Rare-earth metal complexes, stabilization

Related to these catalysts are the systems based on lanthanide metal systems or rare earth metal complexes [46, 47]. The main problem with these catalyst systems is their instability. When the catalyst solution is prepared by reachng a metallocene with an organolithium compound in a polar solvent, the prepared catalyst soluhon is unstable and decomposes quickly, even under a nitrogen atmosphere. The activity of these catalysts can be high only if the catalyst is added to the polymer soluhon immediately after preparation. Attempts have been made to overcome the stability problem by using an additive in the system to improve the stability and the activity of the catalyst [33-35, 41, 57, 58, 61]. Re-... 

Stabilization of the double charge at carbon is known to occur by negative hyperconjugatimi in bis(diphenylihiophDsphinoyl)methanediide [19]. For the rally rare earth metal complex featuring this ligand and for which a theoretical smdy was made, namely [(7)ScQ(py)2], a quite different approach was used. A second-order perturbation analysis was used to quantify and compare the two different stabilization... 

Chiral NMR shift reagents (CSRs) have many practical applications in chemistry, biology, medicine, and related fields. A pair of enantiomers exhibit markedly different biological properties but provide the same chemical shifts in NMR measurements. Since the diastere-omers have different chemical shifts (Wenzel and Wilcox, 2003 Jacobus and Raban, 1969 Viswanathan and Poland, 1995 Parker, 1991), the chiral anions and cations have often been mixed with pairs of enantiomers for signal separation (Jodiy and Lacour, 2000). The effective CSRs of rare earth metal complexes should have high stability in the presence of interfering... 

Kostromina, N. A., Comparative stability of glyconate complexes of rare earth metals, Ukr. Khim. Zhum. 26, 299-304 (1960). 

The crystal radii of the rare earth metal ions decrease in a regular manner along the series. There is vast data suggestive of the formation of predominantly ionic complexes in the case of rare earth ions. Based on electrostatic theory, a direct relationship between the stability constant values and the atomic number of the rare earth metal ion is predicted [12]. In most of the complexes, this correlation of log K with Z holds good for La to Eu although in some cases the europium complexes are less stable than the samarium complexes. Further, this simple relationship is not valid when the heavy rare earth ions Tb to Lu are considered. 

The primarily ionic nature of the RE(III)-carboxylate interaction suggests that a direct relationship between the ionic radii of the RE(III) and the stability of their complexes with car-boxylates should exist the stability constants of the complexes would increase monotonously from La(III) to Lu(III). However, the experimental results obtained indicate that this is only true for light rare earth metals from La(III) to Eu(III). Three different trends are observed for heavy rare earths from Gd(III) to Lu(III), that is, upward, flat, and downward. This is the so called gadoliniumbreak. Acetate, malonate, succinate, glutarate, and adipate complexes fall into the second category. The log Pi of the complexes remain almost unchanged from Gd(III) to Lu(III) (Table 3.2). There have been various interpretations of these trends, and the most widely accepted one is the change in the number of the hydration water molecules [98, 99]. 

The silyl amide type ligands have been used extensively in rare earth chemistry, as well as in actinide and transition metal chemistry, to stabilize electronically unsaturated metal centers due to the available lone pair on the nitrogen donor atom. Because of the relatively larger steric encumbrance, the rare earth complexes with silyl amide type ligands often exhibit low coordination numbers. As a consequence, the large and electropositive rare earth metal centers are accessible to external reagents, which make them more active in many reactions. 

Liu, B., Ctii, D.M., Ma, 1. et al. (2007) Synthesis and reactivity of rare earth metal alkyl complexes stabilized by aniUdo phosphinimine and amino phosphine ligands. Chemistry —A European Journal, 13, 834. 

The reaction between the active compound and the washcoat can be avoided following two approaches. The first approach is to stabilize the active phase by applying it as a complex oxide. The spinel-type oxides, mentioned above, are relatively inactive. On the other hand, perovksites, often AMO3 (A = rare earth metal, e.g.. La, Sr M — transition metal, e.g., Co, Cr, Fe), exhibit promising behavior, which has attracted much attention [55,90,91]. Perovskites may also be used as unsupported oxides, but their surface area is small and not particularly thermostable. The activity of perovskites is dependent mostly on the M cation, but the A cation also has a significant effect [91]. Partial substitution... 

The Zv - complex of the merocyanine system (57) releases the zinc when it is irradiated with visible light and this results in the formation of the colourless closed spiropyranindoline (58, R = H). When the irradiation is stopped the Zn " complex reforms but this does not happen with the nitro derivative (58, R = NO2) in which it is thought that the nitro group stabilises the phenoxide ion in the open form. Other workers have also studied the complexation of spiropyran based merocyanines with transition and rare earth metal ions. An investigation of the influence of Lewis acids (hexa-fluoropropanol, trifluoroethanol and 2-fluoroethanol) on the stability of the coloured form of spiropyran and spirooxazines has been reported. Protonation of the open system produces a form that is photochemically inert and the behaviour of these acids is markedly different from that of acetic acid with such systems. 

One of the most interesting applications of the HSAB concept consists in the prediction of the stability of the complexes formed owing to interaction of alkali metal halides with rare-earth metal halides. These systems are of great interest for the materials science of scintillation materials the said complex halides are now considered among the most promising scintillation detectors and sensors. Besides, the Li- and Gd-based materials are especially convenient as effective detectors of thermal neutrons. The compositions and stability of the formed compounds depend considerably on the kind of acids and bases from which the compound is formed. So, Li+ cation is one of the hardest cation acids, and, therefore, the formation of stable complex halides of Li and lanthanides according to reaction ... 

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