Solids coordination compound reactions

To this point, several types of rate processes that occur in solutions have been described. However, the study of reactions of solid coordination compounds has yielded a large amount of information on behavior in these materials. Several types of reactions of solid complexes are known, but the discussion here will be limited to four common types of processes. 

Many of the experimental studies of the thermal decompositions of coordination compounds have been restricted to non-isothermal measurements primarily directed towards identification of the occurence of a reaction and the characterization of the major products of this change. The improved sensitivity of experimental methods (notably TG and DSC or DTA) has revealed the chemical complexity of the thermal reactions of solid coordination compounds. Much comparative information concerning the relative reactivities of related materials has been obtained. While... 

Dollimore [5] has discussed some aspects of the influence of the central atom on the thermal stabilities of solid coordination compounds. The most fully characterized compounds are those of Co, Cr and Pt. Central atoms of relatively small radius but high oxidation number coordinate most effectively. The possible influences of melting and dependences upon reaction conditions increase the difficulties of identification of the factors which control reactivity. Stabilities are influenced [5] by the electronic structure of the coordinated ion, whether this is a normal or a penetration compound. In the latter species, covalent bonding involves 3dHs4p orbitals, whereas in a normal coordination compound the 4s4pMd orbitals... 

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. 

Low-temperature heat capacities of the solid coordination compounds Zn(Leu)S04 l/2H20(s) and Zn(His)S04T/2H20(s) (Leu = Leucine and His = Histidine) were measured by a precision automated adiabatic calorimeter over the temperature range between T = 78 K and T = 371 K. Di and coworkers [228,229] determined the initial dehydration temperature of the coordination compounds by analysis of the heat-capacity curve. The experimental values of molar heat capacities were fitted to a polynomial equation with the reduced temperatures (x), [x = f (T)], by a least-squares method. Enthalpies of dissolution of both the complexes were determined by isoperibolic solution-reaction calorimetry. 

Many formations and decompositions or other equilibrations of coordination compounds are extremely rapid. The half-life of a reaction such as the replacement ( substitution ) by ammonia of water coordinated to nickel(II) ions is typically microseconds to milliseconds, and there is indeed a convenient distinction (due to Taube) for reactions in solution between kinetically labile and kineti-cally inert systems. On mixing 0.1 M aqueous solutions of the reagents, labile equilibria are fully established within 1 min, whereas inert systems take longer. Many of the ions of the heavier (second and third row) transition elements in several oxidation states (e.g., both Pt + and Pt" +) are inert, as are many spin-paired d ions (Fe +, Co +, Ni" +) and chromium(III) in the first row. Kinetic lability in solution is the rule for coordination compounds containing main group metals. Reactions of solid coordination compounds (like most other solid-state changes) are usually slow. It is this kinetic inertness that has led to the isolation of so many metastable coordination compounds. 

Studies of solid state thermal transformatioiis of metal complexes have led to the development of new and often simple methods for the synthesis of some classes of coordination compounds. Reactions that proceed in the absence of solvent frequently lead to formation of products that differ from those formed in solution. In solution there exist possibilities for solvent molecules, or their deprotonated forms, to function as ligands or reagents. This possibility is excluded when a reaction occurs in the solid state. 

This section is almost entirely concerned with the kinetics of solid phase decompositions of classical coordination compounds, since most of the information available refers to these substances. The hydrates, in which the ligands are water only, are correctly classified under the present heading, but as their dehydrations have been so intensively studied, a separate section (Sect. 1) has been devoted to the removal of water from crystalline hydrates. A separate water elimination step also preceeds many decomposition reactions. 

Solid state photochemical reactions of transition metal coordination compounds. E. L. Simmons and W. W. Wendlandt, Coord. Chem. Rev., 1971, 7.11-27 (88). 

O Brien, P. (1983). Polyhedron 2, 223. An excellent review of racemization reactions of coordination compounds in the solid state. 

The most common reaction exhibited by coordination compounds is ligand substitution. Part of this chapter has been devoted to describing these reactions and the factors that affect their rates. In the solid state, the most common reaction of a coordination compound occurs when the compound is heated and a volatile ligand is driven off. When this occurs, another electron pair donor attaches at the vacant site. The donor may be an anion from outside the coordination sphere or it may be some other ligand that changes bonding mode. When the reaction involves an anion entering the coordination sphere of the metal, the reaction is known as anation. One type of anation reaction that has been extensively studied is illustrated by the equation... 

Reactions in which isomerization of coordination compounds occur in solutions are common, and some reactions of this type in solid complexes have been studied. Generally, there is a change in color of the complex as the crystal field environment of the metal ion changes. Accordingly, some of the color changes that occur when complexes are heated may indicate isomerization, but very few geometrical isomerization reactions in solid complexes have been studied in detail. One such reaction is... 

The chemistry of coordination compounds is a vast field that encompasses many kinds of work. So much of the field is concerned with solution chemistry that it is easy to forget that a great deal is known about solid-state chemistry of coordination compounds. The brief survey presented shows that a great deal is known about some of the reactions, but there is much that needs to be done before many others will be understood. 

Although not all facets of the reactions in which complexes function as catalysts are fully understood, some of the processes are formulated in terms of a sequence of steps that represent well-known reactions. The actual process may not be identical with the collection of proposed steps, but the steps represent chemistry that is well understood. It is interesting to note that developing kinetic models for reactions of substances that are adsorbed on the surface of a solid catalyst leads to rate laws that have exactly the same form as those that describe reactions of substrates bound to enzymes. In a very general way, some of the catalytic processes involving coordination compounds require the reactant(s) to be bound to the metal by coordinate bonds, so there is some similarity in kinetic behavior of all of these processes. Before the catalytic processes are considered, we will describe some of the types of reactions that constitute the individual steps of the reaction sequences. 

It is a useful notion to consider that the reaction of a molecular solid, whether formed of organic, organometallic molecules or coordination compounds, with a vapour is conceptually related to the supramolecular reaction of a crystalline material with a volatile solvent to form a new crystalline solid (Fig. 18). Indeed, the two processes, solid-gas reaction and solid-gas solvation, differ only in the... 

Only one zirconium thiolate coordination compound is known the blue Zr(SPh)4 has been prepared by reaction of ZrCl4 with Al(SPh)3(Et20).470 The dithiolates Zr(SCH2CH2S)-(OCHMe2)2 and Zr(SCH2CH2S)2 have been obtained as white solids from the reaction of stoichiometric amounts of Zr(OCHMe2)4-HOCHMe2 and ethanedithiol in benzene at reflux.471... 

Reactions of coordination compounds in the solid state have been studied extensively for many years. There are very few cases of two complexes reacting with each other the important examples are those in which a complex loses one or more volatile ligand molecules upon heating, or which undergoes rearrangement or racemization without loss of a volatile component. If a volatile ligand is lost, several different results may ensue ... 

Mechanisms for solid-state reactions of coordination compounds have often been formulated based merely on comparisons of activation parameters within a series of compounds. There are, however, many potential problems associated with the use and interpretation of activation... 

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