activated complex
(noun)
A higher-energy species that is formed during the transition state of a chemical reaction.
Examples of activated complex in the following topics:
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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.
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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.
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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.
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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.
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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]-.
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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.
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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.
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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.
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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.
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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.