Octahedral lanthanide complexes

Another well characterized anionic octahedral lanthanide complexes are the hexaisothiocyanato complexes, [M(NCS)6] (35). The conductivity measurements of the hexachlorides, [(C4H9)4N]3 [MCle] and the hexaisothiocyanates in nitrobenzene (Table 2) solution indicate [35,36) a 3 1 electrolyte behaviour of the complexes, showing that the species MX is not appreciably dissociated in solution of reasonably polar solvent. 

It was not until quite recently that the presence of octahedral lanthanide(III) complexes with CN = 6 has been demonstrated. In 1966 Ryan and Jergensen were able to prepare the anionichexahalide complexes of the lanthanides, [MXe] " in non-aqueous (nitriles) solvents [33, 34). Complexes of the type [(C6H5)3PH]3 [MXe] and [C5H5NH]3 [MXe], where M =Pr, Nd, Sm, Eu, Dy, Er, Yb and X- = Cl, Br were obtained as solids. The electronic (/- -/) transitions in these and in hexaiodide, [Mle] -, complexes are extremely weak, except for the magnetic dipole allowed transitions [35). Both electronic and electron transfer spectra (4f- 5d) indicated the presence of octahedral MX species. 

The structure of a genuine six-coordinate lanthanide complex namely, Er(NCS)6 in C4H903NEr(NCS)6 was determined by Martin and coworkers [28]. Earlier spectroscopic studies predicted the octahedral disposition of this complex [29]. The geometrical... 

Empirical correlation of intensities of absorption bands with the structure of complexes in solutions have been made for lanthanide complexes. It has been recognized that forced electric-dipole transitions of low intensities, in some cases lower in intensity than those of magnetic-dipole transitions, may indicate that the ligand field has point group symmetry with a center of inversion. This criterion has been used in the determination of the ligand field by symmetry of Eu3+ aquo ion [202], The absorption band intensity ratios have been used to show the octahedral structure [49] of lanthanide hexahalide complexes, LnXg. ... 

In contrast to the situation with the 3d transition metals in particular, the 4f-4f transitions in the electronic spectra of lanthanide complexes rarely serve any diagnostic purpose. It may be noted, however, that the spectra of the octahedral [LnXe] " ions (X = Cl, Br) have particularly small extinction coefficients, an order of magnitude lower than the corresponding aqua ions, due to the high symmetry of the environment. 

Among other coordination spheres which may be chiral, there is the trivial case of the cA-hw-bidentate octahedral systems such as ciy-[Co(en)2Cl2], 6is-bidentate tetrahedral [M(A-A)2] or Z A-tridentate octahedral [M(A-A-A)2] complexes where the dihedral angle between the two chelate planes is different ih>m 90°, as is often the case. An example, together with the tns-tridentate [M(A-A-A)3] tricapped trigonal prism geometry often found for lanthanide complexes, is shown in Figure... 

Such terms are equivalent to those obtained by Satten et al. (1983) in their study of the excitations of the T,u vibrational mode of the octahedral complex UCI5 at 260 cm L They were aware that their approach would work in a similar way for lanthanide complexes their choice of an actinide was partly determined by their familiarity with the UClj complex. Another important consideration was the high symmetry of the uranium site, which greatly reduces the number of intensity parameters required in the analysis. To assess the situation we note first that the possible pairs (k,t) in expression (94) are (1,2), (3,2), (3,4), (5,4), (5,6) and (7,6). The number of Ti representations of 0(, that occur in the representations of 0(3) are 1, 1,2 and 2 for /c = 1, 3, 5 and 7, so we expect 9 parameters in all. Satten et al. (1983) reduced that number to 6 by carrying out a complete closure over all virtual electronic states, which has the effect of fixing the ratio of the two operators (94) for which t = /c 1. They were able to account quite well for the relative intensities of some 20 or so transitions of the type Aig( H4)-> T, ( Lj), where is a... 

Faulkner TR, Richardson PS (1978) Vibronic coupling model for the intensities of f-f transitions in octahedral lanthanide (111) complexes. Mol Phys 35 1141-1161... 

Transferrin and lactoferrin belong to a family of Fe cation buffer and carrier proteins. X-ray crystallography has shown that both proteins include two distorted octahedral Fe + sites, each of which is coordinated with one aspartate, one histidine, and two tyrosinate residues as well as one exogenous carbonate. Luminescent lanthanides such as Tb + and Eu + cations can be incorporated along with Ee cations into these binding sites to provide water-soluble luminescent lanthanide complexes in sophisticated three-dimensional cages. 

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