Coordination Numbers in Lanthanide Complexes

Coordination Numbers in Lanthanide Complexes 4.5.1 General Principles... 

These ligands are now well represented in complexes with the lanthanides, but were not investigated until 1962, when a study in aqueous solution using a Job s plot method, e.g. at 451 nm for Ho ", showed that a complex Ho(phen)2" was formed. Pr, Nd and Er tripositive ions were also studied, but no solid products were characterized. Indeed, stability constants in water are possibly too low for a solid complex to be isolated. A little later, there were several reports of the isolation of bipyridyl and 1,10-phenanthroline complexes from ethanolic solution. It is perhaps of interest that at this time, the lanthanides were believed not to form stable amine complexes and the role of higher coordination numbers in lanthanide complexes was not fully appreciated. Complexes isolated included the stoichiometries M(N03)s(bipy)2, where M = M(NCS)3(bipy)3, where M = La, Ce, Dy M(MeC02)3(bipy),... 

For certain transitions which have AJ < 2, AL 2, and AS = 0, the intensities are far more sensitive to complexation than for the other transitions. These have been termed hypersensitive transitions by Jdrgensen and Judd (1964). The intensities of these transitions may be up to 200 times greater than the corresponding transition in the aquo ion whereas the intensities of the other transitions are generally approximately the same as in the aquo ion. The sensitivity of these transitions to the environment has led to their use in determining the coordination number for lanthanide complexes in solution. Since the subject of hypersensitivity has been reviewed recently by Henrie et al. (1976) and is covered in ch. 24 (Solution Chemistry), the phenomenon is only mentioned briefly here. 

Useful structural data have been obtained from single crystal X-ray diffraction methods for a number of lanthanide complexes (15,17). In this section, the structures of various complexes with neutral oxygen donor ligands which have been studied by single crystal X-ray diffraction methods are described. For convenience, the structures are discussed in terms of coordination numbers of the lanthanide ions. 

Octacoordination is often encountered in lanthanide complexes. The preferred poly-hedra for eight coordination expected on the basis of interligand repulsivities are square antiprism (D ), dodecahedron with triangular faces (Z)2d), bicapped octahedron (D3(i), truncated octahedron (Z)2ft), 4,4-bicapped trigonal prism (C2v), distorted cube (C2v), and cube (0/,). The most commonly observed polyhedra for this coordination number are, however, the square antiprism and the dodecahedron. 

Fig. 41. A plot of the M—N distances in lanthanide complexes with different coordination numbers...

Although some of the lanthanides form compounds with oxidation states other than + 3, the vast majority of stable species involve the trivalent state. Due to the nature of the weak bonding f electrons, complexes in solution are normally quite labile, and as described below, the preparation and the isolation of pure enantiomers containing a central lanthanide ion and achiral ligands are extremely rare. The coordination number of lanthanide(III) ions is somewhat variable and depends on the size and charge of the coordinated ligands, and the size of the lanthanide ion that varies slightly... 

The solid-state structure of the ethylaluminum oxide metallocene complex (Fig. 30) shows two trivalent samarocene units connected by an [(A EtsO ]2-ethylalumoxane ( EAO ) unit. The latter was described as an adduct of two molecules of triethylaluminum with two [AUfoO]- anions [116]. An unusual asymmetric fx-r q1 (side-on) ethyl coordination mode was observed, which previously has been found only in a small number of lanthanide complexes, i.e., (CsMeshYM/zy 1-Et)AlEt2(THF) [ 144], (C5Me5) 2Sm(THF) (fx-rj1 ijx-Et) AlEt3 (Fig. 9) [115] and (C5Me5)2Sm(THF) (/i-r]1 X-Et) (/z-Cl)AlEt2 [116]. 

Nephelauxetic effects (1 — f ), mean lanthanide-ligand distances (R (Ln-O) and effective coordination numbers of lanthanides (CNeff) in Pr(III) and Nd(III) ODA complexes determined from baricentres (a) of spectral bands ODA = oxydiacetate, OOCCH2OCH2COO- [184],... 

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]. 

With the exception of thorium and protactinium, all of the early actinides possess a stable +3 ion in aqueous solution, although higher oxidation states are more stable under aerobic conditions. Trivalent compounds of the early actinides are structurally similar to those of their trivalent lanthanide counterparts, but their reaction chemistry can differ significantly, due to the enhanced ability of the actinides to act as reductants. Examples of trivalent coordination compounds of thorium and protactinium are rare. The early actinides possess large ionic radii (effective ionic radii = 1.00-1.06 A in six-coordinate metal complexes),and can therefore support large coordination numbers in chemical compounds 12-coordinate metal centers are common, and coordination numbers as high as 14 have been observed. 

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