Transition elements compared with actinides

It is interesting to compare these actinide(IV) cyclooctatetraene complexes with similar compounds of the group IVB transition elements Ti, Zr and Hf. Bis (cyclooctatetraene) complexes of aU three are known although structural data is only available for the first two. All would appear to involve both planar and non-planar COT rings and to exhibit a sHpped sandwich structure rather than the true sandwich structure of uranocene. 

The remaining exceptions concern the lanthanide series, where samarium at room temperature has a particular hexagonal structure and especially the lower actinides uranium, neptunium, and plutonium. Here the departure from simple symmetry is particularly pronounced. Comparing these three elements with other metals having partly filled inner shells (transition elements and lanthanides), U, Pu, Np have the lowest symmetry at room temperature, normal pressure. This particular crystallographic character is the reason why Pearson did not succeed to fit the alpha forms of U, Pu, and Np, as well as gamma-Pu into his comprehensive classification of metallic structures and treated them as idiosyncratic structures . Recent theoretical considerations reveal that the appearance of low symmetries in the actinide series is intimately linked to the behaviour of the 5f electrons. 

U is a member of the actinide series of elements which, together with the rare earths and the transition elements, possess a high heat of oxidation, a low oxide density compared with that of the metal, and the presence of an unfilled d shell in its electronic structure. While the reasons for the high pyrophoric potential of U are not clearly understood, they are thought to be related to these aforementioned properties (see under Pyrotechnics in Vol 8, P511 and Pyrophoric Incendiary Agents , P503-L)... 

The chemistry of the lighter actinides from thorium to americium, all being available in substantial quantities, is now well understood. In the - -4, or higher oxidation states, these elements are best considered as an inner transition series. Their chemistry shows both horizontal similarities within the actinide group and to a lesser degree, some vertical similarities with the group 4, 5, and 6 d-transition elements. All of the actinides in their +3 oxidation states behave in much the same way as the lanthanides. The chemistry of the actinides is reviewed within this context and compared with the corresponding lanthanides. 

Tanthanide chemistry is approaching its 200th Anniversary, but except for data on thorium and uranium the chemistry of the actinides is a comparative youngster of some 30 years. However, the two chemistries are intimately associated because their elements are of the f transition type and thus formally comparable with each other and different from other elements. Indeed, these parallels made it possible to unravel actinide behavior in the early days of transuranium element production. In addition to their chemical similarities, the two series also share the properties of magnetism and radiant energy absorption and emission characteristic of /-electron species. However, important differences exist also, particularly in oxidation states, in bonding, and in complex-ion formation. 

Compared with the lanthanides or the transition metals, the actinide elements introduce a striking array of novel chemical features, displayed most clearly in the chemistry of uranium. There is the variety of oxidation state, and to some extent the chemical diversity, typical of transition metals in the same periodic group, but physical properties which show that the valence electrons occupy /-orbitals in the manner of the lanthanides. This raises the question of the nature of the chemical bond in the compounds of these elements. The configuration of the uranium atom in the gas phase is f3ds2, so it is natural to ask whether there are special characteristics of the bonding that reflect the presence of both/and d valence orbitals. 

Compounds of the stoichiometry MCp 2X2 provide an opportunity to compare actinides with block elements. Several such species have been studied both by gas phase PE experiments as well as by theoretical approaches. Thus, the first comparative study of the PE spectra of a series of transition metal and actinide organometallic compounds with exacdy the same ligand array was carried out on MCp 2X2 (M = Zr, Th, U, and X = CH3 and Cl) complexes. Comparable bonding was found along the series with smaller differences than encountered in the parent series involving transition elements. The major differences between Zr and the actinides are due to the involvement of yin the bonding. 

This chapter is intended to provide a unified view of selected aspects of the physical, chemical, and biological properties of the actinide elements. The f block elements have many unique features, and a comparison of the lanthanide and actinide transition series provides valuable insights into the properties of both. Comparative data are presented on the electronic configurations, oxidation states, redox potentials, thermochemical data, crystal structures, and ionic radii of the actinide elements, together with a miscellany of topics related to their environmental and health aspects. Much of this material is assembled in tabular and graphical form to facilitate rapid access. Many of the topics covered in this chapter, and some that are not discussed here, are the subjects of subsequent chapters of this work, and these may be consulted for more comprehensive treatments. This chapter provides a welcome opportunity to discuss the biological and environmental aspects of the actinide elements, subjects that were barely mentioned in the first edition of this work but have assumed great importance in recent times. 

This relationship, which cannot be derived here, applies well if the unpaired electrons are situated on a nontransition metal atom or an element of the first transition series. It is inapplicable to the rare-earth and actinide ions in solution, for with these, the orbital angular momentum of the highly eccentric / electrons becomes comparable to their spin moment. 

Despite the large amount of experimental data for late transition metals, appreciable differences between values obtained in the same system are observed. It is also important to note that with Rh, Ir, Ru, Os, Mn and Re only a few experiments have been performed with rare-earth elements. Moreover we remark that very few experiments have been performed with plutonium. Thus, it is difficult to define the alloying behavior in the lanthanide series as well as in the actinide series and finally to compare the two series. 

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