Catalysed Reactions

Catalysis is an approach to dealing with extremely slow reactions that otherwise can be driven conveniently only by increasing the temperature pressure and/or reaction time. The concept of catalysis is that the catalyst lowers the activation barrier for a reaction, thus allowing it to proceed at a faster rate; it is not usually considered to influence the position of an equilibrium. The catalyst may be dissolved in the reaction solution (a homogenous catalyst) or present as a solid (a heterogenous catalyst). The former has the advantage of being in the same phase as the reagents; for a heterogenous system, reaction must occur only at the surface of the solid catalysts, so that surface area is an important consideration as is stirring to allow mass transport in the system.

Many catalysts are metals metal oxides or other simple salts or metal complexes. For example formation of platinum (IV) complexes involving ligand substitution is an extremely slow process, due to the kinetic inertness of this oxidation state. However, the addition of small amounts of a platinum (II) complex to the reaction mixture leads to excellent catalysis of the reaction, assigned to mixed oxidation state bridged intermediates that promote ligand transfer.

There are also nonmetallic catalysts of which the best known are the various forms of activated carbon. For example, isomerization of inert, chiral cobalt (III) complexes is accelerated significantly by the presence of carbon; this is assigned to reduction of cobalt (III) on the surface of the carbon to labile cobalt (II) allowing rapid ligand rearrangement, with air subsequently rapidly re-oxidizing the cobalt (II) to cobalt (III) before ligand dissociative processes can become involved.

Complexes of some metal ions show a capacity towards photo-activity, which means that they undergo different chemistry in the presence of light. This is because light can cause electron transfer from metal orbitals to ligand orbitals leading to a reactive excited state that can undergo different chemistry; ruthenium (II) is one metal that exhibits this capacity. Usually, simple complexes exhibit only light (hv) catalysed aquation reactions, but redox chemistry can result with other reagents. Both outcomes are illustrated in the following simple example where both aquation (6.24) and oxidation (6.25) can occur.

Examples of reactions that undergo change in oxidation state are covered in more detail in the following section.

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مواضيع ذات صلة

Reactions Involving the Metal Oxidation State

Chiral Complexes

Substitution Reactions without using a Solvent

Substitution Reactions in Nonaqueous Solvents

Reactions Involving the Coordination Shell

Block f : Inner Transition Metals (Lanthanoids and Actinoids)

Block d: Transition Metals

Block p: Main Group Metals

= Block s : Alkali and Alkaline Earth Metals

A New Face - Oxidation-Reduction

A New Body - Stereochemical Change

Lability and Inertness in Octahedral Complexes