Ionic compounds stereochemistry

Our review of simple ionic compounds shows that the radius-ratio rules are a useful guide to the stereochemistry of halides and oxides, but that the quantitative predictions are not to be taken too seriously. This is no doubt in part attributable to the crudeness of the model from which the rules are derived, and in part to uncertainties in the values of the ionic radii. There are many examples in which the coordination number increases with increasing radius of the metal ion in closely related compounds for example, Mg++ in MgF2 is 6-coordinated but CaF2 has an 8-coordinated structure. It is in accounting, at least semiquantitatively, for such relations that the radius-ratio rules are most important. 

It is well known that the presence of pairs of unshared electrons in the valency orbitals of an atom affects the stereochemistry of the molecule in which it occurs. This is equally evident from the nonplanarity of molecules such as NH3 and PH3 and from the irregular coordination often found about Sn++ or Pb H ions. As in the case of many stereochemical facts, interpretations of the influence of the unshared pair can be given either from the electrostatic or from the covalent point of view. Here we restrict our discussion to the most ionic compounds of the metals and hence use the electrostatic approach which is then more appropriate. We leave for other publications detailed discussions of the quantitative importance of covalency. 

In alkyllithium initiated, solution polymerization of dienes, some polymerization conditions affect the configurations more than others. In general, the stereochemistry of polybutadiene and polyisoprene respond to the same variables Thus, solvent has a profound influence on the stereochemistry of polydienes when initiated with alkyllithium. Polymerization of isoprene in nonpolar solvents results largely in cis-unsaturation (70-90 percent) whereas in the case of butadiene, the polymer exhibits about equal amounts of cis- and trans-unsaturation. Aromatic solvents such as toluene tend to increase the 1,2 or 3,4 linkages. Polymers prepared in the presence of active polar compounds such as ethers, tertiary amines or sulfides show increased 1,2 (or 3,4 in the case of isoprene) and trans unsaturation.4. 1P U It appears that the solvent influences the ionic character of the propagating ion pair which in turn determines the stereochemistry. 

The influence of the nature of the metal ion on the stereochemistry of its environment in octahedral complexes is not well known. Most of the studies have been performed with compounds of cobalt(III). It would be useful to get more information about the influence of crystal field parameters, charge and ionic radii on the stereochemistry of metal complexes. 

Stereochemistry of Ionic Solids ]. D. Dunitz and L. E. Orgel Organometallic Compounds John Eisch and Henry Gilman Fluorine-Containing Compounds of Sulfiir... 

Reaction with unsaturated bicyclic hydrocarbons. From a study of the chlorination of unsaturated bicyclic compounds, Masson and Thuillier1 conclude that the reaction follows, a radical addition mechanism when initiated thermally or photo-chemically. The stereochemistry of the addition is markedly influenced by steric effects. No Wagner-Meerwein rearrangements are observed under these conditions. An ionic mechanism is involved without initiation or in the presence of trifluoroacetic acid in this case the usual carbonium ion rearrangements are observed. 

The coordination numbers and stereochemistry of the ions are given in Table 27-3. In both ionic crystals and in complexes, coordination numbers exceeding 6 are the general rule rather than the exception. Indeed the number of lanthanide compounds in which the coordination number of 6 has been unequivocally established is small many complexes that could have been so formulated are known to have solvent molecules bound to the metal, leading... 

In this review we shall emphasize the ionic viewpoint, but not because we are unaware of the importance of covalent bonding. We believe that only after the irrelevance of much that has been presented as qualitative stereochemical evidence for eovalency has been appreciated, does the importance of quantitative studies, for example of unpaired electron distributions by paramagnetic resonance experiments, become clear. Our program, then, is to interpret as many features as possible of the stereochemistry of metal compounds from the point of view of an ionic theory and only incidentally to draw attention to the modifications that are required if the covalent effects (which, to a greater or lesser extent, influence the properties of all compounds) are to be taken into account. 

In our discussion of the stereochemistry of inert pair ions we shall restrict ourselves to compounds which have structures that may reasonably be regarded as distortions of simple ionic ones. It must be emphasized that a large number of otherwise relevant compounds do not fall into this category although the environment of the metal nearly always lacks a center... 

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