There are particularly strong similarities in the chemistry of cerium(IV) and thorium(IV). Cerium(IV) oxide and thorium(IV) oxide both adopt the fluorite structure. They form isostructural nitrates, M(NO3)4⋅5H2O, where M is Ce or Th, and both form hexanitrato-complex ions [M(NO3)6]2–. The major difference between the two elements in this oxidation state is that thorium(IV) is the thermodynamically stable form of that element while cerium(IV) is strongly oxidizing (Eθ = +1.44 V). There is a mineralogical link between cerium(IV) and thorium(IV). Thorium and the lanthanoids — particularly cerium — are found together in two minerals, monazite and xenotime [18].
Table 13.4 The 4+ ions and their corresponding electron configurations
Ion
Noble Gas Core Electron Configuration
Zirconium(IV)
[Kr]
Cerium(IV)
[Xe]
Hafnium(IV)
[Xe]4f
14
Thorium(IV)
[Rn]
Similarities of the Later Actinoids with the Lanthanoids
There is a generic similarity of the later actinoids with the lanthanoids as they share the common oxidation state of +3. As an example of a close parallel, Xu and Pyykkö have shown that each of the first three ionization potentials of lawrencium is very close to those of the lanthanoid analogue, lutetium [19].
One specific correlation was demonstrated by Thompson et al. in the context of chromatographic elution curves (Figure 13.6). As can be seen there is a remarkable match in peaks for the corresponding pairs of terbium–berkelium, gadolinium–curium, and europium–americium [20].
According to computational studies, one difference between a lanthanoid(III) ion and the corresponding actinoid ion is the degree of bond covalency involving f orbitals. In this investigation, a comparison of the bonding in valence-isoelectronic [EuCl6]3− and [AmCl6]3− was made. Cross et al. concluded that the involvement of the americium 5f electrons in bonding with the 3d electrons of chlorine is far more significant than any involvement of the europium 4f electrons with the 3d orbitals of chlorine [21].
Figure 13.6 A comparison of chromatographic elution curves for three lanthanoids and three actinoids (adapted from Ref. [20]).
Similarity of Nobelium(II) and Group 2 Elements
As was mentioned earlier, nobelium favors the +2 oxidation state. This preference is far more than that of ytterbium, the corresponding lanthanoid, for which the +2 oxidation state is readily oxidized.
Table 13.5 Atom and 2+ ion configurations for radium and nobelium
Element
Atom
Configuration
+2 Ion
Configuration
Radium
[Rn]7s
2
[Rn]
Nobelium
[Rn]7s
2
5f
14
[Rn]5f
14
However, somewhat surprisingly, there is a much greater similarity to the chemistry of the lower alkaline earth metal ion as Maly et al. have stated [22]:
In the absence of oxidizing or reducing agents the chromatographic and coprecipitation behavior of element 102 is similar to that of the alkaline earth elements. … Nobelium is the first actinide for which the +2 oxidation state is the most stable species in aqueous solution.
The similarity in electron configuration with radium can be seen from Table 13.5.
Post-Actinoid Elements
The post-actinoid elements or, more correctly, the super-heavy elements (see Chapter 5), are those from element 104 (rutherfordium) up to yet-to-be-discovered element 126. From rutherfordium to copernicium, these are the 6d members of the transition metal series, then nihonium to oganesson correspond to the filling of the 7p orbitals.
With ever shorter half-lives leading to the ephemeral elements (see Chapter 5), the knowledge of their actual chemistry is very limited. Computational studies of what their chemistry should be increasingly fill the literature, but these unsubstantiated claims will be given only limited space here.
The 6d Elements
With isotope half-lives up to 1.3 hr, aspects of the chemistry of rutherfordium are well established. In particular, the +4 ion seems to be the sole common oxidation state, corresponding to an [Rn]5f14 electron configuration. The compounds characterized to the date of writing this book match those of the heavier Group 4 elements (zirconium and hafnium), specifically: RfCl4, RfBr4, RfOCl2, and K2RfCl6 [23].
Proceeding along the 6d series, the chemistry is based on every more limited data. Dubnium seems to behave like its Group 5 “relatives” niobium and tantalum, and also “pseudo-homologue” protactinium [24]. Likewise, seaborgium appears to form SgO2Cl2 analogous to MoO2Cl2 and WO2Cl2 [25]. Synthesis of Sg(CO)6 matching with Mo(CO)6 and W(CO)6, has also been claimed [26]. Lougheed has commented that, despite predictions of relativistic effects causing dramatic changes in the chemical behavior of these elements, the chemistry of the 6d elements established so far seems to be that of their corresponding earlier group members. Seaborgium, in particular, from its limited chemistry, seems to be just a “normal” Group 6 element [27].
Bohrium, too, follows the pattern of having a matching chemical compound to the other heavy transition metals of Group 7. That is, it forms the compound BhO3Cl, analogous to TcO3Cl and ReO3Cl [28]. Similarly, hassium has been shown to form the characteristic species of Group 8, that is, HsO4 [29].
With such short-lived isotopes, meitnerium seems to mark the current limit of practical chemistry. The chemistry of this element would be of particular interest. Assuming that it did behave chemically as a member of Group 9, it has been proposed from theoretical studies that it might form a compound of meitnerium(IX), that is, [MtO4]+, isoelectronic to HsO4 [30].
A theoretical study of the properties of darmstadtium indicated that it should resemble platinum of Group 10 in its chemistry, in particular in forming a strong bond to the carbonyl ligand [31]. Similarly, roentgenium is predicted to resemble silver of Group 11 in readily forming a +1 species in the [Rg(OH2)2]+ ion [32]. Likewise, according to computational analysis, copernicium is likely to have physical and chemical properties resembling those of mercury [33].
The 7p Elements
All of the presumptions of the chemistry of these elements come from theoretical computations. As such, and as some are contradictory, only a few selected cases will be described. Nihonium is expected to be a typical Group 13 element with a predominant +3 oxidation state [34] while it has been proposed that flerovium is a volatile metal [35]. The most attention has focused on oganesson, the Group 18 member of the period. It has been suggested that this element would be a liquid at
