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Sat Dec 1, 2018, 10:22 PM

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 range of accessible oxidation states (OSs) of an element is fundamental to its chemistry. In particular, high OSs provide an assessment of the propensity, and ultimately the ability, of valence electrons to become engaged in chemical bonding. Until recently, the highest known OS in the entire Periodic Table was VIII, such as in the stable and volatile molecules RuO4 and OsO4. The OS IX was finally realized in the gas-phase complex IrO4+,(1) but neither this moiety nor this highest OS has been isolated in the condensed phase.(1,2) The appearance of distinctive and otherwise inaccessible chemistry in gas-phase species, such as IrIX in IrO4+, is generally attributed to isolation of a moiety that would otherwise be highly reactive with neighbor species in condensed phases.(3) For example, gas-phase PaO22+, which comprises formally PaVI, has been synthesized, but it activates even dihydrogen to yield atomic H and PaO(OH)22+ in which the stable discrete PaV OS state is recovered.(4) In view of its gas-phase reactivity, there is scant chance of isolating PaO22+ in the condensed phase. Another example of a distinctively high OS accessible (so far) only in the gas phase is PrV in PrO2+ and NPrO,(5,6) this being the only known pentavalent lanthanide.

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 μg) 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:

Scheme 1. Generic D2h, C2v, and C2 Symmetry Structures for AnO2(NO3)2–

Apparently this technique has also been applied to lanthanides, motivating this work:

Figure 1. CID mass spectra acquired at a nominal instrumental voltage of 0.5 V for (a) Ce(NO3)4–, (b) Pr(NO3)4–, (c) Nd(NO3)4–, and (d) Tb(NO3)4–. Elimination of NO2 is indicated by arrows. Sequential CID elimination of two NO2 is observed only for Pr(NO3)4– to yield PrO2(NO3)2–.

Mass spectra from the actinides:

The caption:

Figure 2. CID mass spectra acquired at a nominal instrumental voltage of 0.5 V for (a) Pu(NO3)4–, (b) Am(NO3)4–, (c) Cm(NO3)4–, (d) Bk(NO3)4– (with 7% isobaric Cf(NO3)4– from beta-decay of 249Bk), and (e) Cf(NO3)4–. Elimination of NO2 is indicated by arrows. Sequential CID elimination of two NO2 molecules from An(NO3)4– to yield AnO2(NO3)2– is observed in all five cases.

Some calculated structures:

The caption:

Figure 3. Structures of CfIVO2(NO3)2– (top) and CfIIIO2(NO3)2– (bottom) in two perspectives and selected distances in angstrom from CASPT2/DZ calculations.

Results of density functional theory calculations for a curium oxonitride complex:

The caption:

Figure 4. Electron density map of CmO2(NO3)2– from DFT calculations. Charge concentration is indicated by yellow, while charge depletion is indicated by blue.

Molecular orbitals for the plutonium complex in this class:

The caption:

Figure 5. Characteristic molecular orbitals of PuO2(NO3)2– from CASPT2 calculations

The same thing for Berkelium:

The caption:

Figure 6. Characteristic molecular orbitals of BkO2(NO3)2– from CASPT2 calculations.

A text excerpt:

Of the various theoretical approaches, only the AIM model can characterize quantitatively the space between the bonding atoms. Therefore, we performed a topological analysis of the electron density distribution of the AnVO2(NO3)2– complexes in order to see how the density properties of the An–O bonds vary along the 5f row. We were particularly interested in the parameters of An–nitrate interactions, as they may provide a clue on the increasing bend along the actinide row. A graphical representation of the bonding paths, bond and ring critical points of AmVO2(NO3)2– is shown in Figure 7.

Figure 7:

The caption:

Figure 7. Bonding paths (black), bond (green), and ring (small red) critical points of AmVO2(NO3)2–.

Ionization energies:

The caption:

Figure 10. Actinide ionization energies in eV(79) (using corrected value for IE[U3+] as discussed above): (a) fourth IE; (b) fifth IE; (c) sum of fourth and fifth IEs. Dotted lines are approximate upper stability boundaries for (a) AnIV relative to AnIII; (b) AnV relative to AnIV; (c) AnV relative to AnIII.

Some remarks from the conclusion:

Comparison of experimental results for lanthanide and actinide oxide nitrate anion complexes suggested the AnV oxidation state as coordinated actinyl(V) moieties embedded in AnO2(NO3)2– for An = Pu, Am, Cm, Bk, and Cf, this being the first Cm(V) complex. The stability of oxidation state V in these AnO2(NO3)2– complexes has been confirmed by quantum chemical calculations. The relative stability of this oxidation state is particularly notable for Cf and Bk complexes, and therefore the AnIVO2(NO3)2– and AnIIIO2(NO3)2– forms have been explored and their lower stabilities with respect to CfV and BkV have been supported by both CASPT2 and DFT calculations. Whereas pentavalent Cf was expected to be stable due to a half-filled 5f7 configuration, the computations show that this configuration for CfVO2(NO3)2– is not octet with all seven 5f electrons spin-unpaired, but rather sextet with two of the 5f electrons spin-paired in a 5f1+ orbital.

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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Reply Pentavalent States Observed in Curium, Berkelium and Californium. (Original post)
NNadir Dec 2018 OP
dhol82 Dec 2018 #1
NNadir Dec 2018 #3
eppur_se_muova Dec 2018 #2
NNadir Dec 2018 #4
Victor_c3 Dec 2018 #5

Response to NNadir (Original post)

Sat Dec 1, 2018, 10:26 PM

1. I am so impressed by anybody who can comprehend this stuff!

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Response to dhol82 (Reply #1)

Sun Dec 2, 2018, 09:38 AM

3. I am so impressed by anyone who tries to read what he or she can't comprehend.

Unless one does that, one will limit what he or she will ultimately comprehend.


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Response to NNadir (Original post)

Sat Dec 1, 2018, 11:47 PM

2. You know, I was just wondering about that ... :)

Seriously, though, glad to see that people are still pushing out the boundaries, despite the indifference of the culture that surrounds them.

Never did get to do any AIM stuff, not that I'm sure it would have clarified any projects I worked on.

Love seeing all those "extra" nodal surfaces brought in by the high-n and high-l AOs.

Oh, to be doing research again ... *sigh*.

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Response to eppur_se_muova (Reply #2)

Sun Dec 2, 2018, 09:42 AM

4. I still remember my first exposure to orbital pictures when I was in high school.

I had no idea what they meant or from whence they came but now that I do, I still enjoy seeing them, particularly when they represent f-orbital bonding.

It's very wonderful.

I stumbled across this paper while I was wondering about some aspects of plutonium nitride chemistry, also a wonderful molecule with wonderful properties that I think are of supreme importance.

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Response to NNadir (Original post)

Sun Dec 2, 2018, 01:18 PM

5. Thanks for that

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.

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