The case of lanthanides and actinides

THE CASE OF LANTHANIDES AND ACTINIDES 2.8.1 Electronic properties of lanthanides... 

The relaxation equations discussed here and in Section 3.4 and 3.5 take a different form in the case of lanthanides and actinides. For these systems, in fact, the J quantum number substitutes the S quantum number and gj substitutes ge. In the absence of chemical exchange phenomena (r 1 r ) the equations for the... 

In the case of lanthanides and actinides, the ionic radii decrease greatly and the differences between the atomic radii and ionic radii are significant. They are much bigger than those of other elements. If other elements display 10-20% decreases, in lanthanides and actinides the ionic radii would decrease by 50-60%. Also, among lanthanide elements and actinide elements, respectively, the differences between the atomic and the ionic radii are quite small. They are practically equal in most instances (Figures 2.11 and 2.12). 

The actinides. The actinides metals are electropositive and very reactive they are pyrophoric in finely divided form. They tarnish rapidly in air forming an oxide protective coating in the case of Th, but more slowly for the other actinides. The metals react with most non-metals. With steam or boiling water, oxide is formed on the surface of the metal and H2 evolves in this way hydrides are produced that react rapidly with water and facilitate further attack on the metals. The oxidation states observed in the chemistry of lanthanides and actinides are shown in Fig. 5.9. Notice the predominant oxidation state III for the lanthanides... 

Forty years ago, very little was known about lanthanide complexes. By analogy with the d-block metals, it was often assumed that lanthanides were generally six coordinate in then-complexes. We now know that this is not the case, that lanthanides (and actinides) show a wider variety of coordination number than do the d-block metals, and also understand the reasons for their preferred choice of ligand. 

Systems capable of effecting the detection of lanthanide and actinide ions using electrochemical methods have generally consisted of a receptor or ionophore immobilized within a membrane to form an ion-selective electrode (ISE). This electrode is then pertmbed when a cation guest is boimd. On the other hand, optical sensors or optodes are based on a molecule that is either free standing or attached to a polymer matrix. In both cases, the expectation is that an optical response will be produced when the system is exposed to a particular anal5de. Needless to say, for either approach it is beneficial to have a receptor that is well tuned for the lanthanides and actinides. In practice, this has translated to the use of macrocycles rich in O, N, S, or P donor atoms as will be clear from the summary of recent work provided in this article. This article focuses on ISE- and optode-based approaches to lanthanide and actinide cation sensing. 

The amount of unreacted target element that eluted was determined by measuring its radioactivity directly in the case of actinides, and by activation analysis in the case of lanthanides. The distribution of the radioactive neutron capture product was determined by counting both the eluate and the eluted zeolite. All irradiations were done in the Oak Ridge Research reactor in a pneumatic tube facility with a thermal neutron flux of about 4 X 1013 neutrons cm-2 sec-1 or, for a few long irradiations, in a tube adjacent to the reactor core at the fluxes stated in Table VI. 

Visible chemiluminescence has been observed from many reactions of lanthanide and actinide atoms producing metal oxides (M + N02, N20 and 03 [212]), but, in most cases, the spectra are unanalysed and it is not clear even what product MO states are produced. The variation of chemiluminescence intensity with reagent energy has been studied for the reactions Sm, Ho + N20. For Ho + N20, three excited electronic states are produced the relative population of the states decreasing as the energy of the states increases. There is no significant difference in... 

As alluded to above, metal complexes of a number of lanthanide and actinide texaphyrin complexes have also been prepared. In the case of Dy(III) texaphyrin 9.74, as in the case of the bis-pyridine cadmium complex 9.61b, the metal center sits directly within the mean plane of the macrocycle (Figure 9.1.12). This result stands in direct contrast to the highly labile, typically sandwich-type 2 1 or 3 2 complexes observed for porphyrin complexes with these larger metal cations. ... 

Preparation and Characterization of Lanthanide and Actinide Solids. Crystalline / element phosphates were prepared as standards for comparison to the solids produced in the conversion of metal phytates to phosphates. The europium standard prepared was identified by X-ray powder diffiaction as hexagonal EuP04 H20 (JCPDS card number 20-1044), which was dehydrated at 204-234 °C and converted to monoclinic EUPO4 (with the monazite structure) at 500-600 °C. The standard uranyl phosphate solid prepared was the acid phosphate, U02HP04 2H20 (JCPDS card number 13-61). All attempts to prepare a crystalline thorium phosphate failed, though thorium solubility was low. In the latter case the solids were identified as amorphous Th(OH)4 with some minor crystalline inclusions of Th02. 

Although we have treated the hyperfine interaction as an interaction between the nuclear spin I and the intrinsic spin s of the electron, it is in reality the interaction of I with the total angular momentum of the electron. Most lanthanide and actinide ions and some transition metal ions in high symmetry do not have their orbital momentum quenched by the crystal fields. In these cases a complete treatment of the hyperfine interaction must include the following term... 

Speciation and reactivity of actinide compounds comprise an important area for quantum chemical research. Even more so than in the case of lanthanides, f-type atomic orbitals of actinides can affect the chemistry of these elements [185,186] the more diffuse 5f-orbitals [187] lead to a larger number of accessible oxidation states and to a richer chemistry [188]. The obvious importance of relativistic effects for a proper description of actinides is often stressed [189-192]. A major differences in chemical behavior predicted by relativistic models in comparison to nonrelativistic models are bond contraction and changes in valency. The relativistic contribution to the actinide contraction [189,190] is more pronounced than in the case of the lanthanides [191,192]. For the 5f elements, the stabilization of valence s and p orbitals and the destabilization of d and f orbitals due to relativity as well as the spin-orbit interaction are directly reflected in the different chemical properties of this family of elements as compared with their lighter 4f congeners. Aside from a fundamental interest, radioactivity and toxicity of actinide compounds as well as associated experimental difficulties motivate theoretical studies as an independent or complementary tool, capable of providing useful chemical information. 

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