Twelve-coordinate molecules

As for ten- and eleven-coordination, there are no twelve-coordinate complexes containing only unidentate ligands. The most common twelve-coordinate molecules are those of the type [M(bi-dentate)6]. The relevant stereochemistries are shown in Figure 104, and the repulsion energy coefficients listed below ... 

The Kd values for Cs+ -Na+ exchange are Nuclear Tank Wastes. However, substitution of 25 mol% Nb(V) for Ti(fV) eliminates half the Na+ from the tunnels. The Cs+ in the Nb substituted framework forms a twelve coordinate compound2, with eight framework oxygens and four water molecules. Because of this high coordination number the selectivity for Cs+ is sufficiently enhanced to remove Cs+ from nuclear waste solutions. 

The structure (fig. 34, tables 76, 77) of synthetic Ca4AlSi(S04)Fi3 I2H2O consists of SO4 tetrahedra, (Al,Si)Fg (= MFg) octahedra and seven-coordinated Ca polyhedra (Mathew et al. 1981). The Ca polyhedra are monocapped octahedra coordinated by three water molecules (W), the three F2 atoms of the three different MF octahedra and one FI atom shared by the neighboring Ca atoms. The SO4 tetrahedra lie in the cavities of the three-dimensional framework of the MFg octahedra and Ca polyhedra, and are linked to the framework by hydrogen bonds between the twelve water molecules and the four oxygen atoms of the SO4 tetrahedra. Mathew et al. (1981) stated that this appears to be the first example of hydration number twelve observed for the SO4 ion in the crystalline state. Another unusual feature of the structure is that all four faces of the SO4 ion are linked to faces of four different calcium polyhedra via hydrogen bonds. They pointed out that any variation in the Al/Si ratio from one would be restricted by the FI content unless other modes of... 

In a crystal such as FeS04 7H20, six of the water molecules are attached to the iron ion, in the complex Fe(OH2)6 . and the seventh occupies another position, being packed near a sulfate ion of the crystal. In alum, KAl(804)2 I2H2O, six of the twelve water molecules are coordinated about the aluminum ion and the other six about the potassium ion. 

X-ray crystal structures of glutamine synthetase from both Salmonella typhimuriuni and Mycobacterium tuberculosis are very similar. Structures of wild type enzymes and of active site mutants have been determined. All structures have been solved with Mn in the active site. There are twelve identical subunits arranged in two face-to-face symmetrical hexamers. The active sites are in funnel-shaped open-ended cavities located between adjacent subunits of the hexamer. These cavities are 45 A long, 30 A wide at the outer end, and 10 A wide at the inner end and the active site with the two Mn " ions is approximately halfway down the cavity. The metal-metal distance is 5.8 A. The more tightly bound Mn is coordinated to the side chains of Glu-131, Glu-212, Glu-220, and two water molecules, one of which is shared by both metal ions. Glu-129, Glu-357, His-269, and two additional water molecules are bound to the Mn + at the lower affinity site. A schematic view of the active site metal coordination is shown in Figure 36. 

For +2 cations such as zinc(ll) and cadmiunXIl) each metallothionem molecule contains up to seven metal atoms. X-ray studies indicate that the metal atoms are in approximately tetrahedral sites bound to the cysteine sulfur atoms. The soft mer-cury(JI) ion has a higher affinity for sulfur and will displace cadmium from mefallothio-nein. At first the mercury ions occupy tetrahedral sites but as the number increases, the geometries of the metal sites and protein change until about nine Hg(II) atoms are bound in a linear (S—Hg—S) fashion.92 Up to twelve + I cations such as copper(l) find silverfUcan bind per molecule, indicating fi coordination number lower than four, probably three (see Problem 12.34). 

For many solutions, it is not possible to vary the composition of the components over the whole range of mole fractions. This is obviously true of solutions made up of a solid and a liquid. For these systems it is better to choose a standard state which is based on the properties of a dilute solution. This leads to the definition of an ideally dilute solution. Such a system is easily defined on a molecular basis as one in which the solute molecule only comes in contact with solvent molecules, and never with another solute molecule. In the previous discussion of regular solutions it was concluded that, when the two components are of equal size, the coordination number for the other molecules around a central one is twelve. This suggests that an ideally dilute solution must have a solute mole fraction which is less than 1/13, that is, 0.08. 

A coordination compound or complex refers to the grouping that is formed when a metal ion or atom accepts a pair of electrons from a molecule or ion. Metal ions—even in very low concentrations—function as powerful catalysts in many important industrial organic processes, as well as with enzymes (catalysts in living tissues). The total number of electron-donor atoms or donor pairs bonded to a given metal atom or metal cation is referred to as the coordination number. The coordination number of a compound can range from two to twelve and determines geometrical shape and physical properties. 

Twelve different coordinate sets from the single-crystal literature were used to make the maps of the alpha-1,2-, 1,3-, and 1,4-linked polymers in the present work. Another 12 geometries were used for the beta-linked molecules. The minimally allowed atomic radii were ... 

Twelve zinc(II) complexes with thiosemicarbazone and semicarbazone ligands were prepared and characterized by TG-DTA and H and C NMR spectroscopy [227]. Their antimicrobial activities were evaluated by MIC against four bacteria and two yeasts. The authors showed that the properties of the ligands, such as the ability to form hydrogen bonding with a counter anion or hydrated water molecules, or the lesser bulkiness of the N moiety, would be a more important factor for antimicrobial activities than the coordination number of the metal ion for the zinc(II) complexes. 

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