Samarium oxidation states

Samarium, tris(triphenylphosphine oxide)bis-(diethyldithiophosphato)-structure, 1,78 Samarium complexes dipositive oxidation state hydrated ions, 3, 1109 Samarium(III) complexes salicylic acid crystal structure, 2, 481 Sampsonite, 3, 265... 

The hexaborides for which the oxidation state of the metal is 2 -f- are CaB, SrBg, BaB5, EuB, YbB. They are diamagnetic except EuB, which is ferromagnetic (Tc = 12.5 K) . The other rare-earth hexaborides are characterized by the 3-1-oxidation state, except for SmB, in which samarium is in a mixed valence state. They exhibit antiferromagnetic order at low T except LaB, YB and SmB . ... 

There is also the possibihty of the oxidation state of +8 for samarium ... 

The preparation of samarium sulphide, SmS, is of interest because it contains samarium (a lanthanide element) In an unusual oxidation state +2 instead of the more common state +3. Samarium metal In powder form and powdered sulfur are... 

Although all the lanthanides are stable in the solid state as M2+ ions doped into CaF2 crystals, only in the cases of europium, ytterbium and samarium is there any real coordination chemistry, and that is very limited. There is a small but developing organometallic chemistry of the lower oxidation states,641 but that is not within the scope of the present review. Much of the chemistry of the dipositive state depends on solvated species642 and it is convenient to begin with these. 

Another completely different approach consists in choosing a dye, that already possesses aminocarboxylate functions (Meshkova et al., 1992a), such as triphenylmethane dyes. The latter can be used for selective luminescent determination of Nd111 and Ybm in samarium oxide, for instance. As previously described in the section devoted to /3-diketonates (section 3.2.1), the triplet excited states of /i-dikctonatcs lie at energies >20 000-25 000 cm-1, above most of the accepting levels of Lnm ions. As a consequence, determination of Ndm and Yb111 in europium or samarium oxides is difficult using /3-dike to nates since these two ions exhibit luminescence in the NIR, especially Smm with emission lines at 908, 930, 950, and 1038 nm close to the analytical lines of Ndm and Ybm. Therefore, the detection limit of Ndm and Ybm in samarium compounds by luminescence of their ternary complexes with tta and phen is only 0.1-1 wt%. 

In 1956 it was found that europium and ytterbium dissolve in liquid ammonia with the characteristic deep blue color known for the alkali and alkaline earth metals [36-40]. This behavior arises from the low density and high volatility of those metals compared to the other lanthanide elements [41]. Samarium, which normally also occurs in the divalent oxidation state, does not dissolve under... 

Active catalyst species or catalysis intermediates can often be trapped by stoichiometric reactions of the precatalyst with the substrate. The following example describes the successful isolation of such an intermediate with participation of Ln-O cr-bonds. Reduction processes mediated by low oxidation states of the lanthanide elements are of special interest in organic synthesis [256]. One of the most intensively studied reactions is the stoichiometric reduction of arylketones by rare earth metals ytterbium and samarium [277]. Thus formed dianions possess high nucleophilic character and excess lanthanide metal can even accomplish complete cleavage of the C-O double bond (Scheme 36). 

Samarium, like all lanthanide elements, preferentially exists in the + 3 oxidation state. The loss of the three outermost electrons, namely the 5d 6s2 electrons, results in enhanced thermodynamic stability in which a closed-shell... 

Xe-like electronic configuration is adopted. The + 2 oxidation state is most relevant for samarium (f6, near half-filled), europium (f7, half-filled), thulium (f13, nearly filled) and ytterbium (f14, filled). In order to attain the more stable + 3 oxidation state, Sml2 readily gives up its final outer-shell electron, in a thermodynamically driven process, making it a very powerful and synthetically useful single-electron transfer reagent. 

Addition of the same NHC to Eu(thd)3 (thd — tetramethylheptanedioate) affords the europium(III) adduct Eu(thd)3(NHC). The europium-NHC bond distance of 2.663(4) A is shorter than that of the samarium(II) complex and is consistent with the higher oxidation state of the lanthanide centre. The yttrium(III) analogue was also prepared and characterised by NMR spectroscopy. The C2 carbon resonates at 199 ppm in the 13C NMR spectrum, with a yc coupling constant of 33 Hz. This indicates that the NHC remains bound to the metal centre in solution and does not dissociate on the NMR timescale. 

PmCl3.xH20, Pm(N03)3.xH20, and Pm(C2O4)3.10H2O. It would be expected that promethium would form some stable compounds in the +2 oxidation state, though they are unlikely to be made in aqueous solution. No definite evidence has yet been obtained, since studies have been hindered both by the small quantities of the element available and by its radioactivity. The properties of promethium fit neatly into position between neodymium and samarium it is a microcosm of lanthanide chemistry in general. 

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). 

Samarium has two common oxidation states +2 and -h3. Upon solution in toluene under nitrogen, an anionic Sm(II) species, [(—CH2—)5]4-calix-tetrapyrrole Sm(THF)[Li (THF)]2[Li(THF)2]Cl, forms, in part, the compound [(—CH2—)5]4-calix-tetrapyrrole Sm(THF)Li2[Li(THF)](/r -OCH=CH2) . However, this compound is a lithium enolate derived by elimination of THF. In that the metalloorganic reagent is rather similar to what will be discussed in Section XI as part of vanadium enolate chemistry, we fail to understand why in the former case with Sm a lithium enolate is formed but in the latter with V it is an ynolate that is produced. Almost nothing is known to allow comparing the energetics of metal enolates and related ynolates. We note from the enthalpies of... 

Ytterbium has the oxidation states 4-2 and -t-3. A stereochemical dichotomy exists in their enolate chemistry . Yb(II) enolates react with aldehydes to form the erythro-fi-hydroxyketones while Yb(III) enolates yield the threo stereoisomers. We fail to understand this by either thermodynamic or mechanistic reasoning. Analogous to corresponding reaction chemistry for samarium, preformed ytterbium benzophenone dimer, [Yb(Ph2CO) (HMPA)2]2, reacts " with sterically crowded phenols to form in low yield (5%) the enolate complex 22b by dearomatization of benzophenone. The major product (80%) is the... 

Organolanthanide(III) compounds form the bulk of all the known organo-lanthanides. However, in addition to the + 3 oxidation state, the + 2 oxidation state is chemically accessible for samarium, europium and ytterbium (an organocerium(II) [1] and an organoneodynium(II) [2] complex have also been reported but not structurally confirmed) and the +4 oxidation state is accessible for cerium. A few organolanthanide(O) compounds are also known,... 

Organolanthanide chemistry is dominated by the trivalent compounds. " Compounds in oxidation state (II) are restricted to derivatives of europium, samarium, and ytterbium, but they have considerable importance in both organic and organometallic syntheses because of their reducing properties. " Redox transmetalation reactions of organomercurials with lanthanide metals provide convenient syntheses of a number of diorganolanthanides, for example, R2M, R = CgFj or PhCC, M = Yb or Eu. " °... 

All the rare-earth elements occur in the HI oxidation state in compounds, and can be separated and determined in this form to provide what is known as the total REE. Samarium, europium, and ytterbium also occur in the unstable n oxidation state, whereas cerium, praseodymium, and terbium can be found in the IV oxidation state. 

Samarium has only one stable oxidation state (-1-3) within the electrochemical window of water, but it forms several relatively stable compounds with oxygen and hydrogen, which differ in their degree of hydration and in their crystallographic structure. These compounds absorb atmospheric CO2. Nominal degree of hydration indicated by a chemical name/formula reported in the literature does not necessarily reflect the actual degree of hydration. PZCs/IEPs of SiUjO, (nominally) are presented in Tables 3.743 and 3.744. 

Fluorosulphates of most of the lanthanide elements in the oxidation state iii have been prepared by the action of peroxydisulphuryl difluoride on the anhydrous metal carbonates. The oxides of neodymium, samarium, and europium, however, gave indications of partial reaction with S2O6F2 to form mixtures of fluorosulphate with oxide. The vibrational spectra and structure of the lanthanide fluorosulphates suggested the presence of mixed co-ordination type anions. Fluorescence emission, and vibrational spectra of the europium compound indicated octaco-ordination and the presence of ter- and bi-dentate fluorosulphate groups. 

Polarographic studies gave no evidence for the existence of the bivalent oxidation states of selected actinides in acetonitrile solution. Only one wave corresponding to reduction of americium(iii) or curium(iii) to the zero-valent state was observed and experiments with berkelium(iii) and einsteinium(iii) failed to give conclusive results because of rapid radiolysis of the acetonitrile solution. A study of the electrochemical reduction of americium, thulium, erbium, samarium, and europium showed that the elements did assume the bivalent state with the actinide bivalent cations having a smaller stability than the lanthanides. The half-wave potential of nobelium was found to be —1.6 V versus the standard hydrogen electrode for the reaction... 

The series of 15 elements, lanthanium to lutetium, is known as the lanthanide series. These elements all form trivalent ions in solution quadrivalent oxidation states of cerium, praseodymium, and terbium, and bivalent states of samarium and europium are also obtained. 

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