Lanthanide elements high coordination numbers

The lanthanide or rare earth elements (atomic numbers 57 through 71) typically add electrons to the 4f orbitals as the atomic number increases, but lanthanum (4f°) is usually considered a lanthanide. Scandium and yttrium are also chemically similar to lanthanides. Lanthanide chemistry is typically that of + 3 cations, and as the atomic number increases, there is a decrease in radius for each lanthanide, known as the lanthanide contraction. Because bonding within the lanthanide series is usually predominantly ionic, the lanthanide contraction often determines the differences in properties of lanthanide compounds and ions. Lanthanide compounds often have high coordination numbers between 6 and 12. see also Cerium Dysprosium Erbium Europium Gadolinium Holmium Lanthanum Lutetium Praseodymium Promethium Samarium Terbium Thulium Ytterbium. 

The anhydrous compound, whose chemical formula is Er(btc), has not been structurally characterized because of its lack of crystallinity and its physical properties have not been explored in details. This family is characteristic of the problem encoimtered when trying to constmct molecular based microporous materials, that is obstmction of the pores by interpenetration of networks or collapse of the stracture upon removal of the guest molecules intercalated in the cavities. This problem is, of course, also present for coordination polymers based on transition elements but it is much more crucial for lanthanide-based materials. Indeed, lanthanide ions present a high coordination number (generally ranging between 7 and 12) and their coordination sphere most often contains some solvent molecules. These coordination water molecules, when removed upon dehydration, lead to structural rearrangements and, sometimes, to a loss of the microporons character. 

Binary trihalides RX3 are well known for all rare-earth elements with the exception of Eul3 which appears not to exist due to the relative stability of Eul2- Their crystal structures change subject to the lanthanide contraction within the lanthanide series with high coordination numbers for the lighter lanthanides especially with small ligands (fluoride) and vice versa. Table 1 gives an overview of the crystal structures of the trihalides at ambient conditions. 

Very high coordination numbers (9 to 12) are known for some compiex ions of the lanthanide elements. 

LaCrC>3 is one of the family of lanthanide perovskites RTO3, where R is a lanthanide and T is a period 4 transition element. In the cubic unit cell R occupies the cube corners, T the cube centre and O the face-centre positions. The coordination numbers of T and R are 6 and 8 respectively. LaCrC>3 loses chromium at high temperatures, leaving an excess of O2- ions. The excess charge is neutralized by the formation of Cr4+ which results in p-type semiconductivity with hole hopping via the localized 3d states of the Cr3+ and Cr4+ ions. The concentration of Cr4+ can be enhanced by the substitution of strontium for lanthanum. A 1 mol.% addition of SrO causes the conductivity to increase by a factor of approximately 10 (see Section 2.6.2). 

The coordination number, N of lanthanides varies significantly and no intrinsic property of 4/ group atoms predetermines a high propensity for a given N value and symmetry as is common with transition elements like 3d3 Cr(III), 3d6 Co(III), 3d8 Ni(II), 4d6 Rh(III), and 5d6 Ir(III) and Pt(IV). In the case of lanthanides, N takes on values from 3 to 16 as shown by examples given below. 

Compounds of the type [(Me3Si)2N]3M have been prepared for all of the lanthanide elements except Pm, Tb, Dy, Tm, and Er (4). The synthetic method used in their preparation is nucleophilic substitution with three molar equivalents of lithium -or sodium - bis(trimethylsilyl)amide on the metal trichlorides in tetrahydrofuran. The compounds are rather high melting solids (145-170°C) which can be isolated by crystallization from pentane as long needles or by vacuum sublimation (80-100°C). The binary silylamides are monomeric in refluxing benzene solution, in the gas phase (by mass spectrometry), and in the solid state (by x-ray crystallography, see below). Thus, these compounds are three-coordinate, a unique coordination number for the lanthanide elements. 

The first stage of oirr work was the synthesis of lanthanide elements salts (ytterbium and erbium) in a form of acetylacetonates. The rare-earth elements (REE) complexes in most cases have the coordination number (CN) more than six (7, 8, 9, 10 and even 12). CN of REE ions in complexes with organic poly dentate ligands are high and variable [10]. The reason of this phenomenon lies in the big ionic radius, which decreases from 1.06 A (La " ) to 0.88 A (Lu " ) (the effect of lanthanide compression ). The empty site of the coordination sphere is occupied by other ligands water, hydroxyl ions, etc. In IR-spectrum the hydroxyl ion is characterized by a narrow strip at 3700-3600 cm, it has higher frequency than water. Frequency v of water is located in a region of about 3600-3200 cm". ... 

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