Electrostatic Bonds Ions

Electrostatic bonds (ion pair Screening of charge by Lowering the dielectric... 

Unlike the forces between ions which are electrostatic and without direction, covalent bonds are directed in space. For a simple molecule or covalently bonded ion made up of typical elements the shape is nearly always decided by the number of bonding electron pairs and the number of lone pairs (pairs of electrons not involved in bonding) around the central metal atom, which arrange themselves so as to be as far apart as possible because of electrostatic repulsion between the electron pairs. Table 2.8 shows the essential shape assumed by simple molecules or ions with one central atom X. Carbon is able to form a great many covalently bonded compounds in which there are chains of carbon atoms linked by single covalent bonds. In each case where the carbon atoms are joined to four other atoms the essential orientation around each carbon atom is tetrahedral. 

Ion Radius ratio Predicted coordination number Observed coordination number Strength of electrostatic bonds... 

Further, in the case of virtually non-existent ion-solvent interactions (low degree of solvation), so that solute-solute interactions become more important, Kraus and co-workers47 confirmed that in dilute solutions ion pairs and some simple ions occurred, in more concentrated solutions triple ions of type M+ X M+ orX M+X andinhighly concentrated solutions even quadrupoles the expression triple ions was reserved by Fuoss and Kraus48 for non-hydrogen-bonded ion aggregates formed by electrostatic attraction. 

The electrostatic valence rule usually is met rather well by polar compounds, even when considerable covalent bonding is present. For instance, in calcite (CaC03) the Ca2+ ion has coordination number 6 and thus an electrostatic bond strength of s(Ca2+) =. For the C atom, taken as C4+ ion, it is s(C4+) =. We obtain the correct value of z for the oxygen atoms, considering them as O2- ions, if every one of them is surrounded by one C and two Ca particles, z = -[2s(Ca2+) + s(C4+)] = -[2 j + ] = -2. This corresponds to the actual structure. NaN03 and YBOs have the same structure in these cases the rule also is fulfilled when the ions are taken to be Na+, N5+, Y3+, B3+ and 02. For the numerous silicates no or only marginal deviations result when the calculation is performed with metal ions, Si4+ and 02 ions. 

The coordination of an O2- ion is three Al3+ ions within an A1404 cube and one Mg2+ ion outside of this cube. This way it fulfills the electrostatic valence rule (Pauling s second rule, cf p. 58), i.e. the sum of the electrostatic bond strengths of the cations corresponds exactly to the charge on an O2- ion ... 

For the NaCl crystal, the radius ratio is 0.54, which is well within the range for an octahedral arrangement of anions around each cation (0.414 - 0.732). However, because this is a 1 1 compound, there are equal numbers of cations and anions. This means that there must be an identical arrangement of cations around each anion. In fact, for 1 1 compounds, the environment around each type of ion must be identical. We can see that this is so from a very important concept known as the electrostatic bond character. If we predict (and find) that six Cl- ions surround each Na+, each "bond" between a sodium ion and a chloride ion must have a bond character of 1/6 because the sodium has a unit valence, and... 

Rutile, Ti02, which has the structure shown in Figure 7.8, is an important chemical that is used in enormous quantities as the opaque white material to provide covering ability in paints. Because the Ti4+ ion is quite small (56 pm), the structure of Ti02 has only six O2- ions surrounding each Ti4+, as predicted by the radius ratio of 0.39. Therefore, each Ti-O bond has an electrostatic bond character of 2/3 because the six bonds to (ions total the valence of 4 for Ti. There can be only three bonds from Ti4+ to each ()2 ion because three such bonds would give the total valence of 2 for oxygen (3 X 2/3 = 2). 

Consider now the bonds to each O2- ion in the perovskite structure. First, there are two bonds to Ti4+ ions that have a character of 4/6 each, which gives a total of 4/3. However, there are four Ca2+ ions on the corners of the face of the cube where an oxide ion resides. These four bonds must add up to a valence of 2/3 so that the total valence of 2 for oxygen is satisfied. If each Ca-O bond amounts to a bond character of 1/6, four such bonds would give the required 2/3 bond to complete the valence of oxygen. From this it follows that each Ca2+ must be surrounded by 12 oxide ions so that 12(1/6) = 2, the valence of calcium. It should be apparent that the concept of electrostatic bond character is a very important tool for understanding crystal structures. 

RbCaF3 has the perovskite structure with the Ca in the center of the unit cell. What is the electrostatic bond character of each of the Ca-F bonds How many fluoride ions must surround each Ca2+ ion What is the electrostatic bond character of each Rb-F bond How many F ions surround each Rb+ ... 

Ab initio calculations (MP2/6-31G ) of the parent compound of 8 revealed that the most stable arrangement of the dimer adopts Dih symmetry (Fig. 5). Interestingly, the four Li ions and the two phosphorus centers constitute an octahedral skeleton with relatively short Li-Li and Li-P distances of 2.645 and 2.458 A, respectively. Charge analysis (22) undoubtedly supports the electrostatic bonding model for this system because of the high net charges of the natural atomic orbitals (NBO) at Li (+0.768) and P (-1.583), while NBO-Lewis resonance structures support stabilization through delocalization (Fig. 5). 

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