activated complex

Examples of activated complex in the following topics:

• Transition State Theory

  • The species that is formed during the transition state is known as the activated complex.
  • TST is also referred to as "activated-complex theory," "absolute-rate theory," and "theory of absolute reaction rates."
  • The species that forms during the transition state is a higher-energy species known as the activated complex.
  • The mechanism by which the activated complex breaks apart; it can either be converted into products, or it can "revert" back to reactants.
  • Once the activated complex is formed, it can then continue its transformation into products, or it can revert back to reactants.

• Enzyme Catalysis

  • Enzymes are proteins that are able to lower the activation energy for various biochemical reactions.
  • At the active site, the substrate(s) can form an activated complex at lower energy.
  • This change stabilizes the transition state complex, and thus lowers the activation energy.
  • Electrostatic catalysis: electrostatic attractions between the enzyme and the substrate can stabilize the activated complex.
  • An enzyme catalyzes a biochemical reaction by binding a substrate at the active site.

• Biomolecules

  • Coordination complexes are found in many biomolecules, especially as essential ingredients for the active site of enzymes.
  • Coordination complexes (also called coordination compounds) and transition metals are widespread in nature.
  • The transition metals, particularly zinc and iron, are often key components of enzyme active sites.
  • As with all enzymes, the shape of the active site is crucial.
  • The structure of the active site in carbonic anhydrases is well known from a number of crystal structures.

• Metal Cations that Act as Lewis Acids

  • Transition metals can act as Lewis acids by accepting electron pairs from donor Lewis bases to form complex ions.
  • The number of coordinate bonds is known as the complex's coordination number.
  • The product is known as a complex ion, and the study of these ions is known as coordination chemistry.
  • One coordination chemistry's applications is using Lewis bases to modify the activity and selectivity of metal catalysts in order to create useful metal-ligand complexes in biochemistry and medicine.
  • Examples of several metals (V, Mn, Re, Fe, Ir) in coordination complexes with various ligands.

• Transition Metals

  • In complexes of the transition metals, the d orbitals do not all have the same energy.
  • The transition metals and their compounds are known for their homogeneous and heterogeneous catalytic activity.
  • This activity is attributed to their ability to adopt multiple oxidation states and to form complexes.
  • I2•PPh3 charge-transfer complexes in CH2Cl2.
  • From left to right: (1) I2 dissolved in dichloromethane—no CT complex. (2) A few seconds after excess PPh3 was added—CT complex is forming. (3) One minute later after excess PPh3 was added—the CT complex [Ph3PI]+I-has been formed. (4) Immediately after excess I2 was added, which contains [Ph3PI]+[I3]-.

• Applications of Transition Metals to Organic Chemistry

  • The empty and partially occupied d-orbitals that characterize most of these metals enable them to bond reversibly to many functional groups, and thus activate many difficult or previously unobserved reactions, often in catalytic amounts.
  • Finally, benzene and related arenes form stable complexes with Cr(0), either as shown or as a sandwich.
  • This robust complex is easily dissociated by treatment with iodine or by other mild oxidizing agents.
  • These may take two forms, as illustrated, depending again on the nature of the complex.
  • An instructive example of transition metal activation of carbon-carbon double bonds is found in the homogeneous hydrogenation catalyst known as Wilkinson's catalyst.

• Reactions of Alkylidene Complexes

  • The alkylidene complexes described above undergo many interesting and synthetically useful reactions.
  • The next two equations (# 7 & 8) demonstrate the dienophilic activation provided by a Fischer carbene.
  • In most cases the carbene is a stronger activating group than the corresponding ester.

• Reactions of Substituent Groups

  • The benzylic hydrogens of alkyl substituents on a benzene ring are activated toward free radical attack, as noted earlier.
  • The strongest activating and ortho/para-directing substituents are the amino (-NH2) and hydroxyl (-OH) groups.
  • Although the activating influence of the amino group has been reduced by this procedure, the acetyl derivative remains an ortho/para-directing and activating substituent.
  • However, the overall influence of the modified substituent is still activating and ortho/para-directing.
  • Six proposed syntheses are listed in the first diagram below in rough order of increasing complexity.

• Hydrogenation

  • Although the overall hydrogenation reaction is exothermic, a high activation energy prevents it from taking place under normal conditions.
  • Catalysts act by lowering the activation energy of reactions, but they do not change the relative potential energy of the reactants and products.
  • The formation of transition metal complexes with alkenes has been convincingly demonstrated by the isolation of stable platinum complexes such as Zeise's salt, K[PtCl3(C2H4)].H2O, and ethylenebis(triphenylphosphine)platinum, [(C6H5)3P]2Pt(H2C=CH2).
  • Similar complexes have been reported for nickel and palladium, metals which also function as catalysts for alkene hydrogenation.

• Photochemistry

  • The second law of photochemistry, the Stark-Einstein law, states that for each photon of light absorbed by a chemical system, only one molecule is activated for subsequent reaction.
  • Since many photochemical reactions are complex, and may compete with unproductive energy loss, the quantum yield is usually specified for a particular event.
  • For example, irradiation of acetone with 313 nm light (3130 Å ) gives a complex mixture of products, as shown in the following diagram.