Lanthanide complexes formation

In the process of lanthanide complex formation with the porphyrins, the ligand loses two protons and yields lanthanide hydroxy porphyrin or lanthanide porphyrin X, where X = C1, Br, NOJ, etc. Many lanthanide complexes with substituted porphyrins have been prepared by heating a mixture of porphyrin and the lanthanide salt in imidazole melt in the range 210-240°C. When the complex formation is complete the solvent (i.e.) imidazole is eliminated by either sublimation [81] or by dissolution of the mixture in benzene, followed by washing with water [82]. Further purification requires column chromatography. The starting material can be anhydrous lanthanide chloride or hydrated lanthanide acetylacetonate. After purification the final product tends to be a monohydroxy lanthanide porphyrin complex. 

Studies on the thermodynamics of lanthanide complex formation with ligands like ethylenediamine (en) and diethylenetriamine (dien) in acetonitrile to form [M(en) ]31... 

Similar to aqueous solutions studies on lanthanide complex formation in non-aqueous solutions involve complex equilibria characterized by equilibrium constants and stability constants which have been discussed in detail in Chapter 3. To recall, we define the equilibrium constants and stability constants as follows ... 

Many methods have been used in the studies of lanthanide complex formation in non-aqueous solutions. Some commonly used methods are given below ... 

For the synthesis of a REE complex connection, it is a matter of principle to have a pH-medium since the type of a lanthanides complex formation depends on it, and so does the character of a complex formation in water and in the water-organic mediums. When carrying out reaction in water with REE salts it is required that pH of a reaction mass doesn t exceed value at which ytteibium hydroxide is formed. Value of size pH sedimentation of hydroxide of ytterbium and its control in the course of synthesis are necessary conditions when receiving complexes of ytterbium. 

In these studies the enthalpies and entropies of lanthanide complex formation had been measured and the enthalpies and entropies of hydration estimated from theoretical and semiempirical equations. Bertha and Choppin (44) examined the hydration thermodynamics of the lanthanide ions directly and obtained molar entropies of hydration of 338 4 J/mol-K and 401 4 J/mol-Kfor La-Pr and for Dy-Lu, respectively, and a range of 351-392 J/mol-K for Nd-Tb. These observations supported die existence of two dffferendy sized hydration spheres corresponding to the La-Pr and Dy-Lu groups, with Nd-Tb comprising the transition between them. Spedding et al (45) later confirmed die AShyd data of Bertha and Choppin. 

The lanthanides form many compounds with organic ligands. Some of these compounds ate water-soluble, others oil-soluble. Water-soluble compounds have been used extensively for rare-earth separation by ion exchange (qv), for example, complexes form with citric acid, ethylenediaminetetraacetic acid (EDTA), and hydroxyethylethylenediaminetriacetic acid (HEEDTA) (see Chelating agents). The complex formation is pH-dependent. Oil-soluble compounds ate used extensively in the industrial separation of rate earths by tiquid—tiquid extraction. The preferred extractants ate catboxyhc acids, otganophosphoms acids and esters, and tetraaLkylammonium salts. 

The most important minerals of the lanthanide elements are monazite (phosphates of La, Ce, Pr, Nd and Sm, as well as thorium oxide) plus cerite and gadolinite (silicates of these elements). Separation is difficult because of the chemical similarity of the lanthanides. Fractional crystallization, complex formation, and selective adsorption and elution using an ion exchange resin (chromatography) are the most successful methods. 

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... 

Rare earth elements are the general term for 15 kinds of lanthanide elements (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Py, Ho, Er, Tm, Yb, Lu) together with Sc and Y elements. They prefer trivalent states in the complex formation, though three elements (Eu, Sm, Yb) can assume tri- and divalent stateos and Ce a tri- or tetravalent state. Their ionic radii are fairly large (1.0-1.17 A) and their electronegativities are low (1.1-1.2). In fact, the former are much larger than those of... 

EDTA complexes of trivalent metals can be extracted successively with liquid anion exchangers such as Aliquat 336-S by careful pH control. Mixtures of lanthanides can be separated by exploiting differences in their EDTA complex formation constants. 

Kretschmer et al. have described the formation of a lanthanide complex, [Cp6Yb6Cl13] (Cp = cydopentadienyl), which conforms to a truncated octahedron. [36] The anion contains six ytterbium ions, located at the corners of an octahedron, and 12 bridging choride ions. A single chloride ion occupies the center of the shell. 

Tablet. Stability constants for complex formation MA3, MHCitCit2 and MCit for trivalent lanthanides and actinides. Ionic strength 0.15...

We have considered typical examples of lanthanide and actinide solvent extraction by chelate formation, involving complexes with citric acid and with TTA, to prove that the labelling of a stable element by one of its radioactive isotopes can help to produce accurate data on the stability constants for complex formation. The method is applicable to elements with radioisotopes having a half-life allowing an ion concentration of 10 6m or less. Other methods of partition such as radiopolarography and radio-coulometry also result in accurate thermodynamical data when the same procedure of labelling is used (29). 

Most commonly, metal ions M2+ and M3+ (M = a first transition series metal), Li+, Na+, Mg2+, Al3+, Ga3+, In3+, Tl3+, and Sn2+ form octahedral six-coordinate complexes. Linear two coordination is associated with univalent ions of the coinage metal (Cu, Ag, Au), as in Ag(NH3)2+ or AuCL Three and five coordination are not frequently encountered, since close-packing considerations tell us that tetrahedral or octahedral complex formation will normally be favored over five coordination, while three coordination requires an extraordinarily small radius ratio (Section 4.5). Coordination numbers higher than six are found among the larger transition metal ions [i.e., those at the left of the second and third transition series, as exemplified by TaFy2- and Mo(CN)g4 ] and in the lanthanides and actinides [e.g., Nd(H20)93+ as well as UC Fs3- which contains the linear uranyl unit 0=U=02+ and five fluoride ligands coordinated around the uranium(VI) in an equatorial plane]. For most of the metal complexes discussed in this book a coordination number of six may be assumed. 

Various enzymes are capable of degrading the RNA polymer by splitting the bonds between ribose and phosphate. The same reaction has been carried out nonenzymatically with lanthanum and cerium (III) nitrates (2) the mechanism of such a reaction probably involves the formation of a lanthanide complex, followed by cleavage of the phosphate bond ... 

Lactam, ring formation, 288 Lactones, optically active, 31 Lanthanide complexes epoxy ring opening, 234 hetero-Diels-Alder reactions, 217 nitno-aldol reaction, 228 Laudanosine, 36 Leucine hydrocarboxylation, 168 Lewis acid complexes, 212 Ligands ... 

Several of the lanthanide complexes MX3, where X = NSi2Me6, have been shown to give 1 1 adducts MX3L where M = La, Eu and Lu and L = OPPh3.145 The central MN30 unit has virtual C3t symmetry. Evidence has also been presented for the formation of unstable 1 2 adducts MX3L 2 where L = OPMe3.144... 

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