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Organometallic Chemistry an overview of structures and reactions. Peter H.M. Budzelaar. Between organic and inorganic. Organic chemistry: more or less covalent C-X bonds rigid element environments fixed oxidation states (better: valencies ) ??Organometallic chemistry??

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between organic and inorganic
Between organic and inorganic...

Organic chemistry:

  • more or less covalent C-X bonds
  • rigid element environments
  • fixed oxidation states (better: valencies)

??Organometallic chemistry??

Inorganic chemistry:

  • primarily ionic M-X bonds, dative M-L bonds
  • variable and often fluxional environments
  • variable oxidation states

Overview of Organometallic Chemistry

organometallic reactivity
Organometallic reactivity

Since most organometallics are intermediates, the focus in organometallic chemistry is usually on understanding and tuning reactivity

This starts with analyzing reaction mechanisms in terms of elementary steps

The number of possible elementary steps is larger than in"pure organic" chemistry, but the ideas are similar

Overview of Organometallic Chemistry

organometallic structures
Organometallic structures

Knowledge of inorganic and coordination chemistry is useful to understand geometries, electron counts and oxidation states of organometallic compounds

Organometallics are more covalent and often less symmetricthan coordination compounds, so orbital symmetry argumentsare not as important

Overview of Organometallic Chemistry

trends in organometallic chemistry
Trends in organometallic chemistry

Organometallic chemistry is concerned with all metals, in combination with all "organic" elements.

there are many metals !

Generalization is important

the chemistry of e.g. Fe is not much more complicatedthan that of C, but after that there are 80 more metals...

we divide reactions in broader categories than organic chemists do

We concentrate on the M side of the M-C bond,and on how to tune its reactivity

Overview of Organometallic Chemistry

elements of interest
Elements of interest

Organic elements

Main group metals

Transition metals

Overview of Organometallic Chemistry

organic vs organometallic reactivity
Organic vs organometallic reactivity

Organic chemistry:

C-C / C-H : nearly covalent

Cd+-Xd- : polar (partly ionic)

reactivity dominated by nucleophilic attack at C

SN2 and SN1 like reactivity

Organometallic chemistry:

C is the negative end of the M-C bond ("umpolung")

reactivity dominated by electrophilic attack at Cor nucleophilic attack at M

associative and dissociative substitution at M

Overview of Organometallic Chemistry

main group organometallics
Main-group organometallics

s and p orbitals.

8-e rule, usually.

with a lot of exceptions

More electropositive and larger:higher coordination numbers,regardless of the number of electrons.

“Early" groups and not very electropositive:lower coordination numbers.

8-e

10-e

4-e

Overview of Organometallic Chemistry

main group organometallics9
Main-group organometallics

Metal is the "d+" side of the M-C bond.

Chemistry dominated by nucleophilic attack of Cd- at electrophiles.

this is also the main application in organic synthesis

note: this is a simplified pictureof the Grignard reaction

Overview of Organometallic Chemistry

main group organometallics10
Main-group organometallics

M-M multiple bonds are fairly weak and rather reactive

they are a curiosity and relatively unimportant,certainly compared to C-C multiple bonds

Bond strengths in kcal/mol:

C-C 85 C=C 150

N-N 40 N=N 100

P-P 50 P=P 75

Multiple-bonded compounds often have unusual geometries

Overview of Organometallic Chemistry

transition metal organometallics
Transition-metal organometallics

s, p and d orbitals

18-e rule, sometimes 16-eother counts relatively rare

18-e

18-e

18-e

16-e

Overview of Organometallic Chemistry

transition metal organometallics12
Transition-metal organometallics

Lower electron counts if metals are sterically saturated:

12-e

13-e

16-e

Overview of Organometallic Chemistry

transition metal organometallics13
Transition-metal organometallics

Often ligands capable of donating 2-8 electrons

Preference for p-system ligands (good overlap with d-orbitals)

Bonding to neutral ligands (olefin/diene/CO/phosphine)relatively weak

Important for catalysis!

Overview of Organometallic Chemistry

transition metal organometallics14
Transition-metal organometallics

An olefin complex: (Acac)Ir(NCMe)(C2H4)

donation

backdonation

Overview of Organometallic Chemistry

transition metal organometallics15
Transition-metal organometallics

“Forbidden” reactions ?

Overview of Organometallic Chemistry

reactivity of the m c bond
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 -80°C, Me3Sb not even at +50°C.

Overview of Organometallic Chemistry

reactivity of the m c bond17
Reactivity of the M-C bond

Oxidation and hydrolysis: large driving force

Bond strengths in kcal/mol:

Al-C 65 As-C 55 Si-C 74

Al-O 119 As-O 72 Si-O 108

Al-Cl 100 Si-Cl 91

Overview of Organometallic Chemistry

organometallic reaction steps
Organometallic reaction steps

Ligand dissociation / coordination

note: free Me3Aldimerizes to Me6Al2

Overview of Organometallic Chemistry

organometallic reaction steps19
Organometallic reaction steps

Insertion and b-elimination

Overview of Organometallic Chemistry

organometallic reaction steps20
Organometallic reaction steps

Insertion and b-elimination

Overview of Organometallic Chemistry

organometallic reaction steps21
Organometallic reaction steps

Oxidative addition / Reductive elimination

Overview of Organometallic Chemistry

organometallic reaction steps22
Organometallic reaction steps

Oxidative addition / Reductive elimination

Overview of Organometallic Chemistry

organometallic reaction steps23
Organometallic reaction steps

s-bond metathesis

Overview of Organometallic Chemistry

organometallic reaction steps24
Organometallic reaction steps

Redox reactions

Homolysis

Overview of Organometallic Chemistry

organometallic reaction steps25
Organometallic reaction steps

Reactivity of coordinated ligands

Overview of Organometallic Chemistry

factors governing structure and reactivity of organometallic compounds
Factors governing structure and reactivityof organometallic compounds
  • M-C, M-X bond strengths
  • Electronegativity of M (polarity of M-C etc bonds)
  • Number of (d) electrons
  • Coordination number
  • Steric hindrance

Overview of Organometallic Chemistry

trends in the periodic table
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

Overview of Organometallic Chemistry

working with organometallics
Working with organometallics

Synthesis and reactivity studies (inert atmosphere!):

  • Glove box
  • Schlenk line, specialized glassware

Characterization:

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

Not:

  • GC
  • LC

Overview of Organometallic Chemistry