Ytterbium coordination number

For the subject matter of this book, it is of particular interest to consider the situation for a non-crystalline system analogous to that of crystalline ytterbium or strontium under pressure, namely that when a valence and conduction band are separate or overlap slightly. If the degree of overlap can be changed by varying the mean distance between atoms, the composition or the coordination number then a metal-insulator transition can occur. Many examples will be discussed in this book, particularly amorphous films of composition (Mgi- )j(By3, liquid mercury at low densities, and liquid tellurium alloys in which the coordination number changes with temperature. The transition is, we believe, of Anderson type. 

The behaviour of lanthanum in dimethyl formamide (DMF) is quite different from that in methanol and acetonitrile. While perchlorate forms inner sphere complexes with lanthanides in acetonitrile [31], no such complexes are formed in DMF [32]. The coordination properties in DMF solutions were studied by NMR and UV-Vis spectroscopy techniques [33,34], The rate of DMF exchange in the system ytterbium perchlorate-DMF-CD2CI2 was slow enough that 1H NMR resonances permitted the determination of the mean coordination number to be 7.8 0.2. Similar determination in the case of thulium(III) gave a mean coordination number of 7.7 0.2. Thus it was concluded that the predominant species in heavy lanthanides is Ln(DMF)g+ in DMF solutions. In the case of lighter lanthanides, the following equilibrium exists... 

In the complex (C5H4Me)Yb(OC6H2-2,6-But2j4-Me)2(THF) the ytterbium atom is coordinated by one methylcy-clopentadienyl ligand and three O atoms. The formal coordination number around the Yb atom is 6 (Scheme 57). 

The most useful rule in describing the effect of pressure on solids is the so-called Pressure-Coordination Rule (i, 2) according to which the coordination number is increased with pressure. In Table 1 and 2 examples are listed for various crystal structure transformations which follow this qualitative mle at different pressures and temperatures. An exception to this mle is known, however, for ytterbium (2) the cubic face-centered modification (coordination number = 12) of the metal is transformed at 40 kbar into a cubic space-centered stmcture (coordination number = 8). 

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 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". ... 

The structures of the Nd, Y, and La compounds have been reported by Safyanov and Belov (1976) and Safyanov et al. (1977, 1978a,b, 1979) while the erbium and ytterbium compounds have been studied by Rivero et al. (1984, 1985). In all structures the decavanadate is formed by ten edge-sharing VOg octahedra. The coordination number of R is either nine (La and Nd) or eight (Y and Er). The structure of the La compound is unique in that it has two bonds between the R " central ion and decavanadate oxygens. In all other cases the coordination polyhedra around the rare earth is exclusively formed by water oxygens (fig. 116). 

A rare ytterbium(III) complex [Yb(HB(pz)3)3] has an eight-coordinate bicapped trigonal prismatic structure (48) with two tridentate ligands and one bidentate ligand. The complex is stereochemi-cally rigid in solution. 1 A number of thorium and uranium complexes have been reported. These include [U(HB(pz)3)4], [U(H2B(pz)2)4], [U(HB(pz)3)2Cl2], and mono- and di-Cp metal-halogen complexes.49... 

Some salient points to note are (i) the model is in accord with the experimental results, (ii) nuclei at a and yS positions to the coordinating atom show deviations from the model due to contributions of other shift mechanisms. Thus the best procedure for the elucidation of molecular structure by using lanthanide reagents consists of (i) to obtain the relative magnitude of geometrical function, G for different ligand nuclei from the slopes of A bT versus 1 / T plots, (ii) since temperature dependence of ytterbium complexes conforms to the model, use of ytterbium complexes is prudent, (iii) in cases where the temperature dependence is interfered with effects due to chemical equilibrium or exchange, data for a number of lanthanides at room temperature may be obtained and plots of equation... 

The only complexes of lanthanum or cerium to be described are [La(terpy)3][C104]3 175) and Ce(terpy)Cl3 H20 411). The lanthanum compound is a 1 3 electrolyte in MeCN or MeN02, and is almost certainly a nine-coordinate mononuclear species the structure of the cerium compound is not known with any certainty. A number of workers have reported hydrated 1 1 complexes of terpy with praseodymium chloride 376,411,438), and the complex PrCl3(terpy)-8H20 has been structurally characterized 376). The metal is in nine-coordinate monocapped square-antiprismatic [Pr(terpy)Cl(H20)5] cations (Fig. 24). Complexes with a 1 1 stoichiometry have also been described for neodymium 33, 409, 411, 413, 417), samarium 33, 411, 412), europium 33, 316, 411, 414, 417), gadolinium 33, 411), terbium 316, 410, 414), dysprosium 33, 410, 412), holmium 33, 410), erbium 33, 410, 417), thulium 410, 412), and ytterbium 410). The 1 2 stoichiometry has only been observed with the later lanthanides, europium 33, 411, 414), gadolinium, dysprosium, and erbium 33). 

Lanthanide elements have atomic numbers ranging from 57 to 71. With the inclusion of scandium (Sc) and yttrium (Y), a total of 17 elements are referred to as the rare earth elements. A mixture of rare earths was discovered in 1794 by J. Gadolin and ytterbium was separated from this mixture in 1878 by Mariganac, while the last rare earth element promethium (Pm) was separated by a nuclear reaction in 1974. Therefore, a period of more than 100 years separates the discovery of all the rare earth elements. In the latter part of the last century scientists started to focus on the applications of rare earth elements. Numerous interesting and important properties were found with respect to their magnetic, optical, and electronic behavior. This is the reason that many countries list all rare earth elements, except promethium (Pm), as strategic materials. Rare earth coordination chemistry, therefore, developed quickly as a result of this increased activity. 

Lanthanide elements (referred to as Ln) have atomic numbers that range from 57 to 71. They are lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). With the inclusion of scandium (Sc) and yttrium (Y), which are in the same subgroup, this total of 17 elements are referred to as the rare earth elements (RE). They are similar in some aspects but very different in many others. Based on the electronic configuration of the rare earth elements, in this chapter we will discuss the lanthanide contraction phenomenon and the consequential effects on the chemical and physical properties of these elements. The coordination chemistry of lanthanide complexes containing small inorganic ligands is also briefly introduced here [1-5]. 

Owing to nitrates being bidentate, [La(Tp P )(N03)2] has 10-coordinate lanthanum, as has [La(Bp ° 2(N03)], but in [La(Bp( °0 y)2(CF3S03) the presence of a monodentate triflate causes lanthanum to be nine-coordinate. A number of studies have been made of ligands that are not tris(pyrazolyl)borates. [Yb(B(pz)4)3].EtOH has eight-coordinated ytterbium, one pyrazo-lylborate being bidentate. ... 

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