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Transition-metal Organometallics. Peter H.M. Budzelaar. Transition metals are never on time. Early Transition Metals Groups 3,4. Strongly electrophilic and oxophilic Few redox reactions (exception: Ti) Nearly always < 18 e Polar and very reactive M-C bonds (to alkyl and aryl)

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transition metals are never on time
Transition metals are never on time

Transition-metal Organometallics

early transition metals groups 3 4
Early Transition MetalsGroups 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

middle transition metals groups 5 7
"Middle" Transition MetalsGroups 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

late transition metals groups 8 10 and 11
Late Transition MetalsGroups 8-10 (and 11)
  • Many accessible oxidation states
  • Mostly 18e or 16e16e 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

front and back benchers
Front- and Back-benchers

Transition-metal Organometallics

going down
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

m h and m c s bonds
M-H and M-C s-bonds

Transition-metal Organometallics

synthesis of metal alkyls
Synthesis of metal alkyls
  • Metathesis
  • Electrophilic attack on metal
  • Insertion

Transition-metal Organometallics

synthesis of metal alkyls1
Synthesis of metal alkyls
  • Oxidative addition
    • often starts with electrophilic attack

Transition-metal Organometallics

decomposition of metal alkyls
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

how to prevent b hydrogen elimination
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

how to prevent b hydrogen elimination1
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

reactions of metal alkyls
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

synthesis of metal hydrides
Synthesis of metal hydrides
  • Metathesis
  • b-elimination
  • Protonation / oxidative addition

Transition-metal Organometallics

structure of wh 6 p i pr 2 ph 3
Structure of WH6(PiPr2Ph)3

Transition-metal Organometallics

synthesis of metal hydrides1
Synthesis of metal hydrides
  • Hydrogenolysis

Transition-metal Organometallics

reactivity of metal hydrides
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

metal aryls
Metal aryls

Usually much more stable than alkyls

Synthesis:

  • Metathesis
  • Oxidative addition

Reaction/decomposition:

  • Reductive elimination

Transition-metal Organometallics

the other ligands
The other ligands...

Common ligands for transition metals:p ligands, CO, phosphines

General characteristic: s-donating, p-accepting, "soft"

Transition-metal Organometallics

p ligands co phosphines
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

donor and acceptor orbitals
Donor and acceptor orbitals

s-donor

p-acceptor

alkene

CO

phosphine

p

p*

p*

LP

s*

LP

Transition-metal Organometallics

p ligands co phosphines1
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

synthesis of co and p ligand complexes
Synthesis of CO and p-ligand complexes

Stable, neutral ligands:generate empty site(s) in presence of free ligand

1.13

1.37

1.417

1.410

2.141

2.223

1.841

1.913

1.157

1.141

Transition-metal Organometallics

synthesis of co and p ligand complexes1
Synthesis of CO and p-ligand complexes

Variation:reductive synthesis

Transition-metal Organometallics

synthesis of co and p ligand complexes2
Synthesis of CO and p-ligand complexes

Anionic ligands:introduce via metathesis (Cp-), substitutionor oxidative addition (allyl)

Transition-metal Organometallics

synthesis of co and p ligand complexes3
Synthesis of CO and p-ligand complexes

Transition-metal Organometallics

reactivity of p ligand complexes
Reactivity of p-ligand complexes

Ligand activated for nucleophilic attack

both internal and external

Transition-metal Organometallics

reactivity of p ligand complexes1
Reactivity of p-ligand complexes

Change of hapticity

Transition-metal Organometallics

more than 18 e
More than 18 e ?

1.47

1.51

1.40

2.33

2.45

1.22

1.15

1.99

2.97

1.95

2.30-2.32

2.46

1.40

2.35

2.32

1.41

1.41-1.44

1.40

Transition-metal Organometallics

s complexes
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

s complex
s complex ?

2.10

1.92

Transition-metal Organometallics

s complex1
s complex ?

Transition-metal Organometallics

s complexes1
s complexes
  • C-Si bonds

2.63

3.22

103°

Transition-metal Organometallics

s complexes2
s complexes
  • Si-H bonds
  • etc: B-H, Sn-Cl, P-H, ..

??

Transition-metal Organometallics

s complexes3
s complexes

A s complex is an (arrested) intermediate for oxidative addition:

Stable s complexes are formed when the metal is notp-basic enough to enable completion of the addition.

Transition-metal Organometallics