Neodymium coordination number

Compound 29f is formally an ate complex in which one equivalent of LiCl is retained in the product in order to increase the coordination number at the neodymium atom. Neodymium and lithium are bridged by two chlorine atoms and the coordination sphere at lithium is completed by two THF ligands. 

Many factors affect coordination numbers and coordination polyhedra. It was recognized during 1962-1975 that the chemistry of yttrium and lanthanides is dominated by large coordination numbers. Although the structure of Nd(Br0.3>3 9H2O had been determined as early as 1939 by Helmholtz [6] and shown that the central neodymium ion was bound to nine water molecules in a face-centred trigonal prismatic structure, the commonly held opinion was that the rare earth ions formed six-coordinate, octahedral complexes [7]. This notion was based on the known octahedral, six-coordination known at that time for the most... 

Hydrates of rare earth chlorides also have two different crystal systems a triclinic system for lanthanum, cerium, and praseodymium, as well as a monoclinic system for neodymium to lutetium and yttrium. CeCl3-7H20, as an example of the former system, is different from the above infinite polymer as two cerium atoms are connected by two [i2-bridges to form a dimer. The formula for this dimer is [(H20)7Ce([i2-Cl)2Ce(H20)7]Cl4 as shown in Figure 1.18. Therefore, the coordination number of cerium is nine and the polyhedron takes on a destroyed mono-capped square antiprism configuration. 

In silica, however, its fluorescence peaks at 1.088 im [15-19,21,28,29]. This anomolous fluorescence behavior for neodymium in silica is caused by its unusually low coordination number of six in a pure silica host [18,19,27]. In most glasses and crystals, neodymium coordination is seven or nine [15]. 

There is an example of a chlorine ligand with coordination number six, in the compound Nd6(OPr 2)nCl. The chlorine atom lies at the centre of a trigonal prism, formed by the six neodymium atoms which are held together by bridging isopropoxide groups. TTie Nd—Cl distance of 3.05 A is compatible with an ionic formulation, and the stereochemistry does not appear significant. 

The coordination number of neodymium atoms is 21 (fig. 38a). Nickel atoms have the coordination polyhedra in the form of tetragonal antiprisms with one additional atom (fig. 38b). Germanium atoms are situated inside the trigonal prisms and distorted cubo-octahedra (fig. 38c,d). 

A number of oxyanions form complexes (inner- and outer-sphere) in aqueous solution with the rare earth ions. However, the number of authenticated solid complexes is much smaller. By far the greatest number of complexes is formed with the nitrate ion and the structure of some of these were discussed previously (sections 3.9, 3.10). In addition to these it is possible to prepare [(C4H9)4N]j[R(N03)6] for R= La, Ce, Pr, Nd, and Sm and [(C4H9)4N]2(R(N03)5] for R = Ho, Er, Tm, Yb, and Lu and [(C3H7)4N]2 R(N03)5l for R = Nd, Ho, Tm, and Yb (Walker and Weeden, 1973). The role of the size of both the cation and lanthanide ion in determining the coordination number is again demonstrated. At least in the case of neodymium the visible spectra of both salts in nitrobenzene and dichloromethane are identical which indicates that Nd(N03) is unstable in these solvents. 

The tribromides of lanthanum through praseodymium possess the hexagonal UCIj-type structure (Coordination Number of lanthanide CN = 9). The tribromides of neodymium through europium possess the orthorhombic PuBr -type structure (CN = 8), while those of gadolinium through lutetium as well as those of scandium and yttrium possess the rhombohedral FeClj-type structure (CN = 6). 

Although they are not macrocyclic ligands, polyethyleneglycols behave somewhat similarly to the crown ethers regarding lanthanide complexation, as established by a number of crystal structure determinations. Thus a series of neodymium complexes shows consistent 10-coordination, namely [Nd(N03)3(tri-eg)],458 where tri-eg is triethyleneglycol, [Nd(N03)2(penta-eg)]N03,458 and [Nd(N03)2(N03)(tetra-eg)],459 where N03 is monodentate. The larger La3+ ion shows 11-coordination in [La(N03)3(tetra-eg)].460... 

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 (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]. 

Complexes Ln(bipy)2(N03)3 have been studied in more detail than the other bipy complexes all appear to have 10-coordinate structures with all nitrates present as bidentate ligands. The coordination geometry has been variously described as a bicapped dodecahedron and as a sphenocorona. Unlike the La complex, the Lu complex does not possess disorder about the twofold axis. Li N distances are 2.46-2.67(1) A and Lu—O distances in the range 2.426(9)-2.556(9) A. [Y(bipy)2(N03)3] is, like [Y(phen)2(N03)3], isostructural with its lanthanide analogues. Similarly, the structure of [Nd(bipy)2(N03)3] has been shown to be isostructural with [Ln(bipy)2(N03)3] (Ln = Y, La, Lu). A number of compounds Ln(bipy)3(N03)3 (Ln = Ce, Pr, Nd, Yb) have been reported the neodymium complex was shown to be [Nd(bipy)2(N03)3]. bipy, with the third bipy molecule not associating with the neodymium-containing complex. ... 

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