Zinc ionic radii

The most common oxidation state of niobium is +5, although many anhydrous compounds have been made with lower oxidation states, notably +4 and +3, and Nb can be reduced in aqueous solution to Nb by zinc. The aqueous chemistry primarily involves halo- and organic acid anionic complexes. Virtually no cationic chemistry exists because of the irreversible hydrolysis of the cation in dilute solutions. Metal—metal bonding is common. Extensive polymeric anions form. Niobium resembles tantalum and titanium in its chemistry, and separation from these elements is difficult. In the soHd state, niobium has the same atomic radius as tantalum and essentially the same ionic radius as well, ie, Nb Ta = 68 pm. This is the same size as Ti ... 

It is noteworthy that the Be—O distance (1.60 A) is of the same order as that found for other coordination compounds of beryllium and leads to the value of 1.0—1.1 A for the covalent radius of beryllium in this type of coordination.26 Clearly arguments based on relative ionic radii are invalid. Thus the dihydrate of zinc oxinate has been shown to form a distorted tetrahedron with two long Zn—H20 bonds while the lengths Zn—O and Zn—N to the ligand are 2.05 and 2.06 A respectively, whence the zinc radius is 1.38 A. Clearly the use of an ionic radius (Zn2+ = 0.74 A) would be misleading. Similarly the Cu—N bond in compounds of Cu11 with ammonia and ethylenediamine (1.99,2.05,2.01 A) implies a radius of 1.3-1.4 A in these coordination compounds, a value considerably larger than the ionic radius of 0.7 A.23... 

H2O + 2e, 1.216 V. Electron configuration l 22y22pa3s23pic,4r2. Ionic radius Zn+2 0.75 A. Metallic radius 1.33245 A. Other physical properties of zinc are described under Chemical Elements. 

The failure of the Goldschmidt Rules in other cases, such as accounting for the geochemical behaviour of zinc, was attributed to effects of covalent bonding (Fyfe, 1951, 1954). The rules are stated in terms of ionic radius and... 

Von Baumbach and Wagner [4] argued that the zinc interstitial is more probable because of the smaller ionic radius of the Zn++ ion (74 pm) compared with the oxygen ion (138 pm). What could not be decided for decades was the question whether the oxygen vacancy or the zinc interstitial constitutes the donor [5], see Sect. 2.1.1.1. 

Most inorganic chemistry texts list cut-off values for ther+/r ratios corresponding to the various geometries of interstitial sites (Table 2.3). However, it should also be pointed out that deviations in these predictions are found for many crystals due to covalent bonding character. An example for such a deviation is observed for zinc sulfide (ZnS). The ionic radius ratio for this structure is 0.52, which indicates that the cations should occupy octahedral interstitial sites. However, due to partial covalent bonding character, the anions are closer together than would occur from purely electrostatic attraction. This results in an effective radius ratio that is decreased, and a cation preference for tetrahedral sites rather than octahedral. 

Spectra have also been reported for alkaline earth complexes of bipyridyl, terpyridyl, and substituted bipyridyls and phenanthrolines (106,107) of the type M +(L )2. For Be, Mg, Ca, and Sr the spectra show the presence of a ground, orvery lowexcited, triplet state consistent with a divalent metal cation in a tetrahedral environment with one electron on each chelate ligand. The similarity in splittings for Be and Mg is thought to show that interligand interactions prevent the achievement of the small Be + ionic radius. Treatment of bipyridyl with zinc amalgam does... 

Cadmium. Cadmium appears to be compatible or very mildly incompatible, similar to zinc. Almost nothing is known about which minerals it prefers. From a crystal-chemical view, cadmium has similar ionic radius and charge to calcium, but a tendency to prefer lower coordination due to its more covalent bonding with oxygen (similar to zinc and indium). Cadmium in spinel Uierzolites varies from 30 ppb to 60 ppb (BVSP) and varies in basalts from about 90 ppb to 150 ppb (Hertogen et al., 1980 Yi et al., 2000). Cd/Zn is —10 in peridotites (BVSP) and the continental cmst (Gao etal., 1998), and —1.5 X 10 in basalts (Yi etai, 2000). We adopt the mean of these ratios (1.2 X 10" ). 

In (a) and (c) there would be no great difference between the characters of the A-S and B—S bonds in a particular compound, while in (b) the B and S atoms form a covalent complex which may be finite or infinite in one, two, or three dimensions. By analogy with oxides we should describe (a) and (c) as complex sulphides and (b) as thio-salts. Compounds of type (c) are not found in oxy-compounds, and moreover the criterion for isomorphous replacement is different from that applicable to complex oxides because of the more ionic character of the bonding in the latter. In ionic compounds the possibility of isomorphous replacement depends largely on ionic radius, and the chemical properties of a particular ion are of minor importance. So we find the following ions replacing one another in oxide structures Fe, Mg , Mn , Zn, in positions of octahedral coordination, while Na" " more often replaces Ca (which has approximately the same size) than K , to which it is more closely related chemically. In sulphides, on the other hand, the criterion is the formation of the same number of directed bonds, and we find atoms such as Cu, Fe, Mo, Sn, Ag, and Hg replacing Zn in zinc-blende and closely related structures. 

Zinc has a highly concentrated charge in comparison to its relatively small ionic radius (0.65 A) and binds modestly to anions such as carboxylates and phosphates. Its second characteristic is its high affinity for electrons, making it a strong Lewis acid, similar to copper and nickel. However, unlike the other two transition metal ions, it does not show variable valence, which might lead to it being preferred quite simply because it does not introduce the risk of free radical reactions. 

Charles (1954) has compared the entropy of ethylenediamine-tetraacetate (EDTA) complex formation for several elements, including zinc, and finds the entropy change to be a linear function of the partial molal entropy of the complexed metal. In Fig. 10, the entropy changes in the formation of ammonia (Williams, 1954), ethylenediamiue (En) (Davies et al., 1954), and EDTA (Charles, 1954) complexes of Zn, Cu, and Cd are plotted as a reciprocal of the ionic radius minus a term containing the molecular weight (Powell and Latimer, 1951), as a measure of the partial molal entropy of the cations. The AS° values plotted are for the reaction ... 

The SH groups of cysteine residues (177) in the rabbit liver protein metallothionein (MT) and two of its fractions (a-MT, /S-MT) can bind transition metal ions. Thus, formation of the silver complexes Agi2-MT, Agis-MT, Ag6-a-MT and Ag6-/S-MT was determined by circular dichroism (CD), when zinc complexes of metallothionein and its fractions were titrated with Ag(I) ions. The intense CD spectrum of one of the silver complexes was attributed to supercoil formation245. The UVV spectra of bilirubin (168) and its complexes with Cu(II), Ag(I) and Au(III) showed that the complexes had different structures, due to differences of ionic charge and ionic radius between the metal ions238. 

Thallium is a rare element which occurs in the Earth s crust at an estimated abundance of 0.1 to 0.5 ligg (see Part I, Chapter 1). The specific ionic properties of thallium (e.g., ionic radius Tl 0.147 nm) are similar to those of potassium and rubidium (ionic radius K 0.133 nm, Rb" 0.147 nm) thus, thallium occurs ubiquitously as a trace element within the environment, mainly in association with K and Rb. Besides its occurrence in widespread potassium compounds, thallium is a trace component in iron, zinc, copper, and lead minerals (Nriagu 1998). 

In fact, trigonal holes are so small that they are never occupied in binary ionic compounds. Whether the tetrahedral or octahedral holes in a given binary ionic solid are occupied depends mainly on the relative sizes of the anion and cation. For example, in zinc sulfide the ions (ionic radius = 180 pm) are arranged in a cubic closest packed structure with the smaller ions (ionic radius = 70 pm) in the tetrahedral holes. The locations of the tetrahedral holes in the face-centered cubic unit cell of the ccp structure are shown in Fig. 10.36(a). Note from this figure that there are eight tetrahedral holes in the unit cell. Also recall from the discussion in Section 10.4 that there are four net spheres in the face-centered cubic unit cell. Thus there are twice as many tetrahedral holes as packed anions in the closest packed structure. Zinc sulfide must have the same number of S ions and Zn ions to achieve electrical neutrality. Thus in the zinc sulfide structure only half the tetrahedral holes contain Zn ions, as shown in Fig. 10.36(c). 

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