L6: Metal diene complexes

Question 1

Conformations of conjugate diene ligands

Answer 1

• Cisoid (= on same side); almost always coordinated in this form
• Transoid (= alternate); known but extremely rare

Question 2

Changes to pi-system of diene upon binding to metal

Answer 2

• Electron density removed from diene HOMO (psi-2) by L->M donation
• Electron density enters the diene LUMO (psi-3) by M->L back donation
• C-C bond length changes within the diene (outer C-C bonds lengthen and inner C-C bond shortens)
• Some reorientation of the diene pi-orbitals also occurs to maximise overlap with the metal (terminal substituents bend out of C-4 plane)

Question 3

Dynamic processes in diene complexes

Answer 3

• For early metals, ‘flipping over’ is involved via a purely sigma-bonded metallacyclopentene IM
• In late metals, facile in-plane rotation is observed as the only dynamic process

Question 4

Reactivity of diene complexes

Answer 4

• Less reactive than a free diene
• Resist hydrogenation
• Don’t undergo Diels-Alder reactions
• Cationic diene complexes undergo attack by nucleophiles, forming allyl complexes
• Neutral diene complexes undergo attack by electrophiles (e.g. Addition of H or Friedel-Crafts acylation)

Question 5

Agostic interactions in species formed from dienes

Answer 5

• Protonation (electrophilic addition of H) of some neutral diene complexes in the absence of an additional 2 electron ligand can result in species containing an agostic interaction
• This is where a C-H sigma-bond functions as a 2-electron donor (C-H- -M bridge formed)

Question 6

Davies-Green-Mingos rules; about

Answer 6

• Any pi-hydrocarbon ligand bound to a cationic metal centre is more activated towards attack by a nucleophile
• On addition of a nucleophile the hapticity of any pi-ligand is reduced by 1 unit

Classify ligands by…
1. Ligand hapticity (even or odd)
2. Is the pi-system acyclic (open) or cyclic (closed)

Question 7

D-G-M rules; details

Answer 7

Predicting which pi-bound ligand the nucleophile attacks.
Rules by priority…
1. Even before odd
2. Open before closed
3. For open pi-systems. If even (-enes), Nu attack always ata terminal carbon. If odd (-enyls) Nu attacks at a terminal carbon only if LnM+ is strongly electron withdrawing (not a good pi-base)

Question 8

Why is Cp a good spectator ligand?

Answer 8

• It has both odd hapticity and a closed pi-system
• According to D-G-M rules, it is a v. unlikely target for nucleophilic attack