Pentavalent States Observed in Curium, Berkelium and Californium.
The paper I'll discuss in this post is this one: Pentavalent Curium, Berkelium, and Californium in Nitrate Complexes: Extending Actinide Chemistry and Oxidation States (Attila Kovács,*,† Phuong D. Dau,‡ Joaquim Marçalo,§ and John K. Gibson*, Inorg. Chem., 2018, 57 (15), pp 9453–9467.
The shape of the periodic table is actually a quantum mechanical effect. Every electron in an atom must have a unique set quantum configuration numbers, defined by it's primary shell number (which are represented by the rows in the periodic table), and then its suborbital, usually designated for historical reasons as s, p, d, and f, which are represented by its position in a column relative to the "steps" that appear in the table's shape.
In the periodic table both the lanthanides and actinides, the "f elements," appear in the boxes below the main group elements, and the discovery that the actinides, in particular, should go there was the recognition by Glenn Seaborg that they were "f elements" and not, as previously thought as "d elements" that begin with Scandium (Sc) and end with the synthetic element Copernicium (Cn), the "d block elements. The "d block" is actually broken by elements La-Lu (Lanthanum to Lutetium) and Ac-Lw (Actinium to Lawrencium). In fact the "f elements" should represent another "step" in the periodic table, but printing it in this way is logistically difficult since it would be difficult to print on standard paper without making the print too small to read, so they're put in boxes below the "main group" elements.
The heaviest element that has been isolated in a relatively pure form in quantities that are visible is element 99, Einsteinium. It seems theoretically possible to isolate, perhaps, albeit at enormous expense, a visible, if transitory, sample of fermium, element 100, since it is the last element formed by sequential beta decay, but I don't believe it has ever been done, nor will it ever be done. Generally fermium and all of the elements beyond are synthesized on an atom by atom scale in accelerators and are basically known from their decay products and the high energy radiation they produce.
The lanthanide elements, with a few important exceptions generally exhibit the +3 oxidation state, although a few elements like cerium (+4) and europium and samarium (+2) have other oxidation states, but they are all mostly characterized by +3 oxidation state, making their separations from one another somewhat difficult, meaning that their industrial chemistry, important in many modern devices, is at best environmentally suspect at best, environmentally odious at worst.
The chemistry of the lower actinide elements, including those that naturally occur if far richer. In fact thorium is almost always found in the +4 state, protactinium in the +5 state, and uranium in either the +4 or +6 state in the natural environment. For a long time, before Seaborg's discovery, these elements were thought to be "d elements" and in fact, thorium has chemistry much closer to zirconium and hafnium than say, curium, protactinium is more "tantalum like" than curium like, and uranium has many similarities to tungsten. (The presence of billion ton quantities of uranium in oceans only became possible on earth after oxygen appeared in the atmosphere, resulting in the somewhat more soluble +6 uranium oxidation state being formed by oxidation as opposed to the very insoluble +4 state. Uranium, and plutonium, but not generally neptunium, have well characterized +3 states, but thorium, protactinium, do not. (Uranium, neptunium, and plutonium all exhibit volatile hexafluorides (+6) albeit of decreasing stability in sequence; a fact of industrial importance; in the oceans and in certain fresh water supplies, uranium VI is present as the dioxo ion.)
In nuclear technology, the existence of multiple oxidation states among actinide elements greatly simplifies their separations from one another (but not necessarily from fission products), at least in the case where there are only trivial amounts of the transamericium elements, curium and berkelium and californium, all of which can be isolated in gram quantities, and in a the case of curium, kg quantities.
I personally always assumed that except for some exotic chemistry involving +2 states for curium at least, that curium, berkelium and californium most commonly exhibited +3 chemistry and that no higher states existed.
I was wrong.
From the paper cited above:
The early actinides yield ultimate OSs, from AcIII to NpVII, that correspond to engagement of all valence electrons in chemical bonding to yield an empty 5f0 valence electron shell.(7) After Np, the highest accessible actinide OSs, from PuVII to lower OSs beyond Pu, have one or more chemically unengaged valence 5f electron(s), as the nuclear charge increases and energies of the 5f orbitals decrease. The transition from chemical participation of all 5f valence electrons in ubiquitous UVI, to participation of only two valence electrons in prevalent NoII,(8) distinguishes the actinides from the lanthanides for which the relatively low energy of the valence 4f orbitals results in only a few OSs above trivalent.(9) The gas-phase molecular ions BkO2+ and CfO2+ were recently synthesized and their OSs computed as BkV and CfV, which was an advancement beyond oxidation state IV for these elements and extended the distinctive actinyl(V) dioxo moieties into the second half of the actinide series.(10) It is notable that the computed oxidation state in ground-state CmO2+ is not CmV but rather CmIII, which reflects the limited stabilities of OSs above III for the actinides after Am.(10)
A primary goal of the work reported here is to assess stabilities of OSs, particularly the pentavalent OS, of the actinides Cm, Bk, and Cf. These elements represent the transition from the early actinides that exhibit higher OS, including AmVI and possibly also AmVII,(11) to the latest actinides, Es through Lr, that have been definitively identified only in the AnII and/or AnIII OS. The meagre realm of OSs for the late actinides may not be entirely due to intrinsic chemistry because synthetic efforts for these elements have been very limited due to scarcity and short half-lives of available synthetic isotopes. Cm, Bk, and Cf are the heaviest actinides available as isotopes that are both sufficiently abundant (>10 ?g) and long-lived (>100 days) for application of some conventional experimental approaches with relatively moderate procedural modifications.
The higher oxidation states were synthesized in the gas phase by the use of electrospray ionization (ESI) and detected in the mass spectrometer in which the ESI was performed.
The results of the spectra were verified by quantum mechanical computations using AIMAll Software
Some cool pictures from the paper:
The caption:
Apparently this technique has also been applied to lanthanides, motivating this work:
Mass spectra from the actinides:
The caption:
Some calculated structures:
The caption:
Results of density functional theory calculations for a curium oxonitride complex:
The caption:
Molecular orbitals for the plutonium complex in this class:
The caption:
The same thing for Berkelium:
The caption:
A text excerpt:
Figure 7:
The caption:
Ionization energies:
The caption:
Some remarks from the conclusion:
The AnO2(NO3)2– complexes show interesting bonding features. While in the actinyl moiety the ionic character of bonding increases from Pu to Cf (in agreement with experience on several other actinide systems), in the An–NO3– interaction an opposite trend has been observed here. The increasing ionicity in the AnO2 moiety results in charge depletion around An making it more suitable as acceptor for charge transfer from the nitrate oxygens. The increasing covalent character from Pu to Bk ? Cf may be an important factor for the trend observed in the molecular geometries, i.e., a gradual bend of the NO3– ligands (described by the N–An–N angle) around An...
I'm well aware that this may all seem very "out there," and perhaps, in some quarters, generate remarks along the lines of "I couldn't care less."
I assure you though, whether you are inclined to believe it or not, or even if you despise the idea, that the chemistry of the actinides is critical, absolutely critical, to saving whatever is left to save of our rapidly deteriorating environment.
I wish you a rather pleasant Sunday.
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I used to be a simple bench chemist and never did anything at the level you are writing about, but I still enjoy expanding my knowledge.
