Bipyramidal coordination

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). 

Results from an array of methods, including X-ray absorption, EXAFS, esr and magnetic circular dichroism, suggest that in all ureases the active sites are a pair of Ni" atoms. In at least one urease,these are 350 pm apart and are bridged by a carboxylate group. One nickel is attached to 2 N atoms with a fourth site probably used for binding to urea. The second nickel has a trigonal bipyramidal coordination sphere. 

Figure 31.6 The polymeric strucmre of AnFs (An = Pa, U, Np) and Pads, showing the distorted pentagonal bipyramidal coordination of the metal.

Figure 31.7 (a) The octahedral anion in Cs2[U02CU]. (b) Pentagonal bipyramidal coordination of U in dinuclear [U02(02CMe)2L]2 (L = OPPhs, OAsPhs). (c) Hexagonal bipyramidal coordination of U in uranyl nitrate, U02(N03)2-6H20. 

A template synthesis employing Ni(OAc)2, 2,5-dihydroxy-2,5-dimethyl-1,4-dithiane, and 3,3 -iminobis(propylamine) gave the water-soluble five-coordinate complex [Ni(495)], the crystal structure of which shows trigonal bipyramidal coordination of Ni11 with the central amine and terminal thiolates in plane and the two imino nitrogens in axial positions. Solvatochromism of the complex is interpreted in terms of S" H bonding, which may be of relevance to the catalytic cycle in hydrogenases.1341... 

Figure 80 Trigonal bipyramidal coordination capsule with a chemical formula of C H igNdosPdig, a molecular mass of 7,103 Da, and a dimension of 3 x 2.5 x 2.5 nm.

N40 donor set ligands with pyridyl and phenolic arms form mononuclear complexes with approximately trigonal-bipyramidal coordination geometries.888... 

Similarly, Sn—Sn in (ClSnMe2)2, is short at 277.0 pm and there is relatively weak intermolecular Sn... Cl interaction, giving a double chain and trigonal bipyramidal coordination. Similar geometry is found with MeN(CH2CH2CH2)2 which bonds through two C and N in the Sn—Sn linked dimer. 

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