La guerra civ il (1861-1865)

The magnetic properties of actinide ions and relative compounds derive from the spin and orbital angular momenta of unpaired electrons21 _{and in 1932 Van Vleck published the}

theoretical basis for understanding these properties.120 _{However, the interpretation of the}

magnetic data for actinide samples is still challenging and accurate theoretical models are needed. This is principally due to the fact that spin-only approximation, that works well for the first-row TM, loses its validity for actinides because of larger spin-orbit coupling and relativistic effects.121

Ideally, magnetic susceptibility measurements need to be presented together with optical spectroscopic data to have clear information on the electronic structure of actinide compounds. Optical data can be used to resolve the symmetry, then information on the ground state and on the low-lying states can be obtained from magnetic data, such as temperature-dependent magnetic susceptibility and EPR measurements. Moreover, the nature of the metal-ligand bonding can be investigated by focusing on the ligand superhyperfine coupling, using double resonance (electron nuclear double resonance,

ENDOR) and pulsed (electron spin echo envelope modulation, ESEEM)68_{EPR methods.}

Thus, it is possible to estimate the delocalization of the electron spin density into the ligand orbitals, in order to gain useful information on the degree of covalency into the metal- ligand bonding.122

1.5.1 Magnetic Susceptibility of Uranium Ions

Closed shell systems, such as Th4+_{, Pa}5+_{, U}6+_{and [UO}

2]2+ are expected to be diamagnetic,

due to their 1_{S0 (f}0_{) ground state. However, compounds like UF6 and some uranyl} complexes can exhibit temperature-independent paramagnetism (TIP), due to some covalency in metal-ligand bonding. Magnetic susceptibility measurements, reported by

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Eisenstein and Pryce,123_{concluded that in UF}

6 there is a coupling of paramagnetic excited

states with the diamagnetic ground state and probably this is a consequence of a partial

covalent character in the U–F bonding. Similarly, the weak paramagnetism in [UO2]2+,

reported by Denning et al.,124 _{is attributed to the coupling between different states and to}

particular characteristics of the U=O bonding.

Uranium has several paramagnetic oxidation states, 5f1 _U5+_{, 5f}2 _U4+_{, and 5f}3 _U3+_,

therefore an useful analytical method for characterizing uranium complexes is the measurement of magnetic susceptibilities.

Uranium(V) compounds are, as expected for a 5f1 _{system, paramagnetic, usually}

exhibiting Curie-Weiss behaviour, with large Weiss constants.125 _{The absence of electron}

repulsion makes the magnetic data relatively simple to interpret and the magnetic susceptibility is generally reduced by the mixing of higher states and by orbital-reduction effects (covalency).125_{Experimental g-values are, for example, 1.2 in Na3UF8 and 0.71 in}

CsUF6.125

Magnetic susceptibility measurements are often used to confirm the oxidation state of uranium. However, at room temperature, the μeff values of 3.62 and 3.58 μB, often chosen

to distinguish the magnetism for U(III) and U(IV), respectively, are close enough to be within the experimental error of typical measurements.3i, 126 _{Moreover, the histogram in}

Figure 1.8(d) clearly shows that the range of the room-temperature magnetic moments for each of the common paramagnetic uranium ions is large and the overlap is wide.127 _{As a}

consequence, any room-temperature magnetic moment between 1.75 and 3.77 μB could be claimed to be within the full range of U(III), U(IV) or U(V).

The electronic population of the energetic sublevels is also strongly influenced by the temperature and the low temperature depopulation of the sublevels leads to a concomitant decrease in the magnitude of the angular momentum vector which is manifested as a decrease in the magnetic susceptibility.111b, 128 _{Thus, considering that the variable}

temperature magnetic behaviours are distinct and possibly revealing of the oxidation state, and the exact shape of the magnetization response can be quite informative on crystal- field splitting,129 _{it is preferable to obtain magnetic data at variable temperature with a}

Superconducting QUantum Interference Device (SQUID).130 _{In fact, normally, the}

susceptibility of a 5f2 _{U(IV) ion drops drastically toward a diamagnetic ground state at}

low temperature (typically around 50 K). In fact, as the temperature is lowered, the two

unpaired 5f electrons of a U(IV) ion become spin paired in first approximation, leading to the observation of a single magnetic ground state.127 _{This does not occur instead with half-}

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integral 5f3 _{U(III) and 5f}1 _{U(V) ions.}127 _{However, the trend of the data is not always}

consistent with a given ion.131

The magnetic profile of a uranium compound is indeed strictly dependent upon the electronic configuration of the uranium ion. For example, as anticipated, the magnetic profile of 5f2 _{U(IV) compounds can be qualitatively summarized considering a non-} degenerate singlet (S = 0) ground state with Temperature Independent Paramagnetism

(TIP) at low temperatures (to ~ 100 K), followed by a region of temperature-dependent

paramagnetism (Figure 1.8b).127, 129 _{However, this temperature-dependent paramagnetism}

is also dependent on the coordination symmetry.127 _{For example, in an octahedral}

geometry, Oh, there is no contribution to the paramagnetic susceptibility from the first-

order Zeeman term and only the second-order Zeeman term is observed. Species like the

[UCl6]2−ion (and the isoelectronic PuF6) display TIP, caused by the second-order Zeeman

term mixing the T1g excited state with the ground state Ag. Instead, when the symmetry is

lowered from Oh to D4h, it has been shown that the 1st excited state changes from a triplet

T1g to a doublet Eg and both the first- and second-order Zeeman terms contribute to the

susceptibility. In this case, temperature-paramagnetism depends on the splitting of the T1g

excited state, on the value of ΔE and on the thermal population of these excited states.127, 129 _{Scheme 1.5 illustrates the crystal-field splitting on the electronic ground state for a 5f}2 configuration in two different symmetries.85

Scheme 1.5. Qualitative energy-level diagram showing the crystal-field effect on the electronic ground state for a Oh and D4h symmetry.85

Figure 1.8 displays typical variable-temperature magnetic profiles for a) uranium(III), b) uranium(IV) and c) uranium(V).

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Figure 1.8. Representative μeff (μB) versus temperature (K) plots for a) U(III), b) U(IV) and c)

U(V).75 _{d) Histogram of room-temperature magnetic moments for some uranium monometallic}

complexes of the three common oxidation states with U(III), U(IV), and U(V) in white, grey, and

black, respectively. Histogram bin widths are 0.20 μB.127

A typical uranium(III) magnetic profile decreases only slowly until at low temperature, where a drop in the magnetic moment occurs as low-lying states are depopulated, and the uranium ion exhibits an orbital doublet (S = 2) ground state. Notably, however, the magnetic moment is usually higher than what would be expected for the equivalent system with one unpaired electron because some low-lying states are not completely depopulated, even at low temperature. In contrast, uranium(IV) commonly presents a monotonous decrease in the magnetic moment, and the curve tends to zero at low temperatures. At 2

K, there is usually a residual magnetic moment of approximately 0.3 – 0.5 μB, which is

due to TIP. The magnetism of uranium(V) tends to exhibit a more flat line with small variations in the magnetic moment from room temperature (calculated moment of 2.54 μB) to approximately 50 K; below this temperature, there is a rapid drop due to the depopulation of low-lying states, but an appreciable value (ca. 1.1 μB) tends to remain

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because 5f1_{uranium(V) is always an orbital doublet (S = 2). However, the moment is} usually smaller than for uranium(III) because there is less mixing with excited states.

Moreover, it is important to underline that experimentally observed magnetic moments of uranium complexes frequently deviate below ideal behaviour. In Figure 1.8, the magnetic moments at 298 K for uranium(III) and uranium(IV) are approximately 3 μB,

whereas for uranium(V) it is about 2 μB. This effect might be attributed to partial

quenching of L by the crystal-field, which would invoke covalency. However, numerous factors contribute to the observed magnetism in each case, and their inter-relationships are complex and non-linear; therefore, accrediting the unusual magnetism of some uranium complexes as only due to an enhanced covalency is too simplistic.75, 131

The magnetic properties of uranium compounds have attracted interest because a number of 5f3 _{U(III) compounds, with a} 5_{I9/2 ground state, show Single Molecular}

Magnetic (SMM) behaviour,132 _{and some 5f}1 _{uranyl(V) compounds can also exhibit} unusual magnetic behaviour.133 _{Most interestingly it appears that SMM behaviour is an}

intrinsic property of the U(III) ion.134 _{A recent computational study}135 _{has suggested that}

also 5f2 _{U(IV) compounds could present unusual magnetic behaviour, particularly in} tetragonal or trigonal prismatic geometries with the correct ground state.