Calcium ions coordination compounds

A metal-nucleotide complex that exhibits low rates of ligand exchange as a result of substituting higher oxidation state metal ions with ionic radii nearly equal to the naturally bound metal ion. Such compounds can be prepared with chromium(III), cobalt(III), and rhodi-um(III) in place of magnesium or calcium ion. Because these exchange-inert complexes can be resolved into their various optically active isomers, they have proven to be powerful mechanistic probes, particularly for kinases, NTPases, and nucleotidyl transferases. In the case of Cr(III) coordination complexes with the two phosphates of ATP or ADP, the second phosphate becomes chiral, and the screw sense must be specified to describe the three-dimensional configuration of atoms. 

Probably the first scientifically recorded observation of a completely inorganic coordination compound is the formation of the familiar tetraamminecopper(II) ion, [Cu(NH3)4]2+. The 16th century German physician and alchemist Andreas Libavius (15407—1616) noticed that aqua calcis (limewater or saturated calcium hydroxide solution) containing sal ammoniac (ammonium chloride) became blue in contact with brass (an alloy of copper and zinc).8... 

In conclusions, many schemes have been developed for metal ion — phosphate — organic matter interactions in biomineralization. A variety of organic compounds of the kind present in mineralized tissues were found to coordinate calcium ions at neutral or functional sites and in many instances metal ion coordination was accompanied by the binding of phosphate. Although a wealth of information exists on the organic-inorganic interplay, data could not be reduced to a point where a simple model on biological mineralization could be formulated. 

Many ionic compounds of AX2 stoichiometry possess the CaF2 (fluorite), or Na20 (antifluorite) structures shown in Figure 3.15. Fluorite is similar to CsCl, but with every other eight coordinate cation removed. Each fluoride anion is tetrahedrally coordinated by calcium ions. This structure is adopted by several fluorides and oxides. In the antifluorite structure, the coordination numbers are the inverse. Most oxides and other chalcogenides of the alkali metals (e.g. Na2Se, K2Se) possess the antifluorite structure, but so do some more covalent compounds, such as the silicides of Mg, Ge, Sn, and Pb. 

Because the two regioisomeric products 8a and 8b have almost the same molecular dimensions, it is difficult to discriminate between the two isomers with the geometric constraints imposed by the zeolite pores. Considering that calcium ions are apt to form mainly five-membered chelate complexes with polyhydroxy compounds (Fig. 4b) 32,33) and that calcium zeolites have also been employed as sorbents in carbohydrate separations (ii), it is possible to speculate that in the CaY-supported NaN3 system the epoxy alcohol first forms a coordinated structure around a calcium ion, as shown in Fig. 4a, followed by ring opening with an azide anion at the C-3 position of the epoxy alcohol, giving a stable, five-membered chelate complex with the calcium ion. 

Elements with very low electronegativity (alkali metals, alkaline earth metals, such as Na, Ca and Mg) and elements with high electronegativity (halogens such as Cl and I) occur mainly as free ions in biological materials, and are preferably involved in electrostatic interactions. However, even these elements can form less soluble compounds (calcium oxalate), covalent compounds (hormones thyroxine and triiodothyronine are iodinated aromatic amino acids, see Section 2.2.1.2.5) or complex compounds (chlorides as Hgands and some metal ions as central atoms). A ligand is an entity (atom, ion or molecule), which can act as an electron pair acceptor to create a coordinate covalent bond with the central ion. Cd and Hg also tend to form covalent compounds. 

Fluorite, CaF2, shows a radius ratio of 0.96, which predicts that the calcium ions will occupy cubic holes formed by the fiuoride anions. Note in Figure 7.22a that the calcium ions do indeed occupy such sites. However, as required by stoichiometry (see Problem 7.39), half of the cubic holes must be unoccupied. (Note that the center of the unit cell is an unoccupied cubic hole formed by the fluoride ions.) The unit cell of the lattice therefore cannot be the simple cubic of fluorides with one calcium in the body. Rather, a larger unit cell of fee calcium ions with fluorides filling the tetrahedral holes is the more appropriate description. Note that the coordination number of the fluorides is 4, which is consistent with Equation (7.6). Table 7.11 indicates that there is a 90% correlation between the known crystal structure and the calculated radius ratio for compounds that assume the fluorite structure. 

Various other observations of Krapcho and Bothner-By are accommodated by the radical-anion reduction mechanism. Thus, the position of the initial equilibrium [Eq. (3g)] would be expected to be determined by the reduction potential of the metal and the oxidation potential of the aromatic compound. In spite of small differences in their reduction potentials, lithium, sodium, potassium and calcium afford sufficiently high concentrations of the radical-anion so that all four metals can effect Birch reductions. The few compounds for which comparative data are available are reduced in nearly identical yields by the four metals. However, lithium ion can coordinate strongly with the radical-anion, unlike sodium and potassium ions, and consequently equilibrium (3g) for lithium is shifted considerably... 

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