Bonding of Lanthanides and Actinides

In this chapter, we first present an overview of some of the technical difficulties that make f element computational chemistry so challenging, before going on to a number of examples of the uses of different theoretical approaches and the chemical insight they can provide. We shall focus on recent, topical examples that illustrate applications of some of the analytical techniques presented in Volume 1 of this book. 

The Chemical Bond Chemical Bonding Across the Periodic Table, First Edition. 

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The overall distribution of lanthanides in bone may be influenced by the reactions between trivalent cations and bone surfaces. Bone surfaces accumulate many poorly utilized or excreted cations present in the circulation. The mechanisms of accumulation in bone may include reactions with bone mineral such as adsorption, ion exchange, and ionic bond formation (Neuman and Neuman, 1958) as well as the formation of complexes with proteins or other organic bone constituents (Taylor, 1972). The uptake of lanthanides and actinides by bone mineral appears to be independent of the ionic radius. Taylor et al. (1971) have shown that the in vitro uptakes on powdered bone ash of 241Am(III) (ionic radius 0.98 A) and of 239Pu(IV) (ionic radius 0.90 A) were 0.97 0.016 and 0.98 0.007, respectively. In vitro experiments by Foreman (1962) suggested that Pu(IV) accumulated on powdered bone or bone ash by adsorption, a relatively nonspecific reaction. On the other hand, reactions with organic bone constituents appear to depend on ionic radius. The complexes of the smaller Pu(IV) ion and any of the organic bone constituents tested thus far were more stable (as determined by gel filtration) than the complexes with Am(III) or Cm(III) (Taylor, 1972). 

Zinc tetraphenylporphyrinate forms a weak complex with Of in non-aqueous solutions. The bonding in this complex appears to be essentially ionic . We have already mentioned crystal structure determinations of lanthanide and actinide compounds. There is every reason to suppose that these elements have a rich dioxygen complex chemistry and this is confirmed by two recent papers For reasons of space, however, we shall not discuss the dioxygen complex chemistry of these elements. 

Blasse G (1976) The influence of charge-transfer and Rhydberg stales on the luminescence properties of lanthanides and actinides. Struct Bond 26 43-79... 

There is a strong affinity of lanthanides and actinides for oxygen.This property can be helpful for the activation of oxygenated organic functions. An indirect measure of the oxophilicity of f-elements is the bond strengths D for the gas phase dissociation of the diatomic species MO (8). For lanthanides Kcal/mole (799Kj/mole) for... 

P. L. Watson, in Selective Hydrocarbon Activation, J. A. Davies, Ed., VCH Publishers, New York, 1990, pp. 79-112. C—H Bond Activation with Complexes of Lanthanide and Actinide Elements. 

Fig. 1. Atomic volumes of lanthanide and actinide metals. Pa, U, Np and Pu density increased by 5f-bonding. Eu, Es, Yb low density due to divalency.

Shell structure, relativistic and electron correlation effects play an important role for the electronic structure of lanthanide and actinide systems. The importance and the magnitude of these effects have been examplified for the atoms Ce and Th, and the contributions of the Ce 4f and Th 5f shell to chemical bonding have been reviewed for the monoxides CeO and ThO, respectively. Currently quantitatively correct results for lanthanides and actinides can only be obtained from ab initio calculations for the easiest cases, e.g., small systems (atoms, diatomics containing one f element) with a possibly small number of unpaired f electrons and/or problems related only to configurations with the same f occupation number. In other cases ab initio quantum chemistry can at least help to interprete experimental findings. As an example organometallic cerium sandwich complexes such as cerocene were discussed, which may be considered to be molecular analogues of cerium(in)-based Kondo lattice systems. 

Lanthanide and actinide compounds are difficult to model due to the very large number of electrons. However, they are somewhat easier to model than transition metals because the unpaired / electrons are closer to the nucleus than the outermost d shell. Thus, all possible spin combinations do not always have a significant effect on chemical bonding. 

Quantum chemists have developed considerable experience over the years in inventing new molecules by quantum chemical methods, which in some cases have been subsequently characterized by experimentalists (see, for example, Refs. 3 and 4). The general philosophy is to explore the Periodic Table and to attempt to understand the analogies between the behavior of different elements. It is known that for first row atoms chemical bonding usually follows the octet rule. In transition metals, this rule is replaced by the 18-electron rule. Upon going to lanthanides and actinides, the valence f shells are expected to play a role. In lanthanide chemistry, the 4f shell is contracted and usually does not directly participate in the chemical bonding. In actinide chemistry, on the other hand, the 5f shell is more diffuse and participates actively in the bonding. 

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