Octacoordination

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. 

Simple hexahydrated chlorides of Nd, Sm, Eu, Gd, Er and Pu are found to be isostructural, with octacoordinated [M(OH2)6Cl2]+ units. Complete structural analysis for monoclinic (P2/ ) [Eu(OH2)6Cl2]Cl and [Gd(OH2)6Cl2]Cl are now available 106, 107). Bel skii and Struchkov 106) proposed a highly distorted... 

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. 18. The octacoordinated polyhedron for [M(acac)3(OH2)a] unit showing the chelation. The water molecules are shaded...

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...

It has been earlier mentioned that the octacoordinated Pr(III) ion in triclinic NH4[Pr(TTA)4] -HaO crystal (space group Z = 2) possesses essentially... 

Fig. 26. Dimensions of the irregular bicapped trigonal prism around octacoordinated So(III) in [HSc(trop)4]2. The vertical dotted line indicates the approximate 4 axis to illustrate the distortion of this polyhedron towards a dodecahedron...

The lanthanide sulphides show pol5unorphism and contain mixed coordination polyhedra and coordination numbers. The sulphides of La—Dy M2S3) form orthorhombic (A-t5q)e) crystals, where there are two kinds of nonequivalent cations (229, 230), one is heptacoordinated (monocapped triangular prism), and the other is octacoordinated (bicapped trigonal prism). Monoclinic (D-type) sulphides are formed by Dy—Tm and Y (229, 231) and contain both hexacoordinated and heptacoordinated lanthanide ions. The rhombohedral (E-type) sulphides of Yb and Lu contain only hexacoordinated metal ions (229). 

Site 1 of proteinase K (2PRK) is located in the same region as site 2 of the subtilisins, but has different ligands again (Fig. 2g and h). It is one of the few octacoordinate protein Ca -binding sites observed to date, with... 

---