Primary Coordination Environment

Mononuclear biological zinc centers generally exhibit an overall coordination number of 4 and a tetrahedral or distorted tetrahedral geometry [4]. This structural motif enhances the Lewis acidity of the zinc center relative to an octahedral coordination environment. Thus, whereas [Zn(H20)g] has a pfCa of 9.0 [26], the pfCa value for the zinc-coordinated water in carbonic anhydrase 11 is 6.8 (Fig. 8.1) [22]. During catalysis, the coordination number of the active site zinc center in a metaUohydrolase can increase to 5 [13]. 

In synthetic [(N3)Zn-OH2] -type complexes containing neutral nitrogen donor ligands (Fig. 8.2), a pfCa range of 6.2-9.2 is found for the metal-bound 

Howevei as with many of the LZn-OH2 complexes for which pK values have been reported, the solid state structures of [(N(CH2CH2NH2)3)Zn-OH2]X2 and [(N(CH2CH2N(CH3)2)3)Zn-OH2]X2 have not been reported, and thus counterion and/or solvent interactions remain undefined. 

Inclusion of one or more anionic ligands in the coordination sphere of a mononuclear zinc center is expected to raise the pKa of metal-bound water [25]. This is consistent with the observed increase in the pKa of the zinc-bound water 

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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). 

In the sphalerite structure the anions form a cubic close packed array. The structure has a single adjustable parameter, the cubic cell edge. The 0 ions are too small for them to be in contact in this structure (see Fig. 6.4) so ZnO adopts the lower symmetry hexagonal wurtzite structure which has three adjustable parameters, the a and c unit cell lengths and the z coordinate of the 0 ion, allowing the environment around the Zn " ion to deviate from perfect tetrahedral symmetry. In the sphalerite structure the ZnX4 tetrahedron shares each of its faces with a vacant octahedral cavity (one is shown in Fig. 2.6(a)), while in the wurtzite structure one of these faces is shared with an empty tetrahedral cavity which places an anion directly over the shared face as seen in Fig. 2.6(b). The primary coordination number of Zn " in sphalerite is 4 and there are no tertiary bonds, but in wurtzite, which has the same primary coordination number, there is an additional tertiary bond with a flux of 0.02 vu through the face shared with the vacant tetrahedron. 

Fig. 8.5. T1+ coordination number versus anion bonding strength. The curved line represents the coordination number expected if the valence of the Tl-O bonds is equal to the anion bonding strength. The circles are observed coordination numbers. The vertical lines mark the primary coordination number (bottom arrows) and the total coordination number (top arrow) for environments that contain both primary and secondary bonds.

On a more sophisticated level, functional metallobiosite models may also, in principle, be prepared in the same conceptual fashion as applied to purely organic systems. Such systems are much more challenging and their success depends on the extent to which the enzyme activity is related to the nature of the metal first coordination sphere. It becomes, in general, almost exponentially more difficult to control the metallobiosite environment, the further the departure from the metal ion and its primary coordination sphere. 

However, tellurium commonly shows secondary bonding interactions that expand the coordination environment. It can be seen in Figure 8 that both Me2TeCl2 and (CH2)3TeCl2 3 showtwo close Te- -Cl close contacts of3.412-3.478 A and 3.359-3.479 respectively. The coordination environment around tellurium can therefore be considered as AX4Y2E in which X represents the primary bonding interaction, Y the secondary bonding interaction, and E the tellurium lone-pair. 

Amines, hydrazines, and hydroxylamines. Amine complexes are known for tetravalent complexes of the earliest actinides (Th, U), particularly for the halides, nitrates, and oxalates. The complexes are generated either in neat amine, or by addition of amine to the parent compound in a nonaqueous solvent. Some of the known simple amine compounds are presented in Table 6. The molecular structure of ThCl4(NMe3)3 has been determined. The coordination environment about the metal is a chloride capped octahedron. A very limited number of adducts exist in which a tetravalent actinide is coordinated by a hydrazine or hydroxylamine ligand the parent compound is generally a halide or sulfate complex. Cationic metal hydrates coordinated with primary, secondary, or tertiary amines have also been isolated with acetylacetonate, nitrate, or oxalate as counterions. 

Metallothioneins are a unique and widely distributed group of proteins. They are characterized by their low molecular weight (—6000), high cysteinyl content, and the ability to bind substantial numbers of metal ions (43). The proteins bind copper and zinc, thereby providing a mobile pool as part of the normal metabolism of these elements, and offer protection from the invasion of inorganic forms of the toxic elements cadmium, lead, and mercury. In addition, other metals, such as iron and cobalt, can be induced to bind. XAS is ideally suited to probe the environment of these different metal atoms (see Fig. 1), and the structural interpretations obtained from an analysis of the EXAFS data obtained in several such studies are summarized in Table 1(44). Thus, in each case, the data are consistent with the primary coordination of the metal deriving from the cysteinyl residues. 

The crystal chemistry of phosphate minerals has recently been reviewed [9, 10]. These references present a stmctural hierarchy based on the pol5mierization of polyhedra of higher bond-valence, especially tetrahedra and octahedra. In a similar fashion, an extensive stmctural hierarchy of uranyl minerals and inorganic compounds has been developed over the last decade [11, 12]. This chapter follows the concepts and principles of both of these stmctural hierarchies, but places the primary emphasis on actinide coordination. As the coordination environments of the actinides differ with valence state [13, 14], it has been found convenient to discuss the compounds of the lower valence-state actinides separately from those of the higher valence-states. 

Zinc aqua (Zn-OH2) species are prevalent in biological systems. When coordinated to a Zn(II) center, a water molecule can have a pATa value that varies from to 11, with [Zn(OH2)6]2+ having a pKa — 9.0.5 The position of a specific Zn-OH2 unit in this range depends on the primary and secondary ligand coordination environment of the zinc center. A more Lewis acid Zn(II) center, and hence a lower Zn-OH2 pK value, is produced when the total number of primary ligands is low (e.g. 4) and these ligands are neutral donors. For example, the Zn-OH2 moiety in [(THB)Zn(OH2)]2+ (Fig. la) exhibits a pATa value of 6.2.16 This indicates that a tetrahedral (NHis)3Zn(II)-OH2 moiety, as is found in active site of carbonic anhy-drase (CA), could have a pATa at or below physiological pH for the zinc-bound aqua... 

A simple shorthand notation for referring to the unusually complex coordination of Hg(II) was also suggested by Grdenic. A typical example, Hg(SMe)2 (25), is denoted as a [2 + 3] complex indicating two short, primary bonds and three longer secondary bonding interactions from adjacent molecules, yielding an effective coordination number of five. This notation provides a simple, efficient way to describe the local Hg(II) coordination environment. 

In light of recent advances in solid state Hg NMR, Raman, IR, EXAFS, and electronic spectrocopies summarized here, Hg(II) complexes can no longer be considered as spectroscopically silent. A formidable barrier to establishing reliable spectroscopic correlations with the coordination environment still exists because of the small number of well characterized small molecule complexes. Solution Hg NMR, vibrational, and electronic spectroscopies can distinguish complexes with a coordination number of 2 from those with CN = 3 or 4. None of these techniques can readily distinguish between three and four coordination however. Recent advances in solid-state Hg NMR spectroscopy unequivocally demonstrate that Hg-SR complexes with a primary coordination number of three or four can be readily distinguished. 

Transparent mm-sized mesoporous Cu-silica spheres have been synthesized successfully through the introduction of Cu(II)-APTMOS complex into the reaction mixture. The products are characterized by powder XRD, Nj adsorption, TEM, SEM, ESR, and ICP techniques. The pore structure of Cu-silica spheres is disordered. The Cu ions in Cu-silica spheres are in typical six-coordinated environment and cannot be exchanged by other metal ions. The primary experimental results show that the mesoporous Cu-silica spheres exhibit high catalytic activities in the hydroxylation of phenol under the presentation of H2O2... 

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