Pentagonal bipyramid coordination structures

The basic structure of the hydrated oxalato complex, NaNpC CyiVSPkO, is made up of infinite chains of composition [Np02C2O4(H2O)]" connected by Na+ ions and H20 molecules in a three-dimensional network. The neptunium atom is at the centre of a pentagonal bipyramid, coordinated to four O atoms from two C204 groups and a fifth from the water molecule in the equatorial plane.17915... 

Five crystal modifications of anhydrous U02(MeC02)2 have been reported. The structure of the dihydrate consists of chains of pentagonal bipyramidal coordination polyhedra in which half of the MeC02 groups link adjacent bipyramids and the remainder are chelated to individual uranium atoms one water molecule is bonded to each uranium atom.2783... 

In the polymeric acetamide complex of the chromate, [U02Cr04(MeC0NH2)2]B> the structure consists of chains of (U02)0S pentagonal bipyramidal coordination polyhedra in which the equatorial plane is made up from three oxygen atoms from [Cr04]2- tetrahedra and two from the acetamide molecules.300 Similarly, in the dimeric DMA complex with the acetate, [U02(MeC02)2(DMA)]2, the structure consists of two pentagonal bipyramidal polyhedra... 

Figure 12-6 Structure of Fe(DTPA)2- found in the salt Na2[Fe(DTPA)] - 2H20. The seven-coordinate pentagonal bipyramidal coordination environment of the iron atom features three N and two O ligands in the equatorial plane (dashed lines) and two axial O ligands. The axial Fe-O bond lengths are 11 to 19 pm shorter than those of the more crowded equatorial Fe-O bonds. One carboxyl group of the ligand is uncoordinated. [D. C. Flnnen, A. A. Pinkerton, W. R. Dunham, R. H. Sands, and M. O. Funk. Jr..

In the X-ray structure of 82-HgCl2 complex, Hgz+ ion was surrounded by only the five donor oxygens in a pentagonal bipyramidal coordination and the two sulfurs in the crown ring were not coordinated on the other hand, Hgz+ ion was bound by four oxygen and one sulfur atoms in 81-HgCl2 complex. 

Addition of aqueous H2O2 to aqueous HF solutions of (NH4)2W04 adjusted to PH = 3-4 using NH3 yielded (NH4)3F[W0(02)F4] [319]. The crystal structure contains the W0(02)F42 anion with pentagonal-bipyramidal coordination in which the peroxide group bonded side-on in the equatorial plane and the tungsten atom is displaced out of the equatorial plane towards the apical doubly bonded oxygen atom. 

The apparent contradiction between the empirical stoichiometry and the spectral characteristics of these new uranyl complexes was finally resolved by X-ray crystallography. Specifically, a single crystal X-ray structural analysis of the blue-black material formed from the reaction of the anhydrous uranyl chloride and o-dicyanobenzene [112] (Figures 22 and 23) revealed that the complex obtained was in fact an expanded five-subunit superphthalocyanine macrocycle in which a pentagonal bipyramidal coordination geometry pertains about the centrally-bound uranium atom. 

Finally, consider the crystal structure of the rubidium salt of 5,5-dimethyl-l,3-cyclohexanedione-2-sulfonic acid. This species exists as the ketoenol wherein the rubidium ion is surrounded by seven oxygen atoms in pentagonal bipyramidal coordination. We know of no ketoenolate (and hence dianionic) salt of this sulfo-dione with any cation. Perhaps some more highly charged cation would allow for this—note the significantly enhanced stability of rare earth complexes of 2-acetyl-5,5-dimethyl-l,3-cyclohexanedione over the parent diketone . 

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