Lanthanides metal radii

The atom radius of an element is the shortest distance between like atoms. It is the distance of the centers of the atoms from one another in metallic crystals and for these materials the atom radius is often called the metal radius. Except for the lanthanides (CN = 6), CN = 12 for the elements. The atom radii listed in Table 4.6 are taken mostly from A. Kelly and G. W. Groves, Crystallography and Crystal Defects, Addison-Wesley, Reading, Mass., 1970. 

Figure 30.2 Variation of metal radius and 3+ ionic radius for La and the lanthanides. Other data for Ln" and Ln" are in Table 30.2.

The hydrogenation of unfunctionalized alkenes is readily performed by Group III and lanthanide cyclopentadienyl hydride derivatives, one key feature being the high TOFs of these systems (up to 120000 IT1 for hydrogenations catalyzed by Lu, Tables 6.8 and 6.9) [119, 120]. The reaction rate depends heavily on the metal and the ligands. It is inversely proportional to the metal radius (Lu>Sm>Nd>La), and it is faster for the Cp M derivatives than for the ansa di-... 

Symbol Ho atomic number 67 atomic weight 164.93 a lanthanide series rare earth element electron configuration [Xe]4/ii6s2 valence state +3 metallic radius (coordination number 12) 1.767A atomic volume 18.78 cc/mol ionic radius Ho3+ 0.894A one naturally occurring isotope. Ho-165. 

Where the lanthanide ionic radius and the macrocyclic cavity are incompatible, though in hydrous conditions the crown ether is likely to be displaced by water ligands, the crown may still be present in the structure of the crystal as a hydrogen-bonded adduct. This behaviour is seen in [Gd(N03)3(H20)3]-(18-crown-6).445 This type of compound is quite well known in the case of s block metals also, e.g. [Mg(H20)6]Cl2 (12-crown-4)454 and, a more subtle case, [Ca(nitrobenzoate)2(benzo-15-crown-5)]-3H20(benzo-15-crown-5)455 in which an apparent 2 1 complex has only half its crown ligand coordinated to Ca2+. 

Trivalent yttrium and lanthanide metals, except for promethium, have been complexed to octaethylporphyrin by heating at 210 °C in an imidazole melt.17 The complexes obtained as hydroxides, Mm(OEP)(OH), are unstable in acidic media. As the charge radius ratio rule predicts, the early lanthanide metalloporphyrins, MIU(OEP)(OH) (M = La, Ce, PR, Nd), are demetallated during purification, and the middle series (M = Sm, Eu, Gd, Tb, Dy) in 1 % acetic acid in methanol, while the last five (M = Ho, Er, Tm, Yb, Lu) survive in 2% acetic acid in methanol but are dissociated in dilute hydrochloric acid. The Mnl(OEP)(OH) appears to coordinate more than one equivalent of pyridine and piperidine, and dimerizes in noncoordinating solvents such as benzene and dichloromethane at 10 4 M concentration. The dimer is considered to be a di-p-hydroxo-bridged species, different from the p-oxo dimer, Scin(OEP) 20 (Scheme 6). 

The ionic radius of the lanthanide metal significantly influences the nature of the products. For example, with the larger lanthanides La and Ce higher oligomers (preferably trimers) are formed. In the case of Ln = Y the products are generally dimers, but the regioselectivity depends on the substituent R. The dimers RHC=C(H CsCR are obtained with R = Ph or SiMe3. With bulky substituents (R = iPr, fBu) the result is a head-to-tail dimerization of the terminal alkyne, Eq. (18) [82],... 

Fig. 13. Plot of 2 versus radius, r (in pm)/ionic charge, for the complexation of (29) with a series of alkaline earth and lanthanide metal cations.

In recent years, a large number of mono- and dicationic lanthanide alkyl complexes have been found to be efficient catalysts for ethylene polymerization, and in some cases, the dicationic lanthanide derivatives show higher activity and selectivity than their monocationic counterparts. Ionic radii of lanthanide metals also affect the catalytic behavior, and polymerization activity often increases with ionic radius [5, 76],... 

Figure 5-8. Ionic radii and electronegativity of lanthanide metal elements such as Pr, Sm, Gd, and Dy. It is well known that ionic radius decreases and electronegativity increases with atomic number such as from Pr to Dy. (Reproduced with permission from ref 33. Copyright 2003 American Institute of Physics)...

One such a situation is the room-temperature solid solubility of hydrogen in Sc (Azarkh and Funin 1965), Y, Gd, Er, Tm and Lu (Beaudry and Spedding 1975), which are plotted in fig. 22 as a function of the metallic radius. It is immediately seen that the amount of hydrogen dissolved in the rare earth elements with no unpaired 4f electrons is enormously higher than in those lanthanides with unpaired electrons. If the 4f electrons were truly localized and not involved in the chemical bonding one... 

A remark is necessary in connection with fig. 10.14. Lu has a much smaller metallic radius (1.74 A) than La (1.88 A) at normal P (Zachariasen, 1973) due to the lanthanide contraction. For a proper comparison of the melting points one should therefore compress La by external pressure (a pressure of = 100 kbar will suffice), so that its metallic radius equals that of Lu. An extrapolation of the melting curve of La (Jayaraman, 1965a) to 100 kbar shows that the melting point at that pressure is still = 400 K lower than for Lu. Although the melting point anomaly of La becomes, after the above correction, a debatable effect, we believe that it is a significant effect as will become clear from the discussion of Ce (see section 3.2). 

Though yttrium is not a member of the lanthanide series, its chemistry closely mimics that of the lanthanide metal ions. As a result of this similarity, most rare-earth minerals also contain yttrium, some in relatively high abundance. Yttrium is also a product, along with several of the lanthanides, of nuclear fission and so is present in irradiated nuclear fuel. The best estimate of the eight-coordinate cationic radius of Y is 1.019 A, very near to that of Ho (1.015 A) (Shannon 1976). Because the interaction of the lanthanides and Y with solvent and solute molecules is predominantly electrostatic in nature, the solution chemistries of Y and the lanthanides overlap substantially. However, this similarity in the... 

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