Tripositive oxidation state

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. 

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. 

Reference has been made already to the existence of a set of inner transition elements, following lanthanum, in which the quantum level being filled is neither the outer quantum level nor the penultimate level, but the next inner. These elements, together with yttrium (a transition metal), were called the rare earths , since they occurred in uncommon mixtures of what were believed to be earths or oxides. With the recognition of their special structure, the elements from lanthanum to lutetium were re-named the lanthanons or lanthanides. They resemble one another very closely, so much so that their separation presented a major problem, since all their compounds are very much alike. They exhibit oxidation state + 3 and show in this slate predominantly ionic characteristics—the ions. LJ+ (L = lanthanide), are indeed similar to the ions of the alkaline earth metals, except that they are tripositive, not dipositive. 

The most stable oxidation state of thuhum is -i-3. Only the tripositive Tm3+ ion is encountered in aqueous media. The metal also forms compounds in +2 and +4 valence states, but there is no evidence of Tm2+ and Tm + existing in aqueous phase. Thuhum is relatively stable in air at ambient temperature. However, it combines with oxygen on heating forming its sesquioxide, Tm203. 

In contrast to the lanthanide 4f transition series, for which the normal oxidation state is +3 in aqueous solution and in solid compounds, the actinide elements up to, and including, americium exhibit oxidation states from +3 to +7 (Table 1), although the common oxidation state of americium and the following elements is +3, as in the lanthanides, apart from nobelium (Z = 102), for which the +2 state appears to be very stable with respect to oxidation in aqueous solution, presumably because of a high ionization potential for the 5/14 No2+ ion. Discussions of the thermodynamic factors responsible for the stability of the tripositive actinide ions with respect to oxidation or reduction are available.1,2... 

The possibility of dissolving the mixed hydroxide in HNOs and obtaining direct extraction of thorium (and uranium) from the nitrate solution has been studied [155,156], but does not seem to be too promising, possibly due to the partial oxidation of tripositive cerium to the tetrapositive state. Kraitzer [157] was able to separate thorium from the mixed hydroxide cake by extracting the cake with sodium carbonate buffer at pH 9.5—10. Thorium was found to form a soluble carbonate complex and a recovery of better than 99% of thorium was claimed after only four extractions. 

The normally stable oxidation state of the rare earths is the tripositive one with the 4/ ground state configuration. The divalent ions of all rare earths have been prepared by reducing the trivalent ions with y-rays in CaPs matrix. The tetrapositive state of Ce and the higher oxidation states of Pr in PreOn and of Tb in TlfeO were recognized even by the... 

The solubility properties of berkelium in its two oxidation states are entirely analogous to those of Lite actmide and lanLlianide elements in the corresponding oxidation states, Thus in the tripositive state such compounds as the fluoride and the oxalate arc insoluble in add solution, and the tetrapositive slate has such insoluble compounds as the lodate and phosphate in acid solution. The nitrate, sulfate, halides, perchlorate, and sulfide of both oxidation states are soluble,... 

The differences in the nature of the soluble species derived from the positive oxidation states of these elements help to explain the irregular trend in the oxidation potentials going from the tripositive to the penta-positive states. For acid solutions ... 

The highest oxidation states for Ir are VI and V, stabilized by ligands such as F. IrFg (a yellow crystalline sohd, /Xeff = 2.9 /xb at 300 K) is formed by the direct fluorination of Ir metal. Molecules of IrFe are octahedral, and the stmcture has been studied in the gas phase by electron diffraction and in the sohd phase by EXAFS. IrFe is hydrolyzed by water, and reactions (1-4) illustrate its general reactivity. Reaction four represents the formation of the first binary, tripositive metal carbonyl complex. ... 

In aqueous solution, lanthanides are most stable in the tripositive oxidation state, making them difficult to separate and purify. The preference for this oxidation state is due in part to the energy of the 4f electrons being below those of the 5d and 6s electrons (except in the cases of La and Ce). When forming ions, electrons from the 6s and 5d orbitals are lost first so that all Ln + ions have [Xe] 4f electronic configurations. Under reducing conditions, certain lanthanides (europium, samarium, and ytterbium) can be stable as dipositive ions, and cerium can adopt a +4 oxidation state (5). 

Filling of the inner 4f electron shell across the lanthanide series results in decreases of ionic radii by as much as 15% from lanthanum to lutetimn, referred to as the lanthanide contraction (28). While atomic radius contraction is not rmique across a series (i.e., the actinides and the first two rows of the d-block), the fact that all lanthanides primarily adopt the tripositive oxidation state means that this particular row of elements exhibits a traceable change in properties in a way that is not observed elsewhere in the periodic table. Lanthanides behave similarly in reactions as long as the mnnber of 4f electrons is conserved (29). Thus, lanthanide substitution can be used as a tool to tune the ionic radius in a lanthanide complex to better elucidate physical properties. 

Scandium is still a neglected element. It is the most expensive metal in its period (caused by the fact that its even distribution in the earth means that there are no rich ores) and its chemistry is virtually exclusively that of the +3 oxidation state, so that it is not classed as a transition metal and is often silent to spectroscopy and not amenable to study by many of the usual spectroscopic tools of the coordination chemist. Chemists have often either tended to assume that complexes of Sc are just like those of the tripositive ions of the transition metals or that they resemble lanthanide complexes. Neither of these assumptions is correct—how incorrect we are now realizing. Scandium chemistry is starting to exhibit characteristics all of its own, and possibly the burgeoning use of scandium compounds in organic synthesis may drive a real expansion of scandium chemistry. 

The vast majority of the molecular complexes of rare-earth elements (Sc, Y and La-Lu, hereafter abbreviated as R) are in trivalent state, which is the most stable. The oxidation state of the rare earths in these complexes is + 3, and the configuration of the Sc " ", and La + ions is that of the noble gases ([Ar], [Kr] and [Xe], respectively), and that of the remaining rare-earth tripositive ions (Ce to Lu +) is [Xe]4f 5d°6s° (n = l-14) while being formally in the valence shell, the f-electrons in these ions are very contracted and do not normally participate in bonding interactions, which are mostly ionic for all... 

One result of this work is the conclusion that in chromia-alumina, and in other supported oxides, there must be local concentration of the supported oxide. This conclusion is reached because the Weiss constant shows definite indication of exchange interaction at concentrations of the paramagnetic ion too low to cover the surface of the support with even a monolayer. Another conclusion is that the support is sometimes able to modify the relative stabilities of oxidation states in the supported oxide. For instance, manganese oxide supported on gamma-alumina tends to be stabilized in the tripositive state, while on high-area titania it reverts to the tetrapositive state. 

Mendelevium — (Dmitri Mendeleev [1834-1907]), Md at. wt. (258) at. no. 101 m.p. 827°C valence +2, +3. Mendelevium, the ninth transuranium element of the actinide series to be discovered, was first identified by Ghiorso, Harvey, Choppin, Thompson, and Seaborg early in 1955 as a result of the bombardment of the isotope Es with helium ions in the Berkeley 60-inch cyclotron. The isotope produced was Md, which has a half-life of 78 min. This first identification was notable in that Md was synthesized on a one-atom-at-a-time basis. Nineteen isotopes and isomers are now recognized. Md has a half-life of 51.5 days. This isotope has been produced by the bombardment of an isotope of einsteinium with ions of helium. It now appears possible that eventually enough Md can be made so that some of its physical properties can be determined. Md has been used to elucidate some of the chemical properties of mendelevium in aqueous solution. Experiments seem to show that the element possesses a moderately stable dipositive (II) oxidation state in addition to the tripositive (III) oxidation state, which is characteristic of actinide elements. 

Most of the boron group elements exhibit a tripositive (3+) oxidation state however, they can be occasionally found in a unipositive (1+) state (with the exception of boron itself, which we describe in more detail later in this chapter). Keep reading to find out the details of five of the Group 13 (the 13th column on the periodic table) elements boron, aluminum, gallium, indium, and thallium. 

The lanthanide family of elements has played an important role which can be expected to continue in the development of coordination chemistry. In the early history of these elements, their close chemical similarity in the stable tripositive oxidation state made the task of achieving high purity for individual elements very difficult Although the entire lanthanide series had been discovered by 1907 (with the exception of Pm) and mixtures of lanthanides had been found in more than a hundred minerals, it was not until efficient separation methods were developed that detailed and diverse studies of their coordination chemistry could be undert en. 

Uranium differs considerably from tungsten and molybdenum in the chemistry of the lower oxidation states. Uranium (III) has great similarity to the tripositive rare-earth elements and actinium, and uranium (IV) resembles thorium and cerium (IV). Thus uranium (III) and uranium (IV) are not acidic in character they do not tend, like tungsten... 

As mentioned above, a very important point is the presence of seven 5f electrons in stable tripositive curium (element 96), making this element very actinium-like. A series of thoride elements, e.g., would imply stable IV oxidation states in elements 95 and 96 and the presence of seven 5f electrons and the IV state almost exclusively in element 97. A series of this type seems to be ruled out by the now-known instability of americium in solution in the IV state and by the apparent non-existence in aqueous solution of any oxidation state other than III in curium. Moreover, the III state of uranium would be surprising on this basis, because this element would be the second member of a thoride, or IV oxidation state , series. The fact that nearly a year was spent in an unsuccessful effort to separate tracer amounts of americium and curium from the rare earths, immediately following the discovery of these two elements, illustrates how unnatural it would be to regard them as members of a thoride, or IV oxidation state, group. 

The electronic configurations 5f or 4f representing the half-filled f shells of curium and gadolinium, have special stability. Thus, tripositive curium and gadolinium, are especially stable. A consequence of this is that the next element in each case readily loses an extra electron through oxidation, so as to obtain the f structure, with the result that terbium and especially berkelium can be readily oxidized from the III to the IV oxidation state. Another manifestation of this is that europium (and to a lesser extent samarium) -just before gadolinium - tends to favor the 4f structure with a more stable than usual II oxidation state. Similarly, the stable f electronic configuration leads to a more stable than usual II oxidation state in ytterbium (and to a lesser extent in thuUum) just before lutetium (whose tripositive ion has the 4f structure). This leads to the prediction that element 102, the next to the last actinide element, will have an observable II oxidation state. 

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