activated complex

(noun)

A higher-energy species that is formed during the transition state of a chemical reaction.

Related Terms

  • Transition State Theory

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.
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