LECTURE 5: ORGANOMETALLIC REACTIONS I
This presentation is the property of its rightful owner.
Sponsored Links
1 / 24

LECTURE 5: ORGANOMETALLIC REACTIONS I LIGAND SUBSTITUTION PowerPoint PPT Presentation


  • 97 Views
  • Uploaded on
  • Presentation posted in: General

LECTURE 5: ORGANOMETALLIC REACTIONS I LIGAND SUBSTITUTION. LIGAND SUBSTITUTION. LIGAND SUBSTITUTION. Basic premise about metal-catalyzed reactions: Reactions happen in the coordination sphere of the metal Reactants (substrates) come in, react, and leave again

Download Presentation

LECTURE 5: ORGANOMETALLIC REACTIONS I LIGAND SUBSTITUTION

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Lecture 5 organometallic reactions i ligand substitution

LECTURE 5: ORGANOMETALLIC REACTIONS I

LIGAND SUBSTITUTION


Ligand substitution

LIGAND SUBSTITUTION


Ligand substitution1

LIGAND SUBSTITUTION

Basic premise about metal-catalyzed reactions:

  • Reactions happen in the coordination sphere of the metal

  • Reactants (substrates) come in, react, and leave again

  • Binding or dissociation of a ligand is oftenthe slow, rate-determining step

Ligand Substitution


Ligand substitution2

LIGAND SUBSTITUTION

This premise is not always correct, but it appliesin the vast majority of cases.

Notable exceptions:

  • Electron-transfer reactions

  • Activation of a single substrate for external attack

    • peroxy-acids for olefin epoxidation

    • CO and olefins for nucleophilic attack


2 main mechanistic pathways

2 MAIN MECHANISTIC PATHWAYS

ASSOCIATIVE (A):

LnML’ + L’’ slow LnML’L’’ fast LnML’’ + L’

DISSOCIATIVE (D):

L’’

LnML’ slow LnM fast LnML’’

+ L’


Dissociative ligand substitution

DISSOCIATIVE LIGAND SUBSTITUTION

Example:

Factors influencing ease of dissociation:

  • 1st row < 2nd row > 3rd row

  • d8-ML5 > d10-ML4 > d6-ML6

  • stable ligands (CO, olefins, Cl-) dissociate easily(as opposed to e.g. CH3, Cp).

Ligand Substitution


Dissociative substitution at ml 6

DISSOCIATIVE SUBSTITUTION at ML6

16-e ML5 complexes are usually fluxional;the reaction proceeds with partial inversion, partial retention of stereochemistry.

The 5-coordinate intermediates are normally too reactive to be observed unless one uses matrix isolation techniques.

Ligand Substitution


Associative ligand substitution

ASSOCIATIVE LIGAND SUBSTITUTION

Example:

Sometimes the solvent

is involved.

Reactivity of cis-platin:

Ligand Substitution


Trans influence and trans effect

trans influence and trans effect

  • In square planar complexes, some ligands direct substitution to a position trans to themselves.

  • When reaction is controlled by factors influencing the ground state energy of the complex – trans influence

  • Reaction is controlled by factors affecting the transition state energy.


Trans influence

trans influence

Ligands that form strong  bonds or  tend to weaken the metal ligand bond trans to the metal.

In the ground state this property is called the trans influence.

H- > PR3 > SCN- > I-, CH3-, CO, CN- > Br- > Cl- > NH3 > OH-


Trans kinetic effect

trans kinetic effect

  • Tendency of certain ligands to direct incoming groups to trans position with reactions under kinetic control.

    C2H4, CO > CN- > NO2- > SCN- > I- > Br- > Cl- > NH3> OH-


Overall trans effect

overall trans effect

CO, CN-, NO C2H4 > PR3, H- > CH3-, S=C(NH2)2 > Ph- NO2- SCN-, I-, > Br- > Cl- > Py, NH3, OH- H2O


Exercise 7 1

Exercise 7.1


Associative ligand substitution1

ASSOCIATIVE LIGAND SUBSTITUTION

Example:

Sometimes the solvent is involved.

Reactivity of cis-platin:

Ligand Substitution


Lecture 5 organometallic reactions i ligand substitution

rate

Rate = ks[ML4] = k1 [ML4][Y]

Mechanism:

Square pyramidal – trigonal bipyramid – with retention of configuration.


Associative substitution with 18 e systems

Associative substitution with 18 e systems

  • Can occur if the metal can delocalize a pair of electrons onto one of its ligands


Ligand rearrangement

LIGAND REARRANGEMENT

Several ligands can switch between n-e and (n-2)-e situations, thus enabling associative reactionsof an apparently saturated complex:

Ligand Substitution


Dissociative ligand substitution1

DISSOCIATIVE LIGAND SUBSTITUTION

Example:

Rate = k [ML6]

Ligand Substitution


Rate of substitution of ligands

Rate of substitution of Ligands

Rate of substitutions of a particular ligand is a function of ligand type.

Ligands that are nuetral in their free state dissociate rather easily.


Redox induced ligand substitution

Redox-induced ligand substitution

Unlike 18-e complexes, 17-e and 19-e complexes are labile.

Oxidation and reduction can induce rapid ligand substitution.

  • Reduction promotes dissociative substitution.

  • Oxidation promotes associative substitution.

  • In favourable cases, the product oxidizes/reducesthe starting material Þredox catalysis.

Ligand Substitution


Redox induced ligand substitution1

Redox-induced ligand substitution

Initiation by added reductant.

Sometimes, radical abstractionproduces a 17-e species

Ligand Substitution


Photochemical ligand substitution

Photochemical ligand substitution

Visible light can excite an electron from an M-L bonding orbital to an M-L antibonding orbital (Ligand Field transition, LF).This often results in fast ligand dissociation.

Requirement: the complex must absorb, so it must have a colour!

or use UV if the complex absorbs there

Ligand Substitution


  • Login