Hexagonal bipyramidal complexes

Whereas the coordination chemistry of thorium is concerned with only the +4 oxidation state, that of uranium covers oxidation states -1-3 to -1-6. For U(VI), the linear [U02] unit is generally present within the complex and fra -octahedral, pentagonal bipyramidal and hexagonal bipyramidal complexes are usual. For other oxidation states, the coordination polyhedron is essentially determined by the spatial requirements of the ligands rather than electronic factors, and the large size of the U centre allows high coordination numbers to be attained. Complexes involving different oxidation states and coordination numbers include ... 

The majority of U(V1) coordination chemistry has been explored with the trans-ddo s.o uranyl cation, UO " 2- The simplest complexes are ammonia adducts, of importance because of the ease of their synthesis and their versatihty as starting materials for other complexes. In addition to ammonia, many of the ligand types mentioned ia the iatroduction have been complexed with U(V1) and usually have coordination numbers of either 6 or 8. As a result of these coordination environments a majority of the complexes have an octahedral or hexagonal bipyramidal coordination environment. Examples iuclude U02X2L (X = hahde, OR, NO3, RCO2, L = NH3, primary, secondary, and tertiary amines, py n = 2-4), U02(N03)2L (L = en, diamiaobenzene n = 1, 2). The use of thiocyanates has lead to the isolation of typically 6 or 8 coordinate neutral and anionic species, ie, [U02(NCS)J j)/H20 (x = 2-5). 

Among the known structures, the hexagonal bipyramidal (Has) geometry seems to be less common than others and it has only been definitely established for a few uranyl complexes (94—97). Probably an axial colHnear grouping, as in 0—U—0, is necessary for obtaining a geometry for the complexes. None of the octa-coordinated lanthanide complexes has a Hea geometry. 

Chiu prepared monodisperse crystalline particles of metal sulfides, such as lead sulfide (PbS cubes 100 A) (I), cupric sulfide (CuS hexagonal bipyramids 200 A) (2), and zinc sulfide (ZnS multifaceted spheres 0.1-0.4 p,m) (3) by introducing hydrogen sulfide gas into dilute acidic solutions of the ethylenediamine tetraacetic acid (EDTA) complexes of the corresponding metal ions (10 4-10-1 mol dm-3) for several minutes at room temperature. 

A selection of the known dioxouranium(VI) complexes with this type of ligand is given in Table 84. These compounds are precipitated from aqueous solution, usually after the addition of alkali. The coordination geometry of the anion in the ammonium salt of the cupferron complex, NH4[U02 (0N)N(Ph)0 3], is close to hexagonal bipyramidal.332... 

HgCl2 forms a 1 1 complex with 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) with hexagonal bipyramidal coordinated mercury.577 The chlorine atoms are strongly coordinated in the axial positions and the 18-crown-6 in the equatorial position (Hg—Cl = 231.4(1), Hg—0 = 282.5(4) pm).577... 

Structural chemistry of lanthanide complexes dealing with low coordination numbers, 6-, 7- and 8-coordination, dodecahedra, square antiprisms, hexagonal bipyramids, cubes and other structures, 9-coordination, high coordination numbers and organometallic structures is discussed in the fifth chapter. 

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