Bonding Ionic Compounds

Hydrates are ionic compounds bonded to water molecules. They are named by following the name of the compound with a prefix attached to the word hydrate to indicate how many water molecules are bound. 

The co-ordination number in ionic compounds is determined by the radius ratio - a measure of the necessity to minimize cationic contacts. More subtle effects are the Jahn-Teller effect (distortions due to incomplete occupancy of degenerate orbitals) and metal-metal bonding. 

Numerous ionic compounds with halogens are known but a noble gas configuration can also be achieved by the formation of a covalent bond, for example in halogen molecules, X2, and hydrogen halides, HX. When the fluorine atom acquires one additional electron the second quantum level is completed, and further gain of electrons is not energetically possible under normal circumstances, i.e... 

Whether an element is the source of the cation or anion in an ionic bond depends on several factors for which the periodic table can serve as a guide In forming ionic compounds elements at the left of the periodic table typically lose electrons giving a cation that has the same electron configuration as the nearest noble gas Loss of an elec tron from sodium for example yields Na which has the same electron configuration as neon... 

Solvent Effects on the Rate of Substitution by the S 2 Mechanism Polar solvents are required m typical bimolecular substitutions because ionic substances such as the sodium and potassium salts cited earlier m Table 8 1 are not sufficiently soluble m nonpolar solvents to give a high enough concentration of the nucleophile to allow the reaction to occur at a rapid rate Other than the requirement that the solvent be polar enough to dis solve ionic compounds however the effect of solvent polarity on the rate of 8 2 reactions IS small What is most important is whether or not the polar solvent is protic or aprotic Water (HOH) alcohols (ROH) and carboxylic acids (RCO2H) are classified as polar protic solvents they all have OH groups that allow them to form hydrogen bonds... 

X-ray structural studies have played a major role in transforming chemistry from a descriptive science at the beginning of the twentieth century to one in which the properties of novel compounds can be predicted on theoretical grounds. When W.L. Bragg solved the very first crystal structure, that of rock salt, NaCl, the results completely changed prevalent concepts of bonding forces in ionic compounds. 

Broken Bond Model were developed to describe the process of ionization of ionic compounds, especially under primary oxygen-ion bombardment - or gas admission ("O2... 

InPhj mp 208°, TlPhj mp 170°. For Ga and In compounds the primary M-C bonds can be cleaved by HX, X2 or MX3 to give reactive halogen-bridged dimers (R2MX)2. This contrasts with the unreactive ionic compounds of T1 mentioned above, which can be prepared by suitable Grignard reactions ... 

The mp and bp increase in the order of nonpolar to polar to ionic compounds. Associated liquids, in which the molecules are held together by hydrogen bonds, show higher bp than nonassociated polar compounds of similar molecular w eights. 

Ionic compounds are held together by strong electrical forces between oppositely charged ions (e.g., Na+, Cl-). These forces are referred to as ionic bonds. 

Typically, ionic compounds are solids at room temperature and have relatively high melting points (mp NaCl = 801°C, CaCl2 = 772°C). To melt an ionic compound requires that oppositely charged ions be separated from one another, thereby breaking ionic bonds. 

On the other hand, fluorine s high electronegativity and its ability to form mostly ionic chemical bonds, provide materials with several useful properties. First, compared to oxides, fluoride compounds have a wide forbidden zone and as a result, have low electroconductivity. In addition, fluorides are characterized by a high transparency in a wide optical range that allows for their application in the manufacturing of electro-optical devices that operate in the UV region [42,43]. 

In general, the compounds best separated by LSC are those which are soluble in organic solvents and are non-ionic. Water soluble non-ionic compounds are better separated using either reverse-phase or bonded-phase chromatography. 

We shall see in Chapter 2 that the formation of a bond in an ionic compound depends on the removal of one or more electrons from one atom and their transfer to another atom. The energy needed to remove electrons from atoms is therefore of central importance for understanding their chemical properties. The ionization energy, /, is the energy needed to remove an electron from an atom in the gas phase ... 

The nature of a binary hydride is related to the characteristics of the element bonded to hydrogen (Fig. 14.8). Strongly electropositive metallic elements form ionic compounds with hydrogen in which the latter is present as a hydride ion, H. These ionic compounds are called saline hydrides (or saltlike hydrides). They are formed by all members of the s block, with the exception of beryllium, and are made by heating the metal in hydrogen ... 

The hydrated ion [Cu(H20)6]2+ is an example of a complex, a species consisting of a central metal atom or ion to which a number of molecules or ions are attached by coordinate covalent bonds. A coordination compound is an electrically neutral compound in which at least one of the ions present is a complex. However, the terms coordination compound (the overall neutral compound) and complex (one or more of the ions or neutral species present in the compound) are often used interchangeably. Coordination compounds include complexes in which the central metal atom is electrically neutral, such as Ni(CO)4, and ionic compounds, such as K4[Fe(CN)6]. 

For a given molecule and a given intemuclear separation a would have a definite value, such as to make the energy level for P+ lie as low as possible. If a happens to be nearly 1 for the equilibrium state of the molecule, it would be convenient to say that the bond is an electron-pair bond if a is nearly zero, it could be called an ionic bond. This definition is somewhat unsatisfactory in that it does not depend on easily observable quantities. For example, a compound which is ionic by the above definition might dissociate adiabatically into neutral atoms, the value of a changing from nearly zero to unity as the nuclei separate, and it would do this in case the electron affinity of X were less than the ionization potential of M. HF is an example of such a compound. There is evidence, given bdow, that the normal molecule approximates an ionic compound yet it would dissociate adiabatically into neutral F and H.13... 

In the following sections, after a discussion of the properties of ionic compounds and compounds containing electron-pair bonds, the transition from one extreme to the other is considered. It is concluded that in some cases the transition could take place continuously, whereas in others an effective discontinuity would appear. 

There is, of course, a close relation between atomic arrangement and bond type. Thus the four single bonds of a carbon atom are directed toward the comers of a tetrahedron But tetrahedral and octahedral configurations are also assumed in ionic compounds, so that it is by no means always possible to deduce the bond type from a knowledge of the atomic arrangement. 

It is interesting to note, as pointed out to me by Mr. J. L. Hoard, that these considerations lead to an explanation of the stability of trivalent cobalt in electron-pair bond complexes as compared to ionic compounds. The formation of complexes does not change the equilibrium between bivalent and trivalent iron very much, as is seen from the electrode potentials, while a great change is produced in the equilibrium between bivalent and trivalent cobalt. 

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