Vacant sites involved

Intrinsic Defects The simplest crystalline defects involve single or pairs of atoms or ions and are therefore known as point defects. Two main types of point defect have been identified Schottky defects and Frenkel defects. A Schottky defect consists of a pair of vacant sites a cation vacancy and an anion vacancy. A Schottky defect is... 

There are three approaches that may be used in deriving mathematical expressions for an adsorption isotherm. The first utilizes kinetic expressions for the rates of adsorption and desorption. At equilibrium these two rates must be equal. A second approach involves the use of statistical thermodynamics to obtain a pseudo equilibrium constant for the process in terms of the partition functions of vacant sites, adsorbed molecules, and gas phase molecules. A third approach using classical thermodynamics is also possible. Because it provides a useful physical picture of the molecular processes involved, we will adopt the kinetic approach in our derivations. 

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... 

The most active and selective catalysts for both the copolymerisation process and for the apparently simpler ethene carbonylation to monocarbonylated products MP or DEK are cationic square planar Pd(II) complexes in which the metal centre is czs-coordinated by a bidentate P - P ligand, by a Ugand involved in the initial step of the catalysis or in the process of forming the product and with the fourth vacant site coordinated by CO or ethene or a keto group of the growing chain or MeOH (or H2O, always present in the solvent even when not added on purpose) or even by a weakly coordinating anion. 

Insertion and -elimination. A catalytic cycle that involves only one type of elementary reaction must be a very facile process. Isomerisation is such a process since only migratory insertion and its counterpart P-elimination are required. Hence the metal complex can be optimised to do exactly this reaction as fast as possible. The actual situation is slightly more complex due to the necessity of vacant sites, which have to be created for alkene complexation and for P-elimination. 

The preparation of complexes 1 or 2 involves the reaction of Os3(CO)12 in acetonitrile with trimethylamine oxide, which removes CO ligands as C02 the vacant site(s) on the cluster are then filled by acetonitrile rather than the resulting trimethylamine, which is a poorer coordinating ligand. Complexes 1 or 2 are easily isolated in high yields. Their syntheses are reported in detail here, together with the synthesis of a typical derivative Os COJufCsHsN). 

Non-linearities arising from non-reactive interactions between adsorbed species will not be our main concern. In this section we return to variations of the Langmuir-Hinshelwood model, so the adsorption and desorption processes are not dependent on the surface coverage. We are now interested in establishing which properties of the chemical reaction step (12.2) may lead to multiplicity of stationary states. In particular we will investigate situations where the reaction step requires the involvement of additional vacant sites. Thus the reaction step can be represented in the general form... 

These forms show that multiple stationary states will be possible, provided n is greater than unity. As n is the number of vacant sites being recruited into the reaction step, it probably ought to be an integer. Thus multiplicity with simple Langmuir-Hinshelwood adsorption requires at least two vacant sites to be involved in the reaction. 

Fig. 12.3. Stationary-state fractional coverage for adsorption and reaction involving two vacant sites (a) k2/K = 36 showing multiplicity, (b) multiplicity in absence of desorption now one solution corresponding to a fully covered surface exists for all reactant pressures.

In this section we turn to a model where the adsorption and desorption of two reactants occur on similar timescales. The adsorption is competitive, i.e. both reactants are adsorbed on to the same surface sites. Again, a number of vacant sites will be involved in the reaction step. The model is... 

The mechanism by which oscillations occur also involves the vacant site requirement for reaction. For critical values of the partial pressures, the coverage of one of the reactants decreases leading to an increase in the rate of reaction due to the availability of vacant sites. This accelerates the decrease in coverage until the rate of reaction subsides. The large number of vacant sites then increases the rate of adsorption until the surface coverage returns to its previous state to complete the cycle. 

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