Coordination number of ions

We have seen in Chapter 4 that the coordination number of ions in lattices is related to the ratio of the radii of the ions. The same general principles apply to coordination compounds, especially when a single coordination number, such as 4. has two common geometries—tetrahedral and square planar, An extended list of radius ratios is given in Table 12,1. 

E, dementi Determination of Liquid Water Structure. Coordination Numbers of Ions and Solvation for Biological Molecules. Lecture Notes in Chemistry, Vol, 2, Springer-Verlag, Berlin 1976,... 

Notably stronger structural relaxation should accompany the formation of sites on the MgO(OOl) surface that are characterized by a reduced coordination number of ions, such as point (vacancy) or extended (step, edge, comer, etc.) defects [87]. This new situation is adequately reproduced by DF cluster models. For instance, for the edge formed by the intersection of (001) and (100) surfaces of MgO, a notable inward (into the substrate) displacement of Mg and O ions from the bulk-terminated position was computed in the [101] direction, by 15 and 12 pm, respectively [95]. Comer three-coordinated Mgsc cations are predicted to move even further, 32 pm, along the [111] direction, "down" to the three O anion neighbors [60], Opposite displacements of the Mg and O ions, to partly restore the bulk-terminated geometry, take place when an adsorbate effectively repairs the reduction of coordination numbers at these defects on clean MgO surface, e.g. Refs. 60 and 95. 

Figure 4.7 (a) The creation of new surface entails the breaking of bonds across that surface, (ft) Structure of (100) plane in the rock salt structure, (c) Structure of (110) plane in same structure. Note that the coordination number of ions in this plane is 2. which implies that to create a (110) plane, only two bonds per ion would have to be broken. 

Before the discussion of particular dependencies which are encountered in clinker solid solutions it will be useful to remind general principles of solid solutions formation. In crystal chemistry significant importance have the interionic distances in the lattice, which is the basis for ionic radius determination . The radii ratio of neighbouring ions in the lattice is deciding of the numbers of the closest neighbours, then of so called coordination number of ion. For example silicon ion is as a mle surrounded by four oxygen ions (CN=4), located in the tetrahedron comers . 

The maximum coordination number of ions of similar radii and with direct contact between cations and anions is the CsCl structure, coordination number 8 (Figure 2.7a). 

The solvation number of the cobalt(II) ion in aqueous solutions [Ma 68a] and in solutions prepared with polar solvents in general [Mu 64a, Lu 64b, Ma 67, Th 67] is known to be six. Few data are available with respect to how the coordination numbers of ions change if the pure solvent is diluted with some other solvent miscible with it. 

Calculations using such pristine models have been performed and revealed that the (110) and (310) surfaces are more reactive than (111). The result is perhaps intuitive because the coordination number of ions at the surface of (110) and (310) is lower than (111). Thus, simulation was able to offer an important prediction selectively expose the (110) or (310) surfaces to make a more reactive catalyst . However, one is left with a conundrum because the... 

The melting and boiling points of the aluminium halides, in contrast to the boron compounds, are irregular. It might reasonably be expected that aluminium, being a more metallic element than boron, would form an ionic fluoride and indeed the fact that it remains solid until 1564 K. when it sublimes, would tend to confirm this, although it should not be concluded that the fluoride is, therefore, wholly ionic. The crystal structure is such that each aluminium has a coordination number of six, being surrounded by six fluoride ions. 

Aluminum hydroxide and aluminum chloride do not ionize appreciably in solution but behave in some respects as covalent compounds. The aluminum ion has a coordination number of six and in solution binds six molecules of water existing as [Al(H20)g]. On addition of a base, substitution of the hydroxyl ion for the water molecule proceeds until the normal hydroxide results and precipitation is observed. Dehydration is essentially complete at pH 7. 

By far the most important metal containing dyes are derived from OjO-dUiydroxyazo stmctures in which one of the two azo nitrogen atoms and the two hydroxyl oxygen atoms are involved in bonding with the metal ion. Thus these dyes serve as terdentate ligands. In the case of metal ions with a coordination number of four, eg, Cu(H), the fourth position is usuaUy occupied by a solvent molecule (47). 

A hydroxyl group is situated ortho to a carboxyl group which as a bidentate ligand is terminally metallized on the fiber when aftertreated with dichromate. An example is Alizarine Yellow GG [584-42-9] (50) (Cl Mordant Yellow 1 Cl 14025). Cr(III) has a coordination number of six, and therefore normally two dye molecules of the sahcyhc type are chelated to the metal ion. 

The colors obtained depend primarily on the oxidation state and coordination number of the coloring ion (3). Table 1 Hsts the solution colors of several ions in glass. AH of these ions are transition metals some rare-earth ions show similar effects. The electronic transitions within the partially filled d andy shells of these ions are of such frequency that they fall in that narrow band of frequencies from 400 to 700 nm, which constitutes the visible spectmm (4). Hence, they are suitable for producing color (qv). 

Ceitain acid dyes can have thek fastness piopeities impioved by combining the dye with a metal atom (chelation). The most common metal is chromium, although cobalt is sometimes used, and this can be introduced in a number of ways. The basic mechanism is donation of electron pans by groups in the dye (ligands) to a metal ion. For example, has a coordination number of 6, and therefore will accept six lone pans of electrons. Typical ligand groups... 

Metal ion complexation rates have been studied by the T-jump method. ° Divalent nickel and cobalt have coordination numbers of 6, so they can form complexes ML with monodentate ligands L with n = 1—6 or with bidentate ligands, n = 1-3. The ligands are Bronsted bases, and only the conjugate base form undergoes coordination with the metal ion. The complex formation reaction is then... 

Chlorates and bromates feature the expected pyramidal ions X03 with angles close to the tetrahedral (106-107°). With iodates the interatomic angles at iodine are rather less (97-105°) and there are three short I-O distances (177-190 pm) and three somewhat longer distances (251-300 pm) leading to distorted perovskite structures (p. 963) with pseudo-sixfold coordination of iodine and piezoelectric properties (p. 58). In Sr(I03)2.H20 the coordination number of iodine rises to 7 and this increases still further to 8 (square antiprism) in Ce(I03)4 and Zr(I03)4. 

The anhydrous nitrates can be prepared by the action of N2O5 on MCI4. Ti(N03)4 is a white sublimable and highly reactive compound (mp 58°C) in which the bidentate nitrate ions are disposed tetrahedrally around the titanium which thereby attains a coordination number of 8 (Fig. 21.4). Infrared evidence suggests that Zr(N03)4 is isostmctural but hafnium nitrate... 

The anhydrous trihalides are ionic, high melting, crystalline substances which, apart from the trifluorides are extremely deliquescent. As can be seen from Table 30.4, the coordination number of the Ln changes with the radii of the ions, from 9 for the trifluorides of the large lanthanides to 6 for the triiodides of the smaller lanthanides. Their chief importance has been as materials from which the pure metals can be prepared. 

The application of this principle is shown in Table 15.1, where we list the formulas of several complexes formed by platinum(II), which shows a coordination number of 4. Notice that one of the species, Pt(NH3)2Cl2, is a neutral complex rather than a complex ion the charges of the two Cl- ions just cancel that of the central Pt2+ ion. 

---