Lanthanides crystallographic data

Various crown ethers (p. 96) with differing cavity diameters provide a range of coordination numbers and stoichiometries, although crystallographic data are sparse. An interesting series, illustrating the dependence of coordination number on cationic radius and ligand cavity diameter, is provided by the complexes formed by the lanthanide nitrates and the 18-crown-6 ether (i.e. 1,4,7,10,13,16-... 

The use of lanthanide probes for structure determination in small molecules has been extensively emphasized. In practice, bond lengths and angles are taken from crystallographic data, and the rotation angles about single bonds are determined from the nmr studies. The results of small molecule studies have been reviewed elsewhere.7,8... 

Crystallographic data are invaluable in the interpretation of experimental optical spectra, both in identifying the symmetry of the lanthanide ion site and in providing input data to a superposition analysis (Newman, 1970) or to an ab initio crystal-field calculation. Crystallographic data are presented in a short table for each host that lists the following information ... 

Some recent crystallographic data for lanthanide/actinide or alkaline-earth substituted halides. RX , 2 < n <3. 

The ionic radius is a useful parameter with which to correlate numerous physical and thermodynamic properties of the actinide elemoits. Its usefulness for this purpose is not usually dependent on how it is d ned or on the absolute values that are used when comparing members of the series. Nevertheless, the tom radii implies spherical ions, and the modes of deriving such radii from crystallographic data usually assume that these spheres are in contact with spherical anions. When this assumption is not true, as in most real crystals, the derived radii depend on the method of calculation and are somewhat arbitrary. Consequently, there have been published for the actinide elements several tables of radii which differ both in absolute values and in the slope of the curve obtained when they are plotted against atomic number. All of these sets of radii have in common, however, two qualitative features a contraction of the radius with increasing atomic number and a cusp at the half-filled 5f-electron shell. Additional perturbations of the curve at the one-fourth- and three-fourths-fiUed shells have not been established for the actinides, although slight effects were shown to exist for the lanthanides... 

Figure 9.3 Example of a solid state structure of a lanthanide fi-diketonate d-f hybrid. Created using the Mercury 2.4 software from the crystallographic data in Reference [17].

Compared to the wealth of data concerning the solid- and solution-state structures of lithium (di)organophosphides, reports of heavier alkali metal analogues are sparse. Indeed, the first crystallographic study of a homometallic heavier alkali metal (di)organophosphide complex was reported only in 1990 (67) and the majority of such complexes have been reported in the past 3 years. Interest in these complexes stems mainly from their enhanced reactivity in comparison to equivalent lithium complexes, which is particularly useful for the synthesis of alkaline earth, lanthanide, and actinide organophosphide complexes. 

The crystallographic ionic radii of the rare-earth elements in oxidation states +2 (CN = 6), +3 (CN = 6), and +4 (CN = 6) are presented in Table 18.1.3. The data provide a set of conventional size parameters for the calculation of hydration energies. It should be noted that in most lanthanide(III) complexes the Ln3+ center is surrounded by eight or more ligands, and that in aqueous solution the primary coordination sphere has eight and nine aqua ligands for light and heavy Ln3+ ions, respectively. The crystal radii of Ln3+ ions with CN = 8 are listed in Table 18.1.1. 

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