L5: Metal allyl complexes

Question 1

Geometry of eta-3 bonding in allyl

Answer 1

• Metal atom lies below the centroid of the allyl plane
• In other terms, the plane of the allyl is perpendicular to the M-L bond
• It has a pz orbital on each carbon perpendicular to the allyl plane; a t.metal has v.o’s of the correct symmetry to interact w/ all 3 of these MO’s

Question 2

How can bonding strength be improved in metal-allyl bonding (and how is the maximised - two routes)

Answer 2

• Psi-2 is the HOMO of the allyl anion - distortions that increase its overlap with the vacant metal pi-orbital will result in stronger bonding
• This can be done through use of a combination of anti and syn substituents to tilt orbitals away/toward metal - pincer like motion from both sides
• Alternatively, the allyl plane itself can be rotated bringing both orbitals simultaneously into better alignment with the M orbitals (5 to 12 degrees required)

Question 3

Dynamic processes in allyl complexes

Answer 3

1. in plane rotation odf the eta-3 allyl ligand
   As a result, two isomers will intra-convert in solution (rotation is lows on NMR timescale; set of resonances from each is observed)
   e.g. Ni(eta3-C3H5)2
2. syn-anti site exchange
   Rapid exchange of the syn and anti hydrogens occurs in some allyl complexes (this can NOT occur as a result of in-plane rotation). Occurs via high energy, eta-1, 16e^- IM, which undergoes 180 degree bond rotation then returns to eta-3 configuration

Question 4

Stabilisation in syn-anti site exchange. Steric considerations for R groups at allyl termini

Answer 4

• The eta-1 IM may be stabilised by interaction w/ a donor solvent molecule
• As a result, it is typically faster in coordinating solvents (THF< MeCN acetone etc.) than non-coordinating (hydrocarbons)
• With R groups at the allyl termini, there is a thermodynamic preference fort hem to be in a syn site (steric grounds), the conversion of which is facilitated by this site exchange via an eta-1 allyl intermediate

Question 5

Synthesis of metal allyl complexes

Answer 5

1. Metal halide and allyl Grignard
2. Metal carbonyl anion and allyl halide (X not part of complex)
3. ‘Oxidative addition’ of allyl halide (X included in complex product)
4. Nucleophilic addition of hydride to a diene complex
5. Deprotonation of an alkene complex

Question 6

Reactions of eta-1 allyl complexes

Answer 6

1. Conversion to eta-3 allyl by ligand dissociation
2. Protonation to eta-2 alkene cation
   …reactions typical of metal alkyl complexes (e.g. M-C bond cleavage by halogens, insertion of CO)

Question 7

Reactivity of eta-3 allyl complexes

Answer 7

• Conversion to eta-1 allyl by ligand addition
• Attack by nucleophile (cationic allyl complexes are prone to nucleophilic attack; nucleophile usually attacks the face away from the metal)