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Transition-metal Organometallics
Peter H.M. Budzelaar
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Transition metals are never on time
Transition-metal Organometallics
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Early Transition Metals Groups 3,4
Strongly electrophilic and oxophilic Few redox reactions (exception: Ti) Nearly always < 18e Polar and very reactive M-C bonds (to alkyl and aryl) Few d-electrons: preference for "hard" s-donors (N/O/F) weak complexation of p-acceptors (olefins, phosphines) Typical catalysis: Polymerisation Transition-metal Organometallics
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"Middle" Transition Metals Groups 5-7
Many accessible oxidation states Mostly 18e Ligands strongly bound Strong, not very reactive M-C bonds Preference for s-donor/p-acceptor combinations (CO!) Typical catalysis: Alkene and alkyne metathesis Transition-metal Organometallics
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Late Transition Metals Groups 8-10 (and 11)
Many accessible oxidation states Mostly 18e or 16e 16e common for square-planar complexes Easy ligand association/dissociation Weak, not very reactive M-C bonds Even weaker, reactive M-O/M-N bonds Preference for s-donor/weak p-acceptor ligands (phosphines) Typical catalysis: Hydroformylation Transition-metal Organometallics
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Front- and Back-benchers
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Going down... 1st row: often unpaired electrons
different spin states (HS/LS) accessible "highest possible" oxidation states not very stable MnO4- is a strong oxidant 2nd/3rd row: nearly always "closed shell" virtually same atomic radii (except Y/La) highest oxidation states fairly stable ReO4- is hardly oxidizing 2nd row often more reactive than 3rd Transition-metal Organometallics
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M-H and M-C s-bonds Transition-metal Organometallics
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Synthesis of metal alkyls
Metathesis Electrophilic attack on metal Insertion Transition-metal Organometallics
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Synthesis of metal alkyls
Oxidative addition often starts with electrophilic attack Transition-metal Organometallics
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Decomposition of metal alkyls
Dominant: b-hydrogen elimination Alternatives: homolysis a/g/d-eliminations reductive elimination (especially with H or another alkyl) ligand metallation Transition-metal Organometallics
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How to prevent b-hydrogen elimination ?
No b-hydrogen CH3, CH2CMe3, CH2SiMe3, CH2Ph No empty site cis to alkyl Product of elimination unstable Transition-metal Organometallics
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How to prevent b-hydrogen elimination ?
Planar transition state inaccessible even for 5-membered metallacycles b-elimination is difficult ! (basis of selective ethene trimerization) Transition-metal Organometallics
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Reactions of metal alkyls
Insertion, of both polar and non-polar C=X bonds: olefins, acetylenes, allenes, dienes (ketones etc) CO, isocyanides Reductive elimination Transition-metal Organometallics
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Synthesis of metal hydrides
Metathesis b-elimination Protonation / oxidative addition Transition-metal Organometallics
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Structure of WH6(PiPr2Ph)3
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Synthesis of metal hydrides
Hydrogenolysis Transition-metal Organometallics
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Reactivity of metal hydrides
"Hydride is the smallest alkyl" Can react as H+ or H- HCo(CO)4 nearly as acidic as H2SO4 Cp2ScH gives H2 with acids, alcohols, ... Insertion reactions CO insertion rare (endothermic!) Transition-metal Organometallics
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Metal aryls Usually much more stable than alkyls Synthesis: Metathesis
Oxidative addition Reaction/decomposition: Reductive elimination Transition-metal Organometallics
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The other ligands... Common ligands for transition metals: p ligands, CO, phosphines General characteristic: s-donating, p-accepting, "soft" Transition-metal Organometallics
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p ligands, CO, phosphines
s-donor character: phosphines > alkenes, CO p-acceptor character: CO > alkenes > phosphines Depends strongly on the substituents on P, C=C ! Transition-metal Organometallics
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Donor and acceptor orbitals
s-donor p-acceptor phosphine CO alkene p p* p* LP s* LP Transition-metal Organometallics
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p ligands, CO, phosphines
CO is one of the best p-acceptors (p-acids) isocyanides are stronger donors, weaker acceptors PMe3 very weak p-acceptor, good s-donor PF3 nearly as strongly p-acidic as CO C2H4 weak p-acceptor C2(CN)4 very strong p-acceptor even forms stable radical anions Transition-metal Organometallics
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Synthesis of CO and p-ligand complexes
Stable, neutral ligands: generate empty site(s) in presence of free ligand 1.37 1.13 1.410 1.417 2.223 2.141 1.913 1.841 1.157 1.141 Transition-metal Organometallics
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Synthesis of CO and p-ligand complexes
Variation: reductive synthesis Transition-metal Organometallics
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Synthesis of CO and p-ligand complexes
Anionic ligands: introduce via metathesis (Cp-), substitution or oxidative addition (allyl) Transition-metal Organometallics
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Synthesis of CO and p-ligand complexes
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Modification of p ligands by H+/H- addition/abstraction
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Reactivity of p-ligand complexes
Ligand activated for nucleophilic attack both internal and external Transition-metal Organometallics
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Reactivity of p-ligand complexes
Change of hapticity Transition-metal Organometallics
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More than 18 e ? 1.47 1.51 1.40 2.33 2.45 1.22 1.95 1.15 2.97 1.99 2.46 1.40 2.35 2.32 1.41 1.40 Transition-metal Organometallics
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s complexes A s bond as 2-electron donor for a metal.
H2 complexes (non-classical hydrides) C-H bonds Usually intramolecular Sometimes intermolecular In "matrix" Characterized by IR Transition-metal Organometallics
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s complex ? 2.10 1.92 Transition-metal Organometallics
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s complex ? Transition-metal Organometallics
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s complexes C-Si bonds 2.63 3.22 103° Transition-metal Organometallics
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s complexes ?? Si-H bonds etc: B-H, Sn-Cl, P-H, ..
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s complexes A s complex is an (arrested) intermediate for oxidative addition: Stable s complexes are formed when the metal is not p-basic enough to enable completion of the addition. Transition-metal Organometallics
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