5f-orbitals

Uranium is the fourth element of the actinide (SJ series. In the actinide series the electrons are more effectively shielded by the Is and 7p electrons relative to the 4f electrons (shielded by 6s, 6p) in the lanthanide (4p series. Thus, there is a greater spatial extension of 5f orbitals for actinides than 4f orbitals for lanthanides. This results in a small energy difference between and 5/ 6d7s electronic configurations, and a wider range of oxidation states is... 

The actinides ( actinons or actinoids ) are the fourteen elements from thorium to lawren-cium inclusive, which follow actinium in the periodic table. They are analogous to the lanthanides and result from the filling of the 5f orbitals, as the lanthanides result from the filling of the 4f. The position of actinium, like that of lanthanum, is somewhat equivocal and, although not itself an actinide, it is often included with them for comparative purposes. 

Assuming confirmation by further work, these compounds are the first Pu compounds to show Pu-ethylenic bonding. The nature of this bonding is unknown, but participation of 5f orbitals with it orbitals of the ethylene double bond, though unlikely, should be considered. The large and easily polarizable iodide ions could be the key factor in stabilizing the proposed Pu+3-ethylene bonds. 

Figure 4. Dependence of the ratio u u on the number of 5f electrons for light actinide compounds x free ion values, ° experimental values, form band calculations. The hybridisation between 5f and 3d electrons leads to the reduction of the 5f orbital moments (metallic covalency).

Already in the thorium dimer, Th2, we see another pattern. The 7s population is reduced to close to one. The electron is moved to 6d, and a strong quadruple bond is formed, involving three two-electron bonds and two 6d one-electron bonds. We also start to see some population of the 5f orbitals that hybridizes with 6d. 

With the Pa2 dimer, we have reached the maximum bonding power among the actinide dimers. In U2 the bond energy decreases and the bond length increases, which is from the increased stabilization of the 5f orbitals and the corresponding destabilization of 6d. Large transfer of electrons from... 

The f-f overlapping in light actinides may cause broadening of the 5 f wave functions into 5 f bands. On the other hand, from Am on, this overlapping having decreased, this effect occurs much less. It follows that physical properties which depend from 5f orbitals may be better understood, in one case, in the band Umit, in the other case, in the atomic limit. 

Moving across each period the f orbital is progressively filled the 4f orbital is filled for the lanthanides, and the 5f orbital is filled for the actinides. These elements are sometimes referred to as the rare earths, because it was originally difficult to separate and identify these elements. Rare earths are actually not scarce, but the term rare earths is still used for the lanthanides and actinides. A more accurate modern term for these two periods are the inner transition elements. 

Currently, 28 elements filling the 7s, 6d and 5f orbitals have been discovered. In the seventh period, the 14 elements coming after 8gAc are called actinides. All the elements in the 6d block have been discovered, but not all of the elements in the 7p block. 

The lanthanides (and actinides) are those in which the 4f (and 5f) orbitals are gradually filled. At lanthanum, the 5d subshell is lower in energy than 4f, so lanthanum has the electron configuration [Xe] 6s 5d (Table 2.1). 

Early in the actinide series, electrons in the 6d orbitals are lower in energy than there is 5f orbitals, This is clear from the ground-state electronic configurations (Table 9.3) of the atoms, which show that the 6d orbitals are filled before 5f. The 5f orbitals are starting to be filled at protoactinium, and with the exception of curium, the fid orbitals are not occupied again. 

A important relativistic effect is that 5f orbitals of actinides are larger and their electrons more weakly bound than predicted by non-relativistic calculations, hence the 5f electrons are more chemically available . This leads to ... 

More sophisticated MO calculations, especially including relativistic corrections, suggest that whilst uranium 6d orbitals interact with the ligand orbitals, the 5f orbitals are relatively unperturbed. Photoelectron spectra do suggest that 5f interaction increases as more alkyl groups are introduced into the cyclooctatetraene rings. 

The 5f orbitals are gradually filled as the actinide series is crossed and there is general agreement that the 6d orbitals will be filled next, followed by 7p orbitals. Table 14.2 lists predicted electron configurations. 

The absorption spectroscopy in the UV-Vis-NIR is especially rich for the actinides, allowing for fairly simple determinations of the metal oxidation state. The primary absorption bands result from f f transitions, f d and ligand-to-metal charge transfers. The f — f transitions are typically weak since they are forbidden under the LaPorte selection rules. Distortions in symmetry allow for relaxation in these rules and bands in the visible to near-infrared range result. Complexes that contain an inversion syimnetry, for example Pu02CLt, have weaker f- -f transitions (e < 20 cm ). The direct interactions of the 5f orbitals... 

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