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Ch14 Organometallic ReactionsI. Reactions Involving Gain or Loss of Ligands

A. Ligand Dissociation and Substitution1) Ligand (CO) Dissociation can be caused by heat

2) Ligand (CO) Substitution is important for synthesis of new complexesa) 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)

b) Some organometallic substitutions show a two-term rate lawi) Rate = k 1[Complex] + k 2[Complex][L]

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ii) k 1 is due to the dissociative mechanismiii) k 2 is due to a competing associative mechanism

c) Larger metals show more A mechanism; smaller metals mostly Dd) Highly nucleophilic ligands tend to increase A mechanism

3) Phosphine Dissociationa) Cone Angle is defined for phosphine ligands

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b) Larger cone angle = larger steric repulsion = faster ligand dissociation

4) Neither Dissociation nor Substitution reactions change the metal oxidation state

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B. Oxidative Addition1) Coordination Number and Oxidation State of Metal Increase

LnMm+ + X Y L nM (M+2)+ XY

2) Examples:Fe(CO) 5

Dissociation Fe(CO) 4Ox. Add.

I2cis -Fe(CO) 4I2

Fe0, 18e - Fe0, 16e - Fe2+ , 18e -

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3) Cyclometallations incorporate metals into organic rings; may be Ox. Add.

C. Reductive Elimination1) Opposite of Ox. Add.; Oxidation State and Coord. Number are reduced

Ox. Add

Substitution

Ox. Add.(Cp) 2TaH + H 2

Red. Elim.

(Cp) 2TaH 3

Ta 3+ , 16e - Ta 5+ , 18e -

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2) Red. Elim. often results in products like: R H, R R, R X, H H

3) Steric bulk of phosphine ligands can increase the rate of red. elim.

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D. Nucleophilic Displacement1) Nucleophilic Ligands may displace others (Substitution)2) Nucleophilic Complexes may react with electrophiles

E. Summary

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II. Reactions Involving Modification of LigandsA. Insertion Reactions

1) 1,2-Insertionsa) New bonds to adjacent atoms

b) Like 1,2-additions to alkenes in organic chemistry

c) May play a role in some catalyzed polymerizations

M CH 2R + H2C CH 2 M CH 2R

H2C CH 2

1,2-InsertM CH 2CH 2CH 2R

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2) 1,1-Insertionsa) Both new bonds are to the same atom of the inserting group

b) Carbonyl Insertion, Carbonyl Migration, or Alkyl Migration?

i. CO Insertion = direct CO insertion from external CO

ii. CO Migration = movement of bound CO into M R bond

iii. Alkyl Migration = movement of bound R to bound CO

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c) Experimental Evidencei. Free 13CO + complex gives no labeled acyl product

This rules out mechanism #1, CO insertion

ii. Reverse reaction gives 100%cis

13

CO and RBoth mechanism #2 and mechanism #3 are consistent with this result

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iii. Reverse reaction with 13CO cis to acyl group gives 2:1 cis:trans productOnly Mechanism #3 Alkyl Migration is consistent with this data

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B. Hydride Elimination1) Transfer of H from ligand to M; formation of double bond in ligand2) b -elimination is most common, a and g are known

3) Reverse of 1,2-insertion

4) Coordinatively Saturated or Alkyl complexes without b-hydrogens are most stableC. Abstraction = eliminations with no change in metal coordination number

+

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