The Coordination Model

According to the autoionization patterns presented earlier, it might be supposed that OPCl3 could ionize slightly as represented by the equation 

Because the addition of FeCl3 to liquid OPCl3 increases the concentration of the OPCl2+ cation, ferric chloride is an acid in liquid OPCl3, which can be shown according to the solvent concept as 

FeCl3 + OPCI3 i [Cl3Fe - C1P0C12] 5 OPCl2+ + FcCl4 (5.67) 

The simplest way to explain this behavior is to assume that some Cl- is present because of autoionization or that removal of that Cl- by complexing with FeCl3 causes the system represented by Eq. (5.66) to be shifted to the right causing more OPCl2+ to be formed. 

These equations show that FeCLT can form by coordination of the solvent rather than by postulating solvent autoionization according to the solvent concept. In the series of reactions represented by Eq. (5.69), nucleophilic substitution occurs in which a solvent molecule replaces a chloride ion that subsequently interacts with FeCl3. Undoubtedly, a similar situation exists for other reactions in which autoionization appears to occur. Autoionization probably occurs only in solvents in which a proton that is strongly solvated is transferred (H20, HF, NH3, etc.). Although the solvent concept is useful in a formal way, it is unlikely that autoionization occurs for a solvent such as liquid SO2. However, many reactions take place to give the products that would be predicted if autoionization had occurred. We will now describe the chemistry of three of the most extensively studied nonaqueous solvents. 

When an aqueous solution containing Zn2+ or Al3+ has NaOH added, a precipitate of the metal 

When reacting with a base, Zn(OH)2 dissolves by the formation of a complex, Zn(OH)42 . In the reaction with an acid, protons are transferred from H30+ to the hydroxide ions, forming water molecules that remain coordinated to the Zn2+ ion. 

The behavior of Zn2+ in liquid ammonia is analogous to that in water. First a precipitate of Zn(NH2)2 forms when an amide is added, but the precipitate dissolves when either a solution containing NH4+ or Nl l2 is added. This behavior can be shown as follows  

Although this behavior has been illustrated starting with the metal ion, analogous equations can be written starting with the metal oxide as well as the hydroxide. Amphoteric behavior is exhibited in other solvents, as will be illustrated later. 

For some nonaqueous solvents, the autoionization, if it occurs at all, must be to a degree so small that virtually no ions are present. If the ion product constant for a solvent is as low as 10-40, the concentra- 

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The coordination model for non-aqueous solvent behaviour. R. S. Drago and K. F. Purcell, Prog. Inorg. Chem., 1964, 6, 271-322 (125). 

The coordination model provides a way to explain many reactions that occur in nonaqueous solvents without having to assume that autoionization takes place. As shown in Eq. (10.17), the fact that FeCl4 is produced can be explained by substitution rather than autoionization. However, as has been shown earlier in this chapter, it is sometimes useful to assume that the solvent concept is valid, and many reactions take place just as if the solvent has ionized to a slight degree into an acidic and a basic species. 

The chemistry of the specific solvents discussed in this chapter illustrates the scope and utility of nonaqueous solvents. However, as a side note, several other nonaqueous solvents should at least be mentioned. For example, oxyhalides such as OSeCl2 and OPCl3 (described in the discussion of the coordination model earlier in this chapter) also have received a great deal of use as nonaqueous solvents. Another solvent that has been extensively investigated is sulfuric acid, which undergoes autoionization,... 

Briefly, the Coordination Model attempts to account for the various species that form when solutes dissolve in various solvents. At low concentrations, iron(III) chloride, for example, forms [FeClaS] [FeCl2S4]+, [FeCU]- [FeS5Cl]2+ 2 Cl and [FeSe] + 3 Cl depending on the solvent employed. Basically, we wish to understand what solvent properties govern the extent of anion displacement. The overall process can be represented by a series of steps, each of which is exemplified by the general reaction ... 

It should also be mentioned here that many of the chemical reactions which have been "explained with the HSAB model (2) occur in polar solvents and many involve the formation of ionic species. Thermodynamic cycles can be constructed for these reactions which show how many different kinds of effects are operative. When one considers that much of the data involve rate constant and equilibrium constant measurements, the explanation of this data becomes even more complex for there are entropy terms as well as enthalpy terms for all the steps in any cycle that is constructed. Even less information is available concerning these steps than we had above for the coordination model yet explanations are offered based solely on one step (4) — the strength of the bonding. 

The very qualitative nature of the studies encompassed by the Coordination Model (2,4b) caused us to discontinue work on more systems after we had demonstrated the applicability of the model and the essential solvent properties. Om long range goal which is still far from... 

There has been some controversy in the literature over the proper interpretation of reactions in solvents such as phosphorus oxychloride. Drago and coworkers18 have suggested the "coordination model" as an alternative to the solvent system approach. They have stressed the errors incurred when the solvent system concept has been pushed further than warranted by the facts. In addition, they have pointed out that iron(lll) chloride dissolves in triethyl phosphate with the formation of letrachloro-... 

Meek, D. W., Drago, R. S. (1961). Journal of the American Chemical Society, 83, 4322. The classic paper describing the coordination model as an alternative to the solvent concept. 

While in the methods treated before ion solvation represents the sum of various terms of ion-solvent interaction, spectroscopic methods are mainly, if at all, sensitive to the immediate environment of an ion. Due to this the coordination model, representing the primary solvation shell, is not only used for highly charged ions but also for univalent ions. The precise results of the direct ion-solvent interactions made it possible to evaluate equilibrium constants describing the composition in the solvation shell of an ion in mixed solvents. Therefore, the estimation of single ion free ener es of transfer from spectroscopic measurements is the subject of several recent efforts and is theme of Part III. 

The first term on the right hand side refers to the bare ion and disappears because we are engaged in differences of free energies. The second term refers to the coordination model of ion-solvent interaction in the primary solvation shell and the third term takes into account long range interactions. The last contribution may be approximated by the electrostatic interaction of a charged species with the solvent. The radius of the charged species is equal to that of the solvated ion e.g., ionic radius + diameter of the solvent molecules in the primary solvation shell). 

Figure 2. Thermoluttiinescence depicted from the configurational coordinate model (top) and from the band model (bottom). In the coordinate model the trapped electron occnpies an excited state (111) slightly lower in energy than the main excited state (II). Heat can indnce vibrations, which move the electron into the (II) state by direct crossover at C. A transition can then occm to the gronnd state (I), resniting in emission. In the band model heat moves the trapped electron to the condnction band where it can travel thronghont the crystal rmtil it recombines with a hole at the bottom of the band (forbidden energy) gap. [Used by permission of the CRC Press, from Shionoya and Yen (1999) Figs. 49 and 50, p. 90.]...

Drago, R. S., and Purceil, D. F., The Coordination Model for Non-Aqueous Solvent Behavior 6 271... 

Thus the Coordination Model of Non-Aqueous Solutions suggested by Drago is certainly not new. 

The coordination models of the following types suggested for tetraisopropoxoalu-minate derivatives ... 

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