Actinide elements 4-1 oxidation state

Actinides, particularly the lighter ones, display multiple oxidation states and complex chemical behavior, which makes their chemistry quite fascinating. Some isotopes of these elements, such as 232Th, 233,235,238 and 239Pu, are important for the nuclear industry due to their utility as fissile/fertile materials. Therefore, the separation chemistry of different oxidation states of Th, U, and Pu need to be reviewed with respect to both basic as well as applied aspects. Some fundamental chemical properties of the lighter actinides, including oxidation states, hydrolysis, and complexation characteristics form the basis of their separation. 

As stated in the opening section of this chapter, the objective is to discuss and compare the solid-state chemistry and physics of the lanthanide and actinide element oxides. The topics of discussion have been limited to binary oxides of these elements. Therefore, a discussion of the many complex (ternary, mixed, etc.) oxide systems for these f elements, and oxides of actinides representing oxidation states above four that do not have lanthanide counterparts, are not present. 

Evidence other than that of ion-exchange favours the view of the new elements as an inner transition series. The magnetic properties of their ions are very similar to those of the lanthanides whatever range of oxidation states the actinides display, they always have -1-3 as one of them. Moreover, in the lanthanides, the element gado-... 

In the actinides, the element curium, Cm, is probably the one which has its inner sub-shell half-filled and in the great majority of its compounds curium is tripositive, whereas the preceding elements up to americium, exhibit many oxidation states, for example -1-2, -1-3. -1-4, -1-5 and + 6, and berkelium, after curium, exhibits states of -1- 3 and -E 4. Here then is another resemblance of the two series. 

The many possible oxidation states of the actinides up to americium make the chemistry of their compounds rather extensive and complicated. Taking plutonium as an example, it exhibits oxidation states of -E 3, -E 4, +5 and -E 6, four being the most stable oxidation state. These states are all known in solution, for example Pu" as Pu ", and Pu as PuOj. PuOl" is analogous to UO , which is the stable uranium ion in solution. Each oxidation state is characterised by a different colour, for example PuOj is pink, but change of oxidation state and disproportionation can occur very readily between the various states. The chemistry in solution is also complicated by the ease of complex formation. However, plutonium can also form compounds such as oxides, carbides, nitrides and anhydrous halides which do not involve reactions in solution. Hence for example, it forms a violet fluoride, PuFj. and a brown fluoride. Pup4 a monoxide, PuO (probably an interstitial compound), and a stable dioxide, PUO2. The dioxide was the first compound of an artificial element to be separated in a weighable amount and the first to be identified by X-ray diffraction methods. 

Experiments seem to show that the element possesses a moderately stable dipositive (11) oxidation state in addition to the tripositive (111) oxidation state, which is characteristic of the actinide elements. 

Special techniques for experimentation with the actinide elements other than Th and U have been devised because of the potential health ha2ard to the experimenter and the small amounts available (15). In addition, iavestigations are frequently carried out with the substance present ia very low coaceatratioa as a radioactive tracer. Such procedures coatiaue to be used to some exteat with the heaviest actinide elements, where only a few score atoms may be available they were used ia the earHest work for all the transuranium elements. Tracer studies offer a method for obtaining knowledge of oxidation states, formation of complex ions, and the solubiHty of various compounds. These techniques are not appHcable to crystallography, metallurgy, and spectroscopic studies. 

The actinide elements exhibit uniformity in ionic types. In acidic aqueous solution, there are four types of cations, and these and their colors are hsted in Table 5 (12—14,17). The open spaces indicate that the corresponding oxidation states do not exist in aqueous solution. The wide variety of colors exhibited by actinide ions is characteristic of transition series of elements. In general, protactinium(V) polymerizes and precipitates readily in aqueous solution and it seems unlikely that ionic forms ate present in such solutions. 

Actinide ions form complex ions with a large number of organic substances (12). Their extractabiUty by these substances varies from element to element and depends markedly on oxidation state. A number of important separation procedures are based on this property. Solvents that behave in this way are thbutyl phosphate, diethyl ether [60-29-7J, ketones such as diisopropyl ketone [565-80-5] or methyl isobutyl ketone [108-10-17, and several glycol ether type solvents such as diethyl CeUosolve [629-14-1] (ethylene glycol diethyl ether) or dibutyl Carbitol [112-73-2] (diethylene glycol dibutyl ether). 

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 redox behaviour of Th, Pa and U is of the kind expected for d-transition elements which is why, prior to the 1940s, these elements were commonly placed respectively in groups 4, 5 and 6 of the periodic table. Behaviour obviously like that of the lanthanides is not evident until the second half of the series. However, even the early actinides resemble the lanthanides in showing close similarities with each other and gradual variations in properties, providing comparisons are restricted to those properties which do not entail a change in oxidation state. The smooth variation with atomic number found for stability constants, for instance, is like that of the lanthanides rather than the d-transition elements, as is the smooth variation in ionic radii noted in Fig. 31.4. This last factor is responsible for the close similarity in the structures of many actinide and lanthanide compounds especially noticeable in the 4-3 oxidation state for which... 

Table 31.4 Oxidation states and stereochemistries of compounds of the actinides An is used as a general symbol for the actinide elements...

This state is found for the six elements Am and Cf No, though in aqueous solution only for Fm, Md and No. However, for No, alone amongst all the f-series elements, it is the normal oxidation state in aqueous solution. The greater stabilization of the +2 state at the end of the actinides as compared to that at the end of the lanthanides which this implies, has been taken " to indicate a greater separation between the 5f and 6d than between the 4f and 5d orbitals at the ends of the two series. This is the reverse of the situation found at the beginnings of the series (p. 1266). 

Unsubstituted bisphthalocyanines 2 are formed in the presence of several elements which exist in a stable oxidation state of + III or +IV such as titanium, zirconium, hafnium, indium and most of the lanthanide and actinide elements. 

Unusual oxidation states of some actinide and lanthanide elements. L. B. Asprey and B. B. Cunningham, Prog. Inorg. Chem., 1960, 2, 267-302 (245). 

The known oxidation states of plutonium present a 5f -series, starting from f1 [Pu(VII)] up to f5 [Pu(III)]. But contrary to the 4f - and 5f series across the period table, where the properties can be described by some smooth varying parameters, changing of the oxidation states influences the electronic properties drastically. Due to the large range of available oxidation states plutonium represents a favorable element among the actinides to study these effects. 

The only crystalline phase which has been isolated has the formula Pu2(OH)2(SO )3(HaO). The appearance of this phase is quite remarkable because under similar conditions the other actinides which have been examined form phases of different composition (M(OH)2SOit, M=Th,U,Np). Thus, plutonium apparently lies at that point in the actinide series where the actinide contraction influences the chemistry such that elements in identical oxidation states will behave differently. The chemistry of plutonium in this system resembles that of zirconium and hafnium more than that of the lighter tetravalent actinides. Structural studies do reveal a common feature among the various hydroxysulfate compounds, however, i.e., the existence of double hydroxide bridges between metal atoms. This structural feature persists from zirconium through plutonium for compounds of stoichiometry M(OH)2SOit to M2 (OH) 2 (S0O 3 (H20) i,. Spectroscopic studies show similarities between Pu2 (OH) 2 (SOO 3 (H20) i, and the Pu(IV) polymer and suggest that common structural features may be present. 

In terms of gross features of mechanism, a redox reaction between transition metal complexes, having adjacent stable oxidation states, generally takes place in a simple one-equivalent change. For the post-transition and actinide elements, where there is usually a difference of two between the stable oxidation states, both single two-equivalent and consecutive one-equivalent changes are possible. 

Most of the U-series nuclides are metals. Five of them belong to the actinide family corresponding to the filling of the internal orbitals while the orbitals 7s are filled. A sixth, Ra is an alkali earth and shares some chemical properties with other alkali earths, particularly the heavier ones (Sr and Ba), while a seventh, Rn, is a noble gas. The filling of the orbitals prescribes the possible oxidation states of these elements. Their preferred oxidation state is obtained when the electronic configuration is that of the closest rare gas (Rn). 

Asprey, L. B. and Cunningham, B. B., Unusual Oxidation States of Some Actinide and Lanthanide Elements. 2 267... 

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