ic2Topic 3.5 Reaction Mehanism 2.pdf

INORGANIC CHEMISTRY II Reactions and Mechanisms Part II 1 Redox reactions a. Inner sphere (IS), In an inner-sphere redox reaction a ligand is shared to form a transition state; b. Outer sphere (OS), in an outer sphere redox reaction there is no bridg...

INORGANIC CHEMISTRY II Reactions and Mechanisms Part II 1 Redox reactions a. Inner sphere (IS), In an inner-sphere redox reaction a ligand is shared to form a transition state; b. Outer sphere (OS), in an outer sphere redox reaction there is no bridging ligand between the reacting species. Template reactions. REDOX: In complexes, M or L ? The first two steps of an inner-sphere reaction are the formation of a precursor complex and the formation of the bridged binuclear intermediate. These two steps are identical to the first two steps in the EigenWilkins mechanism The final steps are electron transfer through the bridging ligand to give the successor complex, followed by dissociation to give the products. The rate-determining step of the overall reaction may be any one of these processes, but the most common one is the electron-transfer step. However, if both metal ions have a nonlabile electron configuration after electron transfer, then the break-up of the bridged complex is rate determining The different pathways followed by inner- and outer-sphere mechanisms. a. inner-sphere redox reaction - Inner sphere electron transfer is mediated by a bridging ligand. i) Reductant and oxidant share a ligand in the precursor and successor complex. ii) On activation the electron is transferred between the metals. iii) The ligand may transfer between complexes.  In Oh complexes dissociation of a ligand is required to form bridge add Cl- However ligand transfer is not a requirement of inner sphere mechanism i)Formation of the bridging complex can be the rate limiting step (ka). This will be dependent on how inert or labile the complexes are. (kET vs ka). It is also possible that dissociation (kd) is the rate limiting step. ii) Electronic configurations. σ* (‘eg’) orbitals interact strongly with bridging ligand. Orbital symmetries of metal σ* and bridging ligands facilitate electron transfer. Massive acceleration in rates from outer to inner sphere can be achieved. iii) Bridging ligand. Inner sphere electron transfer is very sensitive to bridging ligand. 1) The bridge connects the two metals. 2) Transfer can be a two step process from metal to ligand then ligand to metal. This circumvents the simultaneous reorganization energy of both complexes that is required for outer sphere. OS: No bridging, for this redox reaction IS: No bridging, for this redox reaction  An outer-sphere redox reaction involves electron tunnelling between two reactants without any major disturbance of their covalent bonding or inner coordination spheres; the rate constant depends on the electronic and geometrical structures of the reacting species and on the Gibbs energy of reaction.  Mechanism 1. Formation of precursor complex 2. Activation/reorganization of precursor complex. Electron transfer. Relaxation to successor complex 3. Dissociation of successor complex Formation of precursor complex and dissociation of successor complex are fast. Electron transfer slow. Remember from thermodynamics, driving force. Why Isotopes? So why is there such a large range of rates? Electron transfer between two metal ions in a precursor complex is not productive until their coordination shells have reorganized to be of equal size. (a) Reactants; (b) reactant complexes having distorted into the same geometry; (c) products. Changes in Gibbs Energy of Activation  i) ∆Go‡. Energy is required to reorganize the solvent. Solvents that interact strongly with complexes (e.g. via hydrogen bonding) will reduce the rate of electron transfer. ii) ∆Gi‡. Metal-ligand bond lengths will change when the oxidation state of the metal changes. The Frank-Condon principle states that because nuclei are much more massive than electrons, an electronic transition occurs much faster than the nuclei can respond. Complexes must adjust their M-L bond lengths before electron transfer. Orbital energies must be of equal for electron transfer to occur (but not sole requirement) Why not transfer electron then relax bonds? Self exchange R = potential energy surface of reactants + environment P= " products " At * , the requirement of equal orbital energies is met allowing the possibility of electron transfer a photon could provide the required energy. The only instant at which the electron can transfer within the precursor complex is when both [Fe(OH2)6]3 and [Fe(OH2)6]2 have achieved the same nuclear configuration by thermally induced fluctuations. That configuration corresponds to the point of intersection of the two curves, and the energy required to reach this position is the Gibbs energy of activation, ∆‡G. If [Fe(OH2)6]3 and [Fe(OH2)6]2 differ in their nuclear configurations, ∆‡G is larger and electron exchange is slower. The potential energy curves for electron self-exchange. The nuclear motions of both the oxidized and reduced species (shown displaced along the reaction coordinate) and the surrounding solvent are represented by potential wells. Electron transfer to oxidized metal ion (left) occurs once fluctuations of its inner and outer coordination shell bring it to a point (denoted *) on its energy surface that coincides with the energy surface of its reduced state (right). This point is at the intersection of the two curves. The activation energy depends on the horizontal displacement of the two curves (representing the difference in sizes of the oxidized and reduced forms). - A photon could provide energy to activate the electron  Overlap 2nd and 3rd row metals generally faster that 1st row due to better overlap of 4d and 5d orbitals. (Also due to stronger ligand fields bond length distortions will be smaller). Ligands that have extended π-systems e.g. Phen, bipy etc can assist electron transfer Exercise. Calculate the rate constant of the following redox reaction; k12 = ? k11 k11 k22 k22 How do we distinguish if electron transfer is outer or inner Choosing sphere? Mechanisms Very inert metal ions substitute too 1. Is there a vacant coordination site? slowly to allow 2. Is there a substitutionally labile reactant? bridging: 3. Has ligand transfer occurred? [Ru(NH3)6]2+ 4. Are there large differences in rate on addition or substitution Ligands that are of potentially bridging ligand? able to bridge are required for the inner sphere A good test is to compare electron transfer rates of N3- and NCS- mechanism complexes. Most metals can undergo both types E½ (∆Go ’s) of N3- and NCS- complexes are similar. of reactions, inner- sphere is more likely If kN3- / kNCS- ~ 1 (OS). If kN3- / kNCS- >> 1 (IS). This is because N3 - if the metal is very is symmetric. labile (Cr2+) Comparison with experimental data of known reactions helps decide The Marcus equation can be used to predict the rate constants for outer-sphere electron transfer reactions between different species. the reorganization energy for this reaction is the average of the values for the two self-exchange processes, And With the manipulation of the Marcus cross relation Where k12 is the rate constant K12 is the equilibrium constant obtained from ΔrGO k11 and k22 are the respective self-exchange rate constants for the two reaction partners. f≈1  Z = effective collision frequency in solution Z is the constant of proportionality between the encounter density in solution (in moles of encounters per cubic decimetre per second) and the molar concentrations of the reactants; it is often taken to be 1011 mol-1 dm3 s-1.  template reaction - A reaction in which formation of complex places the ligands in the correct geometry for reaction. Features 1. formation of the complex, will bring the reactants closer with the proper orientation for reaction to proceed. 2. Complexiation changes the electronic structure to promote the reaction. 32 C.Template Reactions Template: organizes an assembly of atoms, with respect to one or more geometric loci to achieve a particular linking of atoms Anchor = organizing entity around which the template complex takes shape, due to geometric requirements. This is often a metal ion. Turn = Flexible entity in need of geometric organization before the desired linking can occur Metal complexes make good templates because many metal ions have strict geometric requirements, and they can often be removed easily after the reaction. 2+ O O NH NH2 NH NH2 NiCl2 H H Ni H2O H2O NH NH2 NH NH2 2+ NH N NH HN - NH HN NaBH4 CN Ni Ni H2O H2O NH N NH HN NH HN  An early example is the dialkylation of a nickel dithiolate 34 Synthesis of phtalocyanin 35 Dialkylations that are templated by alkali metals are called crown ethers Many template reactions are only Example: 18-Crown-6 can be synthesized by the stoichiometric, and Williamson ether synthesis using potassium ion as the the removal of the "templating ion" template cation. can be difficult. The alkali metal- templated syntheses of crown ether syntheses are a notable exception. Another complication is that some so-called template reactions proceed similarly in the absence of the templating ion 36 References : 1. “Inorganic Chemistry” Fifth Ed. Gary L. Miessler, Donald A. Tarr, 2004, Pearson Prentice Hall 2. Shriver and Atkins, Inorganic Chemistry, 5th Ed. Peter Atkins, Tina Overtone, Mark Weller et al. 3. Jaiswal Priyanka Balister, www.slideshare.net End of Lectures