Lanthanides defined

The rare earth elements (REE) are the lanthanides (defined as those elements with valence electrons in 4/orbitals), La, Ce, Pr, Nd, (Pm), Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb. Often included for analysis, because they behave in a chemically similar way, although strictly not REE, are the Group 3 transition metals Y and Lu. The radioactive lanthanide element promethium (Pm) is excluded from analysis, since it is not found in samples because of its short half-life. 

A fundamental study of the extraction of trivalent lanthanides and yttrium nitrates by carboxylic acids in xylene diluent was reported by duPreez and Preston (1992). The steric parameter E of the substituent alkyl group, representing the steric bulk of the carboxylic acid molecule, shows a definite relationship to the extractabilities of the lanthanides defined by their atomic numbers. The extraction reaction of a trivalent lanthanide Ln + by a carboxylic acid dimer is... 

As regards the transition elements, the first row in particular show some common characteristics which define a substantial part of their chemistry the elements of the lanthanide and actinide series show an even closer resemblance to each other. 

Whereas certain rocks of igneous origin formed by melting and recrystallization can include minerals enriched in the lanthanides (4), cerium is usually present as a trace element rather than as an essential component. Only a few minerals in which cerium is an essential stmcture-defining component occur in economically significant deposits. Two minerals supply the world s cerium, bastnasite [68909-13-7] LnFCO., and monazite [1306-41 -8] (Ln,Th)PO. ... 

Lewis acids are defined as molecules that act as electron-pair acceptors. The proton is an important special case, but many other species can play an important role in the catalysis of organic reactions. The most important in organic reactions are metal cations and covalent compounds of metals. Metal cations that play prominent roles as catalysts include the alkali-metal monocations Li+, Na+, K+, Cs+, and Rb+, divalent ions such as Mg +, Ca +, and Zn, marry of the transition-metal cations, and certain lanthanides. The most commonly employed of the covalent compounds include boron trifluoride, aluminum chloride, titanium tetrachloride, and tin tetrachloride. Various other derivatives of boron, aluminum, and titanium also are employed as Lewis acid catalysts. 

Mn(N03)4] , [Fe(N03)4] and [Sn(N03)4], which feature dodecahedral coordination about the metal [Ce(N03)5] in which the 5 bidentate nitrate groups define a trigonal bipyramid leading to tenfold coordination of cerium (Fig. 11.17b) [Ce(N03)6] and [Th(N03)6] , which feature nearly regular icosahedral (p. 141) coordination of the metal by 12 O atoms and many lanthanide and uranyl [U02] complexes. It seems, therefore, that the size of the metal centre is not necessarily a dominant factor. 

The coordination chemistry of the large, electropositive Ln ions is complicated, especially in solution, by ill-defined stereochemistries and uncertain coordination numbers. This is well illustrated by the aquo ions themselves.These are known for all the lanthanides, providing the solutions are moderately acidic to prevent hydrolysis, with hydration numbers probably about 8 or 9 but with reported values depending on the methods used to measure them. It is likely that the primary hydration number decreases as the cationic radius falls across the series. However, confusion arises because the polarization of the H2O molecules attached directly to the cation facilitates hydrogen bonding to other H2O molecules. As this tendency will be the greater, the smaller the cation, it is quite reasonable that the secondary hydration number increases across the series. 

Homoleptic lanthanide(III) tris(amidinates) and guanidinates are among the longest known lanthanide complexes containing these chelating ligands. In this area the carbodiimide insertion route is usually not applicable, as simple, well-defined lanthanide tris(alkyls) and tris(dialkylamides) are not readily available. A notable exception is the formation of homoleptic lanthanide guanidinates from... 

In a different approach three different structurally defined aza-crown ethers were treated with 10 different metal salts in a spatially addressable format in a 96-well microtiter plate, producing 40 catalysts, which were tested in the hydrolysis of /xnitrophenol esters.32 A plate reader was used to assess catalyst activity. A cobalt complex turned out to be the best catalyst. Higher diversity is potentially possible, but this would require an efficient synthetic strategy. This research was extended to include lanthanide-based catalysts in the hydrolysis of phospho-esters of DNA.33... 

In the REC model, the ligand is modelled through an effective point charge situated in the axis described by the lanthanide-coordinated atom axis, at a distance R, which is smaller than the real metal-ligand distance (Figure 2.6). To account for the effect of covalent electron sharing, a radial displacement vector (Dr) is defined, in which the polar coordinate R is varied. At the same time, the charge value (q) is scanned in order to achieve the minimum deviation between calculated and experimental data, whereas 9 and cp remain constant. 

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