Coordination number nine

The lanthanum complex (Scheme 9) has the coordination number nine with two NO3 ions bidentately bound in the solid state, while in solution only one nitrate group is coordinated. H2dapsox is symmetrically pentadentately coordinated in the a-oxyazine form (Scheme 9). 

In addition to these kinetic studies, there has been a considerable amount of work on the thermodynamic parameters associated with this type of reaction. Thus the interaction of [M(edta)aq] with anions of 8-hydroxyquinoline-5-sulfonic acid (oxs ), ida2- and nitrilotriace-tic acid (nta3-) was thoroughly studied for M = Y, La-Lu (except Pm) by calorimetry and pH titration,419 and AG, AH, AS and K found. Some of these values are given in Table 5. It was, however, concluded from the variation of these parameters with atomic number that all M3+ in aqueous solution have the same coordination number (nine) but that the coordination number of the [M(edta)aq] species changes between Sm3+ and Tb. ... 

Coordination Number Nine. There is only one symmetrical arrangement that occurs with regularity for this coordination number. It is derived from a trigonal prism by placing the three additional atoms outside the centers of the three vertical faces, as shown in Fig. 1-11 it retains the D3h symmetry of the trigonal prism. Among the compounds in which this arrangement... 

Preston (1985) described the solvent extraction behavior of a large number of metal cations including rare earth nitrates in solutions of Versatic 10 (2-ethyl-2-methylheptanoic acid), naphthenic, 2-bromodecanoic and 3,5-diisopropylsalicylic acids in xylene. The last two acids extract metal cations under more acidic conditions, pH 1-2. For Versatic 10 the order of extraction of yttrium and lanthanides is La < Ce < Nd < Gd < Y < Ho < Yb and for naphthenic acids it is La < Ce < Y < Nd < Gd k Ho Yb. The lanthanides tend to form complexes of predominantly ionic nature. In the case of Versatic 10, the stability of the complexes increases uniformly with atomic number due to the increase in electrostatic energy as a result of the decrease in ionic radius. The primary branched naphthenic acid allows the formation of complexes with high coordination number, nine for La to Nd, eight and eventually six as the metal ionic radius decreases. In general, the extraction of a metal ion by a carboxylic acid H2A2 can be represented by the reaction... 

The complexes formed by oxalic and malonic acids have been carefully studied and the detailed structural features have been obtained from X-ray diffraction measurements (Hansson, I973e). The oxalate ions in these complexes serve as bridging ligands and the larger metal ions have coordination number nine whereas the smaller ions have coordination number eight. The malonate ions in the rare earth malonates are of two different types, those that are involved in six-membered chelate ring formation and those which are not. 

With diethylenetriamine two types of complexes are formed with nitrate as the anion (Forsberg and Walthen, 1971) R(dien)3(N03)3 (R=La-Gd) and R(dien)2(N03)3 (R = La-Yb, Y). The tris complexes contain only ionic nitrate whereas the bis complexes contain both ionic and coordinated nitrate groups. Coordination number nine was thus assigned to the tris complexes but the coordination number in the bis complexes remains undetermined due to the impossibility of determining the relative numbers of ionic and coordinated nitrate groups solely by means of the infrared spectra. 

Fig. 1. Common geometries for metal coordination numbers from two through nine (see Table 1), where O represents the metal and O, a ligand donor...

Figure 2. Dy(P30io)2 is a lanthanide shift reagent commonly used in biological 7Li NMR experiments. The Dy3+ ion has a coordination number of nine with two P3O10 moieties, acting as tetradentate ligands, and one molecule of H2O coordinated in the first coordination sphere up to seven Li+ ions can bind in the second coordination sphere.

The coordination numbers of the Ln3+ ions in water are now well established from different experimental techniques (214-221). The lighter La3+-Nd3+ ions are predominantly nine-coordinate, Pm3+ Eu3+ exist in equilibria between nine- and eight-coordinate states and the heavier Gd3+-Lu3+ are predominantly eight-coordinate. The change in coordination number is also reflected in the absolute partial molar volumes, U°bs, of several Ln3+ ions determined in aqueous solutions (222,223). 

Probably, the first series of lanthanide complexes with neutral oxygen donor ligands is that of AP with the lanthanide nitrates. In 1913, Kolb (79) reported tris-AP complexes with lighter lanthanide nitrates and tetrakis-AP complexes with heavier lanthanide nitrates. Subsequently, complexes of lanthanide nitrates with AP which have a L M of 6 1 and 3 1 have also been prepared (80-82). Bhandary et al. (83) have recently shown through an X-ray crystal and molecular structure study of Nd(AP)3(N03)3 that all the nitrates are bidentate and hence the coordination number for Nd(III) is nine in this complex. Complexes of AP with lanthanide perchlorates (81, 84), iodides (81, 85), and isothiocyanates (66, 86, 87) are known. While the perchlorates and iodides in the respective complexes remain ionic, two of the isothiocyanates are coordinated in the corresponding complexes of AP with lanthanide isothiocyanates. 

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