibit temperatureindependent paramagnetism, explained by a coupling of paramagnetic excited states with the ground state. Uranium(V) compounds are, as expected for an f 1 system, paramagnetic, usually exhibiting Curie Weiss behaviour, with large Weiss constants; g-values, expected to be 6/7, are modified by the mixing in of higher states and by orbital-reduction effects (covalency), experimental g-values including values of 1.2 in Na UF 8 and 0.71 in CsUF 6. Matters are more complicated for uranium(iv); this f 2 system has a H 4 ground state, the energy level diagram has already been given (Figure 12.4). In a regular octahedral geometry, there is no contribution to the paramagnetic susceptibility from the first-order Zeeman term. Species like the [UCl 6 ] 2 ion (and isoelectronic PuF 6 ) display temperatureindependent paramagnetism, caused by the second-order Zeeman term mixing the T 1g excited state into the ground state. In lower symmetry, such as a D 4h trans-ux 4 L 2 complex, both the first- and second-order Zeeman effects contribute to the susceptibility. If there is a small distortion from a regular octahedron, the splitting of the T 1g excited state is small, so that E in Figure 12.4 is large. There is thus little thermal population of these excited states, so the first-order Zeeman effect is small; the paramagnetic susceptibility shows little or no temperature dependence. As the distortion becomes larger, in complexes like UBr 4 (Et AsO) 2 and UI 4 [(Me 2 N) PO) 2 ], thermal population of a component of the T 1g excited state becomes more feasible, and thus the susceptibility shows a greater temperature dependence. In the case of a cis-ux 4 L 2 complex, with D 4h symmetry, there is no firstorder Zeeman term, so that the second-order Zeeman effect causes temperature-independent paramagnetism. Figure 12.8 shows variable-temperature susceptibility data for some U IV complexes of these types, which is in keeping with these explanations.

8 208 Electronic and Magnetic Properties of the Actinides 9 A 8 χ(mol 10 /cgs emu) B 2 C D Temperature (K) Figure 12.8 Temperature dependence of the magnetic susceptibility of some uranium(iv) complexes: (A) trans- [UBr 4 (Et AsO) 2 ]; (B) trans-[ucl 4 (Et AsO) 2 ]; (C) (Ph 4 P) 2 [UCl 6 ]; (D) cis-[ucl 4 (Ph PO) 2 ] (redrawn from B.C. Lane and L.M. Venanzi, Inorg. Chim. Acta, 1969,, 29 and reproduced by permission of the editor). Few data are available for uranium(iii) compounds, but a number of compounds with the f configuration, like Cs 2 NaUCl 6,haveµ.2 µ B, which is largely temperature independent.