Lanthanum coordination number

Recently, West and coworkers (275) prepared the complex of SONO with lanthanum perchlorate which has a L M of 4 1. Lanthanum in this complex attains a coordination number of eight due to the coordination of both S-0 and N—0 groups to the metal ion (Structure VI). 

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). 

The complex anions [M(edta)(H20)3] where M = Pr, Sm, Gd,410 Tb411 and Dy412 all have similar coordination to the lanthanum complex, but for Er and Yb, [M(edta)(H20)2] , which are eight-coordinated, are obtained.412,413 The change in coordination number occurs at some point in the Ho area. Of course, it is by no means certain that a change, if there is one, occurs at exactly the same position in the series in solution. 

FIGURE 25 Projection of the La4NijPb4 structure onto theab plane. Lanthanum, nickel, and lead atoms are drawn as medium gray, black filled, and open circles, respectively. The lead chains and the triangular nickel units are emphasized. The bottom drawing shows the polyhedra of coordination number 12 for the La2 (gray shading) and Pb2 atoms. 

LaCrC>3 is one of the family of lanthanide perovskites RTO3, where R is a lanthanide and T is a period 4 transition element. In the cubic unit cell R occupies the cube corners, T the cube centre and O the face-centre positions. The coordination numbers of T and R are 6 and 8 respectively. LaCrC>3 loses chromium at high temperatures, leaving an excess of O2- ions. The excess charge is neutralized by the formation of Cr4+ which results in p-type semiconductivity with hole hopping via the localized 3d states of the Cr3+ and Cr4+ ions. The concentration of Cr4+ can be enhanced by the substitution of strontium for lanthanum. A 1 mol.% addition of SrO causes the conductivity to increase by a factor of approximately 10 (see Section 2.6.2). 

The electronic structure and chemical bonding in the 15-crown-5 complex of lanthanum trichloride having a coordination number of 8 has been studied. The molecular structure of LaCl3(15-crown-5) and the charge distribution of the complex, ligand and metal chloride are shown in Figs 5.23 and 5.24, respectively. 

In the orthorhombic perovskite-like compounds, LaFe03, the standard deviation of the average cation-oxygen bond distances has been used as a criterion for obtaining the coordination number [158]. For lanthanides Sm-Lu, there are eight oxygen atoms in the coordination sphere, and for lanthanum the coordination number is nine in the compound. 

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... 

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 the presence of 15-crown-5, reaction of lanthanum nitrate with bipy in MeOH-MeCN led to [La(bipy)(N03)3(H20)2(Me0H)] 15-crown-5, which has 11-coordinate lanthanum. The increase in coordination number in comparison with [La(bipy)2(N03)3] results in an slight (and possibly not statistically significant) increase in La—N distance from 2.66 A to 2.70 A, and in La—O from 2.56-2.63 A to 2.69 A. The presence of the coordinated MeOH molecule supplies a fifth hydrogen atom so that hydrogen bonds can be formed to all the crown ether oxygens. In another synthesis in the presence of a crown ether, a 10-coordinate complex, [La(bipy) (N03)3(H20)2] benzo-15-crown-5, is obtained. ... 

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