12.Reaction Mechanisms (2) - Electron transfer.pdf -...

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Inorganic Reaction Mechanisms (2) – Electron Transfer * Electron transfer is the other well-studied reaction mechanism for "classical" coordination complexes (i.e., non-organometallic); this is the process by which electrons are transferred from one metal center to another, resulting in a change in oxidation state for each * As with substitution, both thermodynamic and kinetic issues at play: Thermo: * What oxidation states are more/less stable for different metals w/ different ligands? * Where does the equilibrium for electron transfer (ET) lie? * What are the thermodynamic redox potentials for metal complexes? Kinetic: * What are the mechanisms of ET? * What are the rates of ET? * How can we relate these things to what we know about structure, bonding, reactivity? * Here we will be interested in mechanism (= kinetics) 2 Big Categories Inner-sphere ET Outer-sphere ET e tunnel from one metal to other through a bridging ligand complexes come into contact without sharing a bridging ligand less common because of necessity for bridging ligand e tunnel through intervening medium (solvent, periphery of ligands, etc.) Inner-sphere ET Classic example: [CoCl(NH 3 ) 5 ] 2+ + Cr(H 2 O) 6 2+ [CrCl(H 2 O) 5 ] 2+ + Co(NH 3 ) 5 (H 2 O) 2+ inert labile inert labile * The electron transfer from Cr 2+ to Co 3+ proceeds along with transfer of a Cl atom * Additionally, it has been found that added 36 Cl does not get incorporated into the Cr 3+ product (because it's inert) * The reaction proceeds quickly, but it's difficult to imagine how that could happen with one inert reactant and one inert product! i.e., for Cl transfer to occur first, Cl would have to dissociate readily from the inert Co 3+ complex; for Cl transfer to occur second, there would have to be dissociation of H 2 O from the inert Cr 3+ complex * How does it happen? Concerted ligand and electron transfer!
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Inorganic Reaction Mechanisms (2) – Electron Transfer * Basic mechanism: * When looking at the rate of an inner-sphere ET reaction, it's important to note that there are several elementary steps included in the process: + bridge formation (including the possible dissociation of a ligand from one reactant) + electron transfer + dissociation of products from bridged intermediate
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Inorganic Reaction Mechanisms (2) – Electron Transfer * Typically the electron transfer step is rate-limiting; however, there are cases where either of the other steps can be rate-limiting: If both reactants are somewhat kinetically inert, then the bridge formation is likely to be rate- determining since the dissociation of a ligand to allow bridge formation will be slow: ex: V(H 2 O) 6 2+ + CoCl(NH 3 ) 5 2+ V(H 2 O) 5 Cl 2+ + Co(H 2 O)(NH 3 ) 5 2+ If at least one reactant and one product is labile, then electron transfer is likely the limiting step;
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  • Fall '19
  • Redox, Electron Transfer

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