Ionic compounds relative stabilities

The structure of the few indium(II) compounds identified by early workers is one of the classical problems in group III chemistry, (cf. gallium, Section 25,3.3.3). It is now clear that both M—M-bonded compounds and ionic mixed valence species exist, but the factors which govern the relative stability of these species still remain to be elucidated. 

Evidently the most - practically the only - stable oxidation state of La in ionic compounds is III. Does this hold for the later members of the lanthanide series Fig. 5.1 suggests that the I oxidation state has little prospect of stability, given the high atomisation enthalpies and the relatively low second and third ionisation energies. The II oxidation state has better prospects, however. Consider the disproportionation ... 

These points are well illustrated by comparing Cu, Ag and Au with respect to the relative stabilities of their oxidation states. Although few compounds formed by these elements can properly be described as ionic, the model can quite successfully rationalise the basic facts. The copper Group 1 Id is perhaps the untidiest in the Periodic Table. For Cu, II is the most common oxidation state Cu(I) compounds are quite numerous but have some tendency towards oxidation or disproportionation, and Cu(III) compounds are rare, being easily reduced. With silver, I is the dominant oxidation state the II oxidation state tends to disproportionate to I and III. For gold, III is the dominant state I tends to disproportionate and II is very rare. No clear trend can be discerned. The relevant quantities are the ionization energies Iu l2 and A the atomisation enthalpies of the metallic substances and the relative sizes of the atoms and their cations. These are collected below / and the atomisation enthalpies AH%tom are in kJ mol-1 and r, the metallic radii, are in pm. 

According to the ionic model, the relative stability of Ln2+ and Ln3+ compounds in determined by a balance between the third ionisation energy (I3) of the lanthanide, and the difference of lattice (or... 

The lower electronegativities of the S—Po elements lessen the ionic character of compounds that are formally analogous to those of oxygen, alter the relative stabilities of various kinds of bonds, and drastically lessen the importance of hydrogen bonding, although weak X—H - S and S—H—X bonds do exist. 

For metal hydride clusters, much less is known about electronic and geometrical size effects, except for very small clusters that can be relatively easily traced with computational methods. Results on other ionic compounds, such as ZnS clusters, indicate less well defined size effects, and only for small cluster sizes [11]. As the size dependence will be clearly different than for metal clusters, potentially there is a large influence of size on the stability differences between small hydride and the corresponding metal clusters. However, the practical impact of these effects is probably limited, as in the bulk experimental preparation of light metal (hydride) clusters, generally, polydisperse samples are obtained, and hence pronounced effects of... 

For these s-block elements, the stability of complexes is often affected by the difference in the size of the ionic radii relative to the cation and anion pair. Large anions are generally better stabilized by large cations for example, barium peroxide is more stable than beryllium peroxide. In fact, the barium peroxide compound is so stable that it forms spontaneously in air. 

In the recent past, diverse opinions have been put forward to explain the occurrence of the various structural types [21, 22, 652-654] Madhukar [654] has shown that in these mainly covalent compounds the small variation in the ionicity of the bonds competes against the polarization contribution of the bonding orbitals in determining the relative stability of octahedral versus trigonal-prismatic cation coordination. In a plot of the ratio of the single-bond covalent radii of the cation and anion versus bond ionicity, all compounds with trigonal-prismatic... 

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