Complexes octacoordinated

The complex Tb(TTFA) (o-phen) with TTFA = thenoyltrifluoroaceto-nate is a octacoordinate rare earth chelate which contains one o-phen and three TTFA ligands (45-46). The latter are related to acetylacetonate. 

The last vanadium derivative that we will consider is the N206 octacoordinated complex called Amavadin. It consists of a vanadium(IV) ion and two trideprotonated molecules of A -hydroxyl-2,2/-iminodipro-pionic acid (H3hidpa), [V(hidpa)2]2-. 

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. 

Other techniques, such as C.D. spectral change, have been used to demonstrate the presence of octa coordination for lanthanide ion in a system containing Eu(FOD)3 and alcohols or ketones (28). However, the anionic tetrakis complexes e.g. Eu(acac)i, Eu(benzac)i, Eu(DBM)i, Eu(BTFA)4, tend to dissociate into the tris-complex and L in alcoholic solution. The degree of dissociation depends on the complex as well as the polarity of the medium. In alcohol-DMF medium the dissociation is enhanced compared to the alcoholic solutions (29). The end product of these dissociation reaction may well be an octacoordinated species. Fluorescence emission from the coordinated europium ion was also helpful in estabhshing the nature of the species in solution 29). 

Octacoordination is now recognized to be common for many lanthanide and actinide complexes. Among the < -transition metal ions, however octacoordination is less frequent. The d, d, d and d configurations are known to produce eight coordinated complexes (97). In an excellent review Lvppard (92) surveyed the chemistry of the octa coordinated complexes, especially stereochemical and bonding aspects, and included most of the published material upto 1967. 

Among the chelated species, octacoordination is often encountered. The tetrakis-acetylacetonate complex of U(IV) (a-form) and Ce(IV) are isostructural. A two-dimensional X-ray analysis showed a slightly distorted square antiprismatic geometry [109—111) for Ce(acac)4 belonging to space group P2i/c [Ctn] with an average Ce—O (acac) bond length of 2.40 A and an <0—Ce—0 of 72°. The a-form of Th(acac)4 is found to be isomorphous (770) with Ce(acac)4 (Th—0=2.41 A)... 

Fig. 19. The octacoordinated polyhedron of M0eN2 in [Eu(DPM)3(Py)2) and [Ho(DPM)8 (4-Pic) 2] showing the mode of chelation of the DPM ligand. The bond distances for the Eu(III)-complex are given...

Fig. 21. The octacoordinated polyhedron of diaquo-tris-nicotinic acid Ho (III) complex in hexathiocyanatochromate salt. The dimensions and angles of the polyhedron are shown. The water molecules are shaded...

Formal octacoordination ( 4 + 4 )229 in a silicon complex was recently demonstrated by Corriu s group, who prepared 201 (equation 53). The Si-N distances (Table 27) are long relative to those common in penta- and hexacoordinate compounds, but are in the same range as those found in the heptacoordinate compounds discussed here. Also, the geometry resembles the heptacoordinate complexes, in that it is basically a tetrahedron with dimethylamino donor groups pointing toward the center, in what may be termed a tetracapped tetrahedron . 

TABLE 27. X-ray crystallographic and 29Si NMR data for hepta- and octacoordinate silicon complexes and tetravalent model compounds... 

Unlike Si and Ge acetylacetonates (acac), in which the central M atom is hexacoordinated, the coordination number of the Sn atom in RSn(acac)3 is 7173. Hexacoordinated Si, Ge, Sn, Pb complexes with tropolone (Tp) are known. Tp4M molecules, where the metal atom is octacoordinated for M = Sn, Pb or hexacoordinated for M = Si, Ge, are of particular interest. The latter compounds correspond to the structure Tp3M+Tp- and exist as salts or ion pairs174. 

The shapes and intensities of spectral bands were used in complexation studies of lanthanide nitrates in organic solvents. The degree of complexation of nitrate was in the order water > methanol > acetonitrile. The tendency of the nitrate anion and the solvent molecules to compete for coordination followed the order DMF > TBP > NOJ > H2O > C2H5OH > dioxane. The 4f-4f spectral band of Nd(C104>3 in DMF, DMSO and DMSO-H2O mixtures split into two components whose relative intensities were attributed to different amounts of nona- and octacoordinated Nd(III) solvated ion. The major species are [Nd(DMF)9]3+/[Nd(DMSO)9]3+ and the minor species are [Nd(DMF)8]3+/[Nd(DMSO>9]3+. Addition of water to the equilibrium mixture resulted in a shift of the equilibrium to predominantly nonacoordinated species due to substitution of DMF/DMSO by water along with a red shift of the spectral band [224,225]. 

Absorption spectra of lanthanides in the 4f-4f region were used in the studies on complexation by haloacetates in DMF, DMSO, methanol and in mixtures of the solvents. Octacoordination in the solvents was inferred. The band intensities in pure solvents were higher than in solvent mixtures. Haloacetate appears to form an inner sphere complex in organic solvents and an outer sphere complex in aqueous media [227-231],... 

Absorption spectra of Pr(M) and Nd(III) complexes with alkyl, aryl, fluorine substituted P-diketones and the associated hypersensitive transitions were studied in detail. Mono, bis, tris and tetrakis complexes in benzene, DMF, DMSO and ethanol were also studied. Spectra both in solids and solution permitted the interpretation of the state of p-diketones based on the trends in the intensity and energy interaction parameters [238,244]. The various complexes were isolated and their IR, 1H NMR, stability constants were studied as well. Octacoordination in the tetrakis complex and tris complex associated with two solvent molecules was noted. 

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