Ligand Exchange Chemisorption

Ligand exchange is the process by which ions are held at a solid phase surface by covalent bonding. The term chemisorption is often used to describe this process in order to distinguish it from physisorption, where coulombic bonds are involved and the process is ion exchange (see previous section). 

Phosphate, silicate, borate, arsenate, selenite, chromate, and fluoride are anions for which ligand exchange is important. Nitrate, chloride, bromide, and perchlorate are not held, while sulfate and selenate may be weakly held. As a consequence, leaching of nitrate and sulfate from soil in drainage water can be significant, but very little phosphate is lost in solution. Of the trace metals, Co, Cu, Ni, and Pb are strongly held on oxide surfaces by chemisorption, but the process is much less important for Cd and Zn. 

Humic compounds can react with metal cations to form organometallic complexes  

The stability of the metal complex can be expressed as the equilibrium constant (see also Chapter 4). For example, if Equation (5.43) is simplified to 

While experimentally [M] can be measured and [ML] calculated, it is difficult to quantify [L], given the polymeric and chemically diverse nature of soil organic matter. 

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Chemisorption Adsorbate Orientation Competitive Chemisorption Adsorbate Exchange Adsorbate Reactivity Electrocatalvsis Synthesis Mode of Coordination Ligand Substitution Ligand Exchange Ligand Reactivity Homoqeneous Catalysis... 

It is also termed chemisorption (especially for gases), inner sphere adsorption and, in the case of ligands, ligand exchange. The binding constants, Kf and for the surface complexes show the same stability trend as do the constants for the equivalent complexation reactions in solution. 

Figure 14 Chemisorption of a phosphate anion on to an iron oxide surface by ligand exchange...

In oxidizing soil solutions, Sb is likely to form the anionic molecule Sb(OH)6, above pH 4, and SbCOH) in more acid solution. As an anion, Sb(OH)6 may adsorb by ligand exchange on oxides and silicate clays. Antimony associates with ferric hydroxide in soils, perhaps a result of chemisorption of the anion on this mineral. 

Chemical bonding to the support surface is essential to achieve adsorption control in ALE on porous materials. Suntola [5] in his review of the ALE process, describes in a general way the effect of activation energy on bond formation in ALE reactions, and he considers the desorption of precursors from the surface. However, in most of the reactions of metal compounds with porous supports that have been studied the reaction is directed towards chemisorption through ligand exchange. An equilibrium condition does not therefore exist where adsorption and desorption take place equally. Desorption is more likely to occur when elemental precursors are used, such as metallic zinc in the preparation of Zn/zeolite catalysts [24]. 

To verify the covalent bond formation (chemisorption), the results from several different analyses (element determinations, spectroscopic analysis) must be combined. Element determinations can provide an indication of ligand exchange reactions by showing whether the number of ligands in the surface complex is fewer than in the original metal complex, in the following way ... 

A notable exception are chemisorbed complexes in zeolites, which have been characterized both structurally and spectroscopically, and for which the interpretation of electronic spectra has met with a considerable success. The reason for the former is the well-defined, although complex, structure of the zeolite framework in which the cations are distributed among a few types of available sites the fortunate circumstance of the latter is that the interaction between the cations, which act as selective chemisorption centers, and the zeolite framework is primarily only electrostatic. The theory that applies for this case is the ligand field theory of the ion-molecule complexes usually placed in trigonal fields of the zeolite cation sites (29). Quantum mechanical exchange interactions with the zeolite framework are justifiably neglected except for very small effects in resonance energy transfer (J30). ... 

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