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Dissociative Associative
Principal mechanisms of ligand exchange in octahedral complexes Dissociative Associative
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(5-coordinated intermediate)
MOST COMMON Dissociative pathway (5-coordinated intermediate) Associative pathway (7-coordinated intermediate)
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Experimental evidence for dissociative mechanisms
Rate is independent of the nature of L
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Experimental evidence for dissociative mechanisms
Rate is dependent on the nature of L
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Inert and labile complexes
Some common thermodynamic and kinetic profiles Exothermic (favored, large K) Large Ea, slow reaction Exothermic (favored, large K) Large Ea, slow reaction Stable intermediate Endothermic (disfavored, small K) Small Ea, fast reaction
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Labile or inert? LFAE = LFSE(sq pyr) - LFSE(oct)
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Why are some configurations inert and some are labile?
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Substitution reactions in square-planar complexes
the trans effect (the ability of T to labilize X)
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Synthetic applications
of the trans effect Cl- > NH3, py
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Mechanisms of ligand exchange reactions in square planar complexes
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Electron transfer (redox) reactions
-1e (oxidation) M1(x+)Ln + M2(y+)L’n M1(x +1)+Ln + M2(y-1)+L’n +1e (reduction) Very fast reactions (much faster than ligand exchange) May involve ligand exchange or not Very important in biological processes (metalloenzymes)
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Reactions ca. 100 times faster than ligand exchange
Outer sphere mechanism [Fe(CN)6] [IrCl6]2- [Fe(CN)6] [IrCl6]3- [Co(NH3)5Cl]+ + [Ru(NH3)6]3+ [Co(NH3)5Cl] [Ru(NH3)6]2+ Reactions ca times faster than ligand exchange (coordination spheres remain the same) r = k [A][B] Tunneling mechanism
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Inner sphere mechanism
[Co(NH3)5Cl)]2+:::[Cr(H2O)6]2+ [Co(NH3)5Cl)] [Cr(H2O)6]2+ [Co(NH3)5Cl)]2+:::[Cr(H2O)6]2+ [CoIII(NH3)5(m-Cl)CrII(H2O)6]4+ [CoII(NH3)5(m-Cl)CrIII(H2O)6]4+ [CoIII(NH3)5(m-Cl)CrII(H2O)6]4+ [CoII(NH3)5(m-Cl)CrIII(H2O)6]4+ [CoII(NH3)5(H2O)]2+ + [CrIII(H2O)5Cl]2+ [CoII(NH3)5(H2O)]2+ [Co(H2O)6] NH4+
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than outer sphere electron transfer (bridging ligand often exchanged)
Inner sphere mechanism Reactions much faster than outer sphere electron transfer (bridging ligand often exchanged) r = k’ [Ox-X][Red] k’ = (k1k3/k2 + k3) Tunneling through bridge mechanism
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Organometallic Chemistry
Brooklyn College Chem 76/76.1/710G Advanced Inorganic Chemistry (Spring 2008) Unit 6 Organometallic Chemistry Part 1 General Principles Suggested reading: Miessler/Tarr Chapters 13 and 14
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Elements of organometallic chemistry
Complexes containing M-C bonds Complexes with p-acceptor ligands Chemistry of lower oxidation states very important Soft-soft interactions very common Diamagnetic complexes dominant Catalytic applications
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The d-block transition metals
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Main types of common ligands
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A simple classification of the most important ligands
X L L2 L2X L3
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Counting electrons Method A Determine formal oxidation state of metal
Deduce number of d electrons Add d electrons + ligand electrons (A) Ignore formal oxidation state of metal Count number of d electrons for M(0) Add d electrons + ligand electrons (B) Method B The end result will be the same
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Why is this relevant? Stable mononuclear diamagnetic complexes generally contain 18 or 16 electrons The reactions of such complexes generally proceed through 18- or 16-electron intermediates Although many exceptions can be found, these are very useful practical rules for predicting structural and reactivity properties C. A. Tollman, Chem. Soc. Rev. 1972, 1, 337.
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Why 18 electrons? antibonding
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Organometallic complexes
18-e most stable 16-e stable (preferred for Rh(I), Ir(I), Pt(II), Pd(II)) <16-e OK but usually very reactive > 18-e possible but rare generally unstable
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A closer look at some important ligands
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Typical -donor ligands
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Other important C-donor ligands
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Other important ligands
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Other important ligands
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The M-L-X game
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Each X will increase the oxidation number of metal by +1.
Each L and X will supply 2 electrons to the electron count.
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Now looking at compounds having a charge of +1 to obey 18 e rule.
Elec count: 4 (M) +2 (NO) +12 (L6) = 18 NO+ is isoelectronic to CO X increases O N by 1 Elec Count: 4 (M) + 4 (L2) + 10 (L5)
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Actors and spectators Actor ligands are those that dissociate or undergo a chemical transformation (where the chemistry takes place!) Spectator ligands remain unchanged during chemical transformations They provide solubility, stability, electronic and steric influence (where ligand design is !)
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Organometallic Chemistry 1.2 Fundamental Reactions
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Fundamental reaction of organo-transition metal complexes
D(FOS) D(CN) D(NVE) Association-Dissociation of Lewis acids ±1 Association-Dissociation of Lewis bases ±2 Oxidative addition-Reductive elimination Insertion-deinsertion FOS: Formal Oxidation State; CN: Coordination Number NVE: Number of valence electrons
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Association-Dissociation of Lewis acids
D(FOS) = 0; D(CN) = ± 1; D(NVE) = 0 Lewis acids are electron acceptors, e.g. BF3, AlX3, ZnX2 This shows that a metal complex may act as a Lewis base The resulting bonds are weak and these complexes are called adducts
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Association-Dissociation of Lewis bases
D(FOS) = 0; D(CN) = ± 1; D(NVE) = ±2 A Lewis base is a neutral, 2e ligand “L” (CO, PR3, H2O, NH3, C2H4,…) in this case the metal is the Lewis acid Crucial step in many ligand exchange reactions For 18-e complexes, only dissociation is possible For <18-e complexes both dissociation and association are possible but the more unsaturated a complex is, the less it will tend to dissociate a ligand
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D(FOS) = ±2; D(CN) = ± 2; D(NVE) = ±2
Oxidative addition-reductive elimination D(FOS) = ±2; D(CN) = ± 2; D(NVE) = ±2 Vaska’s compound Very important in activation of hydrogen
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Oxidative addition-reductive elimination
H becomes H- Concerted reaction Vaska’s compound via Ir: Group 9 cis addition CH3+ has become CH3- SN2 displacement trans addition Also radical mechanisms possible
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Oxidative addition-reductive elimination
Not always reversible
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Insertion-deinsertion
D(FOS) = 0; D(CN) = 0; D(NVE) = 0 Mn: Group 7 Very important in catalytic C-C bond forming reactions (polymerization, hydroformylation) Also known as migratory insertion for mechanistic reasons
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Migratory Insertion Also promoted by including bulky ligands in initial complex
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Insertion-deinsertion The special case of 1,2-addition/-H elimination
A key step in catalytic isomerization & hydrogenation of alkenes or in decomposition of metal-alkyls Also an initiation step in polymerization
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Attack on coordinated ligands
Very important in catalytic applications and organic synthesis
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Some examples of attack on coordinated ligands
Nucleophilic addition Electrophilic addition Nucleophilic abstraction Electrophilic abstraction
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