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Ch14 Organometallic Reactions. Reactions Involving Gain or Loss of Ligands Ligand Dissociation and Substitution Ligand (CO) Dissociation can be caused by heat Ligand (CO) Substitution is important for synthesis of new complexes Rate is independent of incoming ligand = D mechanism (for most)

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ch14 organometallic reactions
Ch14 Organometallic Reactions
  • Reactions Involving Gain or Loss of Ligands
    • Ligand Dissociation and Substitution
      • Ligand (CO) Dissociation can be caused by heat
      • Ligand (CO) Substitution is important for synthesis of new complexes
        • Rate is independent of incoming ligand = D mechanism (for most)

Ni(CO)4 Ni(CO)3 18e- to 16e- (slow)

Ni(CO)3 + L Ni(CO)3L 16e- to 18e- (fast)

        • Some organometallic substitutions show a two-term rate law

i) Rate = k1[Complex] + k2[Complex][L]

slide2
k1 is due to the dissociative mechanism
      • k2 is due to a competing associative mechanism
    • Larger metals show more A mechanism; smaller metals mostly D
    • Highly nucleophilic ligands tend to increase A mechanism
  • Phosphine Dissociation
    • Cone Angle is defined for phosphine ligands
slide3
Larger cone angle = larger steric repulsion = faster ligand dissociation
  • Neither Dissociation nor Substitution reactions change the metal oxidation state
slide4
Oxidative Addition
    • Coordination Number and Oxidation State of Metal Increase

LnMm+ + X—Y LnM(M+2)+XY

    • Examples:
slide5
Cyclometallations incorporate metals into organic rings; may be Ox. Add.
  • Reductive Elimination
    • Opposite of Ox. Add.; Oxidation State and Coord. Number are reduced

Ox. Add

Substitution

slide6
Red. Elim. often results in products like: R—H, R—R’, R—X, H—H
  • Steric bulk of phosphine ligands can increase the rate of red. elim.
slide7
Nucleophilic Displacement
    • Nucleophilic Ligands may displace others (Substitution)
    • Nucleophilic Complexes may react with electrophiles

E. Summary

slide8
Reactions Involving Modification of Ligands
    • Insertion Reactions
      • 1,2-Insertions
        • New bonds to adjacent atoms
        • Like 1,2-additions to alkenes in organic chemistry
        • May play a role in some catalyzed polymerizations
slide9
1,1-Insertions
    • Both new bonds are to the same atom of the inserting group
    • Carbonyl Insertion, Carbonyl Migration, or Alkyl Migration?
      • CO Insertion = direct CO insertion from external CO
      • CO Migration = movement of bound CO into M—R bond
      • Alkyl Migration = movement of bound R to bound CO
slide11
Experimental Evidence
    • Free 13CO + complex gives no labeled acyl product

This rules out mechanism #1, CO insertion

    • Reverse reaction gives 100% cis13CO and R

Both mechanism #2 and mechanism #3 are consistent with this result

slide12
Reverse reaction with 13CO cis to acyl group gives 2:1 cis:trans product

Only Mechanism #3 Alkyl Migration is consistent with this data

slide13
Hydride Elimination
    • Transfer of H from ligand to M; formation of double bond in ligand
    • b-elimination is most common, a and g are known
    • Reverse of 1,2-insertion
    • Coordinatively Saturated or Alkyl complexes without b-hydrogens are most stable

C. Abstraction = eliminations with no change in metal coordination number

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