Hydroxide bridges, double, structure

Figure 1. The crystal structure of Zr2(0H)2(SO4)3(H2O)4> reprinted with permission from Ref. 5, copyright 1966, American Chemical Society. Zirconium atoms are shown as solid circles, oxygen atoms as open circles. The Pu compound is isomorphous, Zr being replaced by Pu. la shows the manner in which the bridging sulfates link Pu atoms to form layers, lb shows the manner in which layers are linked through the double hydroxide bridges.

M(0H)2SOi, H2O where M=Zr (8), Hf (12) also have been determined and reveal the presence of almost planar zigzag chains of metal atoms joined by double hydroxide bridges. The single exception to this trend toward formation of double hydroxy-bridged metal dimers or chains is the compound which is best described as CeOSOif,H20 (17). However, even in this structure the cerium ions form chains which are linked by bridging oxide ions. 

The only crystalline phase which has been isolated has the formula Pu2(OH)2(SO )3(HaO). The appearance of this phase is quite remarkable because under similar conditions the other actinides which have been examined form phases of different composition (M(OH)2SOit, M=Th,U,Np). Thus, plutonium apparently lies at that point in the actinide series where the actinide contraction influences the chemistry such that elements in identical oxidation states will behave differently. The chemistry of plutonium in this system resembles that of zirconium and hafnium more than that of the lighter tetravalent actinides. Structural studies do reveal a common feature among the various hydroxysulfate compounds, however, i.e., the existence of double hydroxide bridges between metal atoms. This structural feature persists from zirconium through plutonium for compounds of stoichiometry M(OH)2SOit to M2 (OH) 2 (S0O 3 (H20) i,. Spectroscopic studies show similarities between Pu2 (OH) 2 (SOO 3 (H20) i, and the Pu(IV) polymer and suggest that common structural features may be present. 

The crystal structure of Zr(0H)2(N03)2(H20)4 has been determined (338). Chemically the structure may be considered as consisting of chains of [Zr(0H)2(N03)(0H2)2 ]M, which are joined by nitrate groups through hydrogen bonds. The zirconium atoms are joined by double bridges of hydroxide ions. The coordination number of the zirconium is 8 and the coordination polyhedron is a dodecahedron with triangular faces. There are two formula units per unit cell in the triclinic structure which has a =7.408 A, 6= 11.227 A, c = 6.741 A, with a = 60.91°, j3= 99.78°, and y= 99.78°. 

Oxygen forms binary compounds with nearly all elements. Most may be obtained by direct reaction, although other methods (such as the thermal decomposition of carbonates or hydroxides) are sometimes more convenient (see Topic B6). Oxides may be broadly classified as molecular, polymeric or ionic (see Topics B1 and B2). Covalent oxides are formed with nonmetals, and may contain terminal (E=0) or bridging (E-O-E) oxygen. Especially strong double bonds are formed with C, N and S. Bridging is more common with heavier elements and leads to the formation of many polymeric structures such as Si02 (see Topics FT and F4). 

When the pH of the solution is raised, the first step is the loss of one H to give the unit AlOH (H2O)This ion has the same octahedral structure as that in Figure 1, but the distance between the aluminum and the hydroxide ion is slightly less than the AI-H2O distance. This monomeric complex ion has been reported by many workers. In our experiments, and in the work of others, however, these monomeric ions showed a strong tendency to polymerize into larger units. The basis of the polymerization process is the formation of a double OH bridge between adjacent aluminum ions (8,16). 

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