Dehydration-anation reactions

Dehydration-anation reactions of coordination compounds. House [174] has proposed a general mechanism for the solid state dehydrations and anations of coordination compounds. The generation of point defects, analogous to Schottky or Frenkel defects (Chapter 1), is considered as the transition state. Reactions with low , values may occur by an S l dissociative mechanism and reactivity is influenced by the availability of free space in the crystal structure. Corbella and Ribas [175] found support for such mechanisms [174,176],... 

Dehydration/anation reactions yielding binuclear coordination compounds... 

FIGURE 7.4 Rate plots for the dehydration-anation reaction of [Co(NH3)jH20]Cl3. 

Kinetic data for the deaquation-anation reactions [33] of a range of mixed amine coordination compound salts of chromium(III) were discussed with a view to determining the mechanisms of the reactions observed. From the results it was concluded that behaviour is influenced by the available free space in the crystal structure and that dehydration, accompanied by isomerization, involves the simultaneous rupture of Cr-N (organic ligand) and Cr-0 (water) bonds. 

In the hydrated coordination compounds, one water molecule underwent an anation reaction to yield the aquapentacyano coordination compounds which were dehydrated between 370 and 470 K ... 

In addition to the thermal transformations described in the preceding sections, a variety of other reaction t)rpes may be induced by heat treatment. These include linkage isomerizalion,84 thermal conversions of coordinated ligands,85 reductive elimination reactions, thermal conversions of counterions, and the transformations of square-planar and octahedral complexes through counter-ion anation reactions or via dehydration. ... 

Several interesting observations have been made on this reaction. First, the rate of isomerization was found to be the same as the rate of dehydration. All attempts to dehydrate the starting complex by conventional techniques were found to lead to isomerization. On the basis of this and other evidence, the mechanism proposed involves the aquation in the complex followed by anation. In this process, water first displaces Cl- in the coordination sphere and then is displaced by the Cl-, possibly by an SN1 mechanism. A trigonal bipyramid transition state could account for the Cl- reentering the coordination sphere to give an cis product. The rate law for this reaction is of the form... 

Corbella and Ribas [33], in a systematic study of the factors that influence the kinetics of dehydration and anation in solid coordination compounds, identified the availability of free space in the crystal structure as an important controlling factor in reactions proceeding by a dissociative S l mechanism. 

The removal of ligands (including H2O) from coordination compounds may be accompanied by recrystallization and either a change in coordination number, structure or ligands present through formation of new bonds with the anions, anation, etc. Constituent water in crystalline hydrates may be as coordinated ligands and/or structural water. A scheme for the classification of dehydration reactions is described in Table 7.3. 

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