Eight coordination

Eight coordination frequently occurs in lanthanide and actinide fluoride complexes. The preponderance of these octacoordinate compounds can be assigned to one of two idealized coordination polyhedra, namely the square antiprism, symmetry (D4, ), or the dodecahedron (Dg ) with triangular faces. Of the remaining idealized eight-coordinate polyhedra, some are not represented and others have but few representatives in this class of compounds. 

The antiprism can be characterized by two parameters, i.e. js, the ratio of the length of the side of a triangular face to that of the edge of the square face, and 0, the angle between the 8 axis and the vector from 

Three parameters are necessary to characterize the dodecahedron with triangular faces (see Fig. 6b). Hoard and Silverton chose for these parameters 0a, the angle between the M—A bond vector and the 4 axis of the polyhedron 0b, the corresponding angle made with the M—B bond vector and the ratio of the bond lengths, M—A vector/M—B vector. The most favorable values for these parameters are 0a = 35.2°, 0B == 73.5° and M—A/M—B = 1.03. The corresponding values for the hard sphere model are M—A = M—B, 0a = 36.9° and 0b = 69.5°. 

The dodecahedron requires a larger radius ratio than the antiprism (i.e., 0.665 versus 0.645), and it is tempting to say that it is favored over the antiprism as a coordination type for the larger (with respect to the d-transition series) actinide elements. The lack of a sufficiently large number of well-determined structures makes such a suggestion tenuous at this time. However, there is some support for this thesis among the one dimensional polymers, as explained in the following section. 

The eight-fold coordination type with the ligands situated at the corners of a cube maximizes non-bonded repulsions (20, 58, 59,64), and it, therefore, is not an appealing coordination t 7pe. The cube is, indeed, unknown as a structure type among the molecular coordination compounds of the d-transition elements although it is prevalent among inonic compounds with the CaF2 and CsCl lattice types. 

---

Sohd uranium—phosphate complexes have been reported for the IV and VI oxidation states, as well as for compounds containing mixed oxidation states of U(IV) and U(VI). Only a few sohd state stmctures of U(IV) phosphates have been reported, including the metaphosphate U(P03)4, the pyrophosphate U(P202), and the orthophosphate, CaU(PO4)2. The crystal stmcture of orthorhombic CaU(POis similar to anhydrite (194). Compounds of the general formula MU2(PO4)3 have been reported for M = Li, Na, and K, but could not be obtained with the larger Rb and Cs ions (195). In the sohd state, uranium(IV) forms the triclinic metaphosphate, U(P03)4. Each uranium atom is eight-coordinate with square antiprismatic UOg units bridged by... 

Uranium pentafluoride [13775-07-0], UF, has been isolated under different conditions, leading to two different modifications, d and p. The former is a grayish white soHd, which is synthesized from the interaction of UF [7783-81-5] and HBr or by heating UF [10049-14-6] and UF to 80—100°C. The yellowish white P-modification is also obtained by reacting UF and UF, but at higher temperatures (150—200°C). The two different modifications of UF have both been stmcturaHy characterized. The a-form consists of infinite chains of octahedral UF units. The P-form has eight-coordinate uranium atoms with the fluorides in a geometry between dodecahedral and square antiprismatic. 

Bismuth Trifluoride. Bismuth(III) duoride is a white to grey-white powder, density 8.3 g/mL, that is essentially isomorphous with orthorhombic YF, requiring nine-coordination about the bismuth (11). It has been suggested that BiF is best considered an eight-coordinate stmcture with the deviation from the YF stmcture resulting from stereochemical activity of the bismuth lone-pair electrons. In accord with its stmcture, the compound is the most ionic of the bismuth haUdes. It is almost insoluble in water (5.03 0.05 x 10 M at pH 1.15) and dissolves only to the extent of 0.010 g per 100 g of anhydrous HF at 12.4°C. 

For halides the cation should have a charge of 2+ rather than 4+ for tetrahedral coordination. The only fluoride compound capable of containing two-coordinate F and four-coordinate cations is Bep2. For ZrF, the radius ratio rule predicts that Zr" " is eight-coordinate if all fluorine atoms are two - c o o rdinate. 

Eight coordination chemistry. S. J. Lippard, Prog. Inorg. Chem., 1967,8,109-193 (3811. 

Spot tests, 1, 552 Square antiprisms dodecahedra, cubes and, 1, 84 eight-coordinate compounds, 1,83 repulsion energy coefficients, 1, 33, 34 Square planar complexes, 1,191, 204 structure, 1, 37 Square pyramids five-coordinate compounds, 1,39 repulsion energy coefficients. 1,34 Squares... 

Indelli and Guaraldi propose the acid-independent path to involve eight-coordinate iron (six CN plus two I ligands) and the acid-catalysed path to involve seven-coordinate iron, viz. 

TiCRjtftc) was prepared starting from Ti(NR2)4 4,5) or TiCl4 (6). The structure of the ethyl compound was elucidated by X-ray studies (7, S). The titanium atom is eight-coordinated by the sulfur atoms of the four chelating ligands and the coordination geometry closely approximates to that of a dodecahedron. 

Nb(Et2C c)4X is a 1 1 electrolyte in which the metal is probably eight-coordinated by sulfur atoms from four bidentate dithiocarbamate groups. Infrared data suggest that the ligands are unsymmetrically bonded. 

The third category is the heavy eight-coordinate trivalent lanthanides, whose lability decreases with the progressive filling of the 4f orbitals and the resulting lanthanide contraction, and which are very labile as a consequence of their large rM (7,10,11). 

The 15 trivalent lanthanide, or/ -block, ions La3+, Ce3+, Pr3+, Nd3+, Pm3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, and Lu3+, which may be collectively denoted Ln3+, represent the most extended series of chemically similar metal ions. The progressive filling of the 4/orbitals from La3 + to Lu3 + is accompanied by a smooth decrease in rM with increase in atomic number as a consequence of the increasingly strong nuclear attraction for the electrons in the diffuse / orbitals (the lanthanide contraction). Thus, the nine-coordinate rM decrease from 121.6 to 103.2 pm from La3+ to Lu3+, and the eight-coordinate ionic radii decrease from 116.0 to 97.7 pm from La3+ to Lu3+ (2). Ligand field effects are small by comparison with those observed for the first-... 

---