Uranium atom, coordination number

The stereochemistry of uranium alkoxides is very diverse, and it is possible to conclude that the geometry of the metal atom is independent on its oxidation state and the coordination numbers are in general lower than those observed for the inorganic compounds. In the series of monomeric U(IV) molecules — e.g., [U(OC6H3Bu 2-2,6)4] [1669], [UI2(OC6H3Bu 2-2,6)2THF] [79],... 

For the order of complexing ability for ligands DMSO > py > phen > bipy (30) is consistent with that suggested above. Because of their low complexing ability with this group, bipyridyl and phenanthro-line yield only protonated cations in acidic solutions containing UO " (514, 516) but in aqueous or alcoholic solution, chelate complexes may be formed in which the uranium atom can exhibit a coordination number of either 6 or 7 (496, 514-516, 601, 635). 

Depending upon nature and numbers of the Li ligands, these complexes can be either electroneutral (e.g. [U02S04(H20)2]) or positively charged (e.g. [U02(H20)5] ). In most coordination compounds, there are well-defined uranyl ions. However, in some compounds it is impossible to clearly identify uranyl ions. For instance, in the structure of hexakis(neopentoxy)uranium(VI) XEBVOP, uranium atoms have sixfold coordination consisting of six equivalent bonds with r(U-O) = 2.00 A. 

The stmcture of UP04C1(H20)2 is also adopted by its Br analog [33]. From the Wyckoff positions of the atoms, the actinide site in the stmcture appears to contain UO6X2 (X = Cl, Br) polyhedra that approximate a Johnson snub disphenoid, also known as a Siamese dodecahedron, but as the halogen site is half-occupied, the actual coordination number is seven [35]. These polyhedra share vertices with phosphate tetrahedra to form an open framework with voids that extend parallel to the [100] direction. The halogen atoms that coordinate uranium line these voids (Fig. 5). 

A), possesses in phenacite a coordination number of 6. For zirconium, a coordination number of 8 is possible because the sp d hybridization is easily accessible. This hybridization will be more easily reached by heavier atoms thorium and uranium, which will replace zirconium atoms in the altered variety of zircon-the so-called malacon. The sp d hybridization imposes a quadratic symmetry. 

The uranium(VI) has achieved a maximum coordination number of 8 with the 6 oxygen atoms from the ligands lying in the equatorial plane. The two TBP ligands are trans, yielding a resultant dipole moment of zero for the molecule... 

Uranium forms tris and tetrakis-cyclopentadienyls CpjU and Cp U. In the latter there are four fy -cyclopentadienyl groups so that the metal atom is formally 12-coordinate. This high coordination number is consistent with essentially ionic (electrostatic) bonding. This may also apply to the complexes MCp (M = Ti, Zr) where the structures apparently depend on the size of the metal atom and, for Cp Zr, the 18-electron rule does not apply. 

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