Coordination environments

Evans and co-workers investigated the effect of a number of -symmetric bis(oxazoline) ligands on the copper(II)-catalysed Diels-Alder reaction of an N-acyloxazolidinone with cyclopentadiene. Enantiomeric excesses of up to 99% have been reported (Scheme 3.4). Evans et al." suggested transition state assembly 3.7, with a square planar coordination environment around the central copper ion. In this scheme the dienophile should be coordinated predominantly in an cisoid fashion in... 

The stmcture of Pmssian Blue and its analogues consists of a three-dimensional polymeric network of Fe —CN—Fe linkages. Single-crystal x-ray and neutron diffraction studies of insoluble Pmssian Blue estabUsh that the stmcture is based on a rock salt-like face-centered cubic (fee) arrangement with Fe centers occupying one type of site and [Fe(CN)3] units randomly occupying three-quarters of the complementary sites (5). The cyanides bridge the two types of sites. The vacant [Fe(CN)3] sites are occupied by some of the water molecules. Other waters are zeoHtic, ie, interstitial, and occupy the centers of octants of the unit cell. The stmcture contains three different iron coordination environments, Fe C, Fe N, and Fe N4(H20), in a 3 1 3 ratio. 

There is Httie known chemistry of tetrakis-Cp thorium complexes. Pseudotetrahedral molecules, (rj-ring) Tb (15), where ring = Cp [1298-75-5] or Ind [11133-17-6], have a measurable dipole, resulting from deviation of symmetry (88). Difference of coordination environments, eg, v] in the indenyl system and in the Cp system, appears to indicate great steric crowding about the thorium center, which probably limits the reactivity and synthetic derivatization of these complexes. 

The known uranium(VI) carbonate soHds have empirical formulas, 1102(003), M2U02(C03)2, and M4U02(C03)3. The soHd of composition 1102(003) is a well-known mineral, mtherfordine, and its stmcture has been determined from crystals of both the natural mineral and synthetic samples. Rutherfordine is a layered soHd in which the local coordination environment of the uranyl ion consists of a hexagonal bipyramidal arrangement of oxygen atoms with the uranyl units perpendicular to the orthorhombic plane. Each uranium atom forms six equatorial bonds with the oxygen atoms of four carbonate ligands, two in a bidentate manner and two in a monodentate manner. 

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). 

Fluorides. Uranium fluorides play an important role in the nuclear fuel cycle as well as in the production of uranium metal. The dark purple UF [13775-06-9] has been prepared by two different methods neither of which neither have been improved. The first involves a direct reaction of UF [10049-14-6] and uranium metal under elevated temperatures, while the second consists of the reduction of UF [10049-14-6] by UH [13598-56-6]. The local coordination environment of uranium in the trifluoride is pentacapped trigonal prismatic with an 11-coordinate uranium atom. The trifluoride is... 

Figure 4.17 Crystal and molecular structure of (LiMe)4 showing (a) the unit cell of lithium methyl, (b) the LijCj skeleton of the tetramer viewed approximately along one of the threefold axes, (c) the 7-coordinate environment of each C atom, and (d) the (4 -I- 3 -I- 3)-coordinate environment of each Li atom. After ref. 93, modified to include Li—H contacts.

Figure 16.13 Structures of some tetrahalides of Se and Te (a) Sep4 (gas), (b) crystalline Sep4, and schematic representation of the association of the pseudo-tbp molecules (see text), (c) coordination environment of Te in crystalline Tep4 and schematic representation of the polymerized square pyramidal units, (d) the tetrameric unit in crystalline (TeCl4)4, and (e) two representations of the tetrameric molecules in Te4li6 showing the shared edges of the Telg octahedral subunits.

Figure 16.18 Some coordination environments of Se and Te in their organohalides.

Metal polysulfido complexes have attracted much interest not only from the viewpoint of fundamental chemistry but also because of their potential for applications. Various types of metal polysulfido complexes have been reported as shown in Fig. 1. The diversity of the structures results from the nature of sulfur atoms which can adopt a variety of coordination environments (mainly two- and three-coordination) and form catenated structures with various chain lengths. On the other hand, transition metal polysulfides have attracted interest as catalysts and intermediates in enzymatic processes and in catalytic reactions of industrial importance such as the desulfurization of oil and coal. In addition, there has been much interest in the use of metal polysulfido complexes as precursors for metal-sulfur clusters. The chemistry of metal polysulfido complexes has been studied extensively, and many reviews have been published [1-10]. 

Destructive techniques have been widely applied to determine the concentration of key elements In cells and other biota, but beside being Incapable of use in vivo, they offer no Information on the chemical nature of the element In question. For example, acid digestion of cells which have accumulated various organotln species, and subsequent traditional analysis by atomic absorption (AA) spectroscopy or element-specific spectrofluorlmetry, will produce quantitative data on the amount of tin present, but will reveal nothing about the coordination environment of the metal on the cell surface prior to destruction. 

From an analysis of the EPR spectra, Blumberg and Peisach (4) suggested that the coordination environment of Rieske clusters must include one or more atoms which are less electron donating than sulfur. This was significantly substantiated by studies of the Rieske... 

Fig. 5.10 (a) Coordination environment of the Cs cation in Cs[(Zr6B)Cl2.2ln.8]- (b) Distorted octahedral halide environment of the Na cation in Na[(Zr6B)Cl3.9lio.i] (a), thermal ellipsoids with 50% probability. 

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