Samarium divalent

Although rare-earth ions are mosdy trivalent, lanthanides can exist in the divalent or tetravalent state when the electronic configuration is close to the stable empty, half-fUed, or completely fiUed sheUs. Thus samarium, europium, thuUum, and ytterbium can exist as divalent cations in certain environments. On the other hand, tetravalent cerium, praseodymium, and terbium are found, even as oxides where trivalent and tetravalent states often coexist. The stabili2ation of the different valence states for particular rare earths is sometimes used for separation from the other trivalent lanthanides. The chemicals properties of the di- and tetravalent ions are significantly different. 

Other catalytic uses of rare-earth compounds have not reached the same development. Neodymium salts are, however, used for mbber manufacturing (22). Divalent samarium haHdes are employed in organic synthesis (23). 

While ytterbium(II) benzamidinate complexes have been known for many years/ the synthesis of the first divalent samarium bis(amidinate) required the use of a sterically hindered amidinate ligand, [HC(NDipp)2l (Dipp = C6H3Pr2-2,6)/ As illustrated in Scheme 54, the dark green compound Sm(DippForm)2(THF)2 (DippForm = [HC(NDipp)2] ) can be prepared by three different synthetic routes. Structural data indicated that hexacoordinated... 

Table 11. Ethylene polymerization by divalent samarium complexes...

Divalent metals, ferrites of, 24 543 Divalent samarium halides, 24 649 Divalent zinc-silver oxide batteries, 3 454-455 Divergence, 24 656... 

The divalent compounds of samarium primarily are halides, the reddish-brown crystalline dichloride, SmCb [13874-75-4] the dark-brown diiodide, Sml2 [32248-43-4] and the dark brown dibromide, SmBr2 [50801-97-3]. Samarium also forms a difluoride, SmF2 [15192-17-3]. The trivalent salts of these halogens are more stable than their divalent counterparts. 

The reduction to the divalent state involves samarium, europium, and ytterbium. In 1906 C. Matignon and E. Gazes obtained samarium(II) chloride by reducing the trichloride with hydrogen. In 1911, G. Urbain and F. Bourion prepared europium(II) chloride by a comparable reduction involving gydrogen, and in 1929 ytterbium(II) chloride was similarly obtained by W. Klemm and W. Schuth. 

H.B. Kagan and J.L. Namy, Preparation of divalent ytterbium and samarium derivatives and their use in organic chemistry 525... 

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... 

The divalent samarium complexes (CsMe5)2Sm and (C5Me5)2Sm(THF)2 have also been found to be effective precatalysts in these hydroamina-tion/cyclization reactions [68]. In this case the initial step is the formation of samarium(III) intermediates via allylic C-H activation [Eq. (15)]. 

Hill reported the first divalent, homoleptic rare earth bis(iminophos-phorano)-methanide complex which exhibited two samarium-methanide bonds in the solid state.49 A one pot reaction between samarium diiodide,... 

The standard electrode potentials for all the rare earths have similar values and are comparable with the redox potentials of alkaline earth metals [144], Thus the lanthanides are strong reducing agents, and form trivalent ions easily. Both europium and samarium can exist in both trivalent and divalent states and the divalent states are not stable in aqueous solutions. Cerium can exist in both tetravalent and trivalent states in solution but Ce(III) is the most stable. 

P. Girard, J. L. Namy, and H. B. Kagan, Divalent lanthanide derivatives in organic synthesis. 1. Mild preparation of samarium iodide and ytterbium iodide and their use as reducing or coupling agents,. /. Am. Chem. Soc., 102 (1980) 2693-2698. 

In the last few years, only a few studies have been devoted to the optical properties of Sm3+ ions in fluoride glasses [138,139], The main result concerns the excitation of the 4G5/2 level by a three-fold up-conversion process including a direct two-photon absorption mechanism. Izumitani et al. have succeeded in stabilizing divalent samarium in fluoroaluminate and fluorohafnate glasses [140], Absorption spectra reveal a strong 4f - 5d transition whose maximum is located around 320 nm. Emission of Sm2+ ions in fluoroaluminate glass occurs in the red, between 680 nm and 810 nm, from the 5D0 excited state to the 7Fj (J = 0, 1, 2, 3, 4) levels. 

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