Actinide elements divalent state, stability

It might be expected that lanthanides which exhibit divalent states, like Eu and Yb, would be most likely to have their sesquioxides vaporize via the generation of monoxide vapor species. Instead these sesquioxides tend to form atomic vapor (e.g., R) species and oxygen due to the greater stability of R versus RO. Thus, for the sesquioxides of actinides with a greater tendency towards divalency or that are divalent metals (e.g., Cf, Es and Fm), the more likely it is that the vaporization mode of their sesquioxides is via generation of elemental vapor plus oxygen. 

It is generally accepted that the stability of the divalent state increases for the second half of the actinide series of elements. Starting with californium, the potentials of the M(iii)/M(ii) couples increase regularly in the order No > Md > Fm > Es > Cf, with values ranging from 1.45 V for nobelium to -1.60 V for californium [172]. 

Californium is the first element in the sequence of actinides to show strong divalent tendencies. The progressive stabilization of the divalent ground-state (7), which is apparently complete at Fm and perhaps may occur already for Es (the estimated crystal entropy for Es is plotted in Figs. 1, 2, and 4 as being divalent). 

Noteworthy by their virtual absence in this review are the lanthanides [158] and actinides, whose crown thioether chemistry is as yet untouched. The propensity of these ligands to stabilize low oxidation states if carried over to the lanthanides engenders the possibility of synthesizing, e.g. complexes of divalent lanthanides (e.g. Sm(II), Eu(II), Yb(II)). While the lanthanides and actinides as class a elements... 

It is not feasible to consider conventional-type studies (structure, vaporization behavior, etc.) for the transeinsteinium oxides, given their scarcity and short lifetimes. Presumably their oxide chemistry would be very similar to that of the lanthanide oxides, especially for the trivalent oxidation state. However, there is an increased tendency for actinides in the second half of the series toward divalency. This suggests that the monoxides of the higher actinides (e.g., transeinsteinium elements) could be potentially more stable, perhaps approaching the stability of Eu monoxide. With No (element 102 homolog of Yb), the monoxide could be its most stable oxide. 

Several properties of californium were considered in making correlations of the 5f-electron status across the series [81,82,85-88,91-93] one prediction was that californium metal would be close to the divalent-trivalent metallic boundary [85]. Californium is the first element in the actinide series to show strong divalent tendencies, owing to the progressive stabilization of the divalent ground state [92] that is probably complete at fermium. 

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