Organometallic Chemistry

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Organometallic Chemistry

• betweenorganicandinorganic(?)

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Why Organometallic Chemistry ?

• Main use in organic synthesis and (homogeneous)
  catalysis.
• Using metals, you can make complicated organic
  structures that would be hard to make otherwise.
• This is because, compared to "standard organic
  chemistry", metals display new and unusual
  reaction types.
• To understand the "rules" you have to be able to
  "count electrons".

The "18-electron rule"...
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• Organic Chemistry
• more or less covalent C-X bonds
• rigid element environments
• fixed oxidation states
• ?Organometallic Chemistry?
• Inorganic / Coordination Chemistry
• primarily ionic M-X bonds
• variable and often fluxional environments
• variable oxidation states

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Organometallic Chemistryvs. Organic Chemistry

• Many similarities
• Many differences
• Both can be understood"
• You dont have to learn everything by heart, but
  some basic knowledge is required to get anywhere.

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Organometallic Chemistry

• There are many metals !
• The chemistry of Fe is not much more complicated
  than that of C, but after that there are 80 more
  metals... Generalization is important.
• We concentrate on the M sideof the M-C bond, and
  on how to tune its reactivity.
• Applications synthesis, catalysis, materials,
  pharmaceuticals.

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Organic Chemistry

• Chemistry of C-X bonds
• C-C / C-H nearly covalent
• Cd-Xd- polar (partly ionic)
• SN1, SN2 and analogous additions/eliminations
• Electrocyclic reactions
• Woodward-Hoffmann rules

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Organic Chemistry

• Carbon is tetrahedral, trigonal or linear
• Reactive intermediates / transition states

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Organometallic ChemistryMain group metals

• s and p orbitals.
• 8-e rule, usually.
• Metal is the "d" side of the M-C bond.
• "Umpolung" of the organic fragment.

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Organometallic ChemistryMain group metals

• More electropositive and largerhigher
  coordination numbers,regardless of the number of
  electrons.
• Early" groups and not very electropositive
  lower coordination numbers.

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Organometallic ChemistryMain group metals

• Multiple bonds relatively weak and reactive.
• C-C 85 CC 150N-N 40 NN 100P-P 50 PP 75

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Organometallic ChemistryTransition metals

• s, p and d orbitals
• 18-e rule, sometimes 16 eother counts relatively
  rare

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Organometallic ChemistryTransition metals

• Metal is usually "d"
• Metals are sterically saturated

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Organometallic ChemistryTransition metals

• Often ligands capable of donating 2-8 electrons
• Preference for p-system ligands
• Bonds relatively weak
• Catalysis !

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Forbidden reactions ?
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Reactivity of the M-C bond

• Polar Þ reactive towards e.g.
• Water
• Me3Al explodes with water Me4Sn does not react.
• Oxygen
• Me2Zn inflames in air Me4Ge does not react.
• Carbonyl groups
• MeLi adds at -80C, Me3Sb not even at 50C.

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Oxidation and hydrolysislarge driving force

• Al-C 65 As-C 55 Si-C 74
• Al-O 119 As-O 72 Si-O 108
• Al-Cl 100 Si-Cl 91

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Type of reactions in organometallic chemistry (1)

• Ligand dissociation / coordination

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Type of reactions in organometallic chemistry (2)

• Metal as a ligand

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Type of reactions in organometallic chemistry (3)

• Insertion
• b-elimination

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Type of reactions in organometallic chemistry (4)

• Oxidative addition / Reductive elimination

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Type of reactions in organometallic chemistry (5)

• s-bond metathesis

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Type of reactions in organometallic chemistry (6)

• Redox reactions
• Homolysis

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Type of reactions in organometallic chemistry (7)

• Reactivity of coordinated ligands

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Factors governing structure and reactivity of
organometallic compounds

• M-C, M-X bond strengths
• Electronegativity of M
• Number of (d) electrons
• Coordination number
• Steric hindrance

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Trends in the periodic table

• Main group metals
• left and down more electropositive
• down higher oxidation states less stable
• Transition metals
• middle strongest preference for 18-e
• 2nd and 3rd row strong preference forpaired
  electrons (low-spin states)
• down higher oxidation states more stable

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Techniques ofOrganometallic Chemistry

• Xray diffraction Þ structure Þ bonding
• NMR Þ structure en dynamic behaviour
• (calculations)
• IR
• MS
• EPR

• Not
• GC
• LC