Metal-allyl complexes

Question 1

Why are they important?

Answer 1

Important precursors/intermediates during homogeneous catalysis catalytic cycles

Question 2

General propertjes of allyl ligands

Answer 2

Can be anionic, neutral (radicals) or cationic
Contain an even number of valence electrons (pi electrons)
Contain an odd number of carbon atoms (3C and 5C most important)
Unsaturated
Binary allyl complexes are labile - the metal is released by dimerisation of the organic ligand (i.e. so are important precursors for homogeneous catalysis)

Question 3

Methods of synthesis of metal-allyl complexes (with anionic allyl ligand)

Answer 3

1. Substitution
2. Sigma-allyl to pi-allyl rearrangement
3. Pi-olefin to pi-allyl rearrangement

Question 4

1. Substitution reactions for synthesis of metal-allyl complexes

Answer 4

1. X- ligand replaced by allyl- ligand
2. Metal salt + organometallic reagent = a type of salt metathesis (d.f.), good way of preparing binary allyl complexes (therefore synthesis and isolation must be carried out at low T because these complexes are thermolabile - will decompose to give oligomers of the allyl ligand and M(0))

Question 5

2. Sigma-allyl to pi-allyl rearrangement for synthesis of metal-allyl complexes

Answer 5

1. Carbonyl metalate + allyl halide. Driving force = salt formation
2. Metal hydride + diolefin

Question 6

Metalate

Answer 6

= any complex anion that contains a metal ligated to several atoms/small ligands

Question 7

3. Pi-olefin to pi-allyl rearrangement for synthesis of metal-allyl complexes

Answer 7

1. Electrophilic/nucleophilic addition to diolefins (requires a Lewis base to donate electron density and provide a more stable coordination number)
2. Hydrogenation of an electron-excess complex

Question 8

Psi1

Answer 8

0 nodes

Sigma-bond from ligand to metal s, pz and dz2 orbitals

Question 9

Psi2

Answer 9

1 node
(HOMO?)
Pi-bond from ligand to metal py and dyz orbitals

Question 10

Psi3

Answer 10

2 nodes
(LUMO?)
Pi-backbonding from metal to ligand px and dxz

Question 11

Metal-ligand interactions in metal-allyl complexes

Answer 11

The MO interactions result in an electronic rotational barrier
Syn and anti protons are inequivalent, although dynamic behaviour is often observed in solution so they appear equivalent on the NMR timescale

Question 12

Dynamic behaviour in metal-allyl complexes

Answer 12

Pi-sigma-pi rearrangement

C-C and C-M bond rotation in the sigma-complex allows syn and anti protons to change positions

Question 13

Reactions of metal-allyl complexes

Answer 13

MOST metal-allyl complexes are attacked by nucleophiles (basically anything with a lone pair)
Fe allyl anions attack electrophiles (i.e. Fe attacks E+, the complex itself acts as a nucleophile)

Question 14

Dienyl and trienyl allyl complexes

Answer 14

Eta5 and eta7 complexes are prepared using similar reactions to those of the eta3 complexes
e.g. hydride addition to an eta6 ligand —> goes to eta5 allyl

Question 15

Pentadienyl complexes

Answer 15

e.g. Cp

Can exist as: closed (i.e. Cp itself), open bridged and open

Question 16

Open pentadienyl complexes

Answer 16

Opening leads to more steric bulk - open metallocenes are more sterically shielded
Form coordinatively unsaturated species i.e. they are less likely to form (eta5-C5H7)2MLn
More stable than closed analogues

Question 17

Why are complexes formed with open pentadienyls more stable than closed pentadienyls?

Answer 17

Ring opening perturbs the pi MOs on Cp
Cleaving the bonding pi MOs results in their energy being raised
Cleaving the associated anti-bonding MOs lowers their energy
The HOMO/LUMO gap decreases - therefore open pentadienyls are better pi donors (higher energy HOMOs) and better sigma acceptors (lower energy LUMO)
An open cyclopentadienyl therefore has stronger M-L bonds, but the ligand is also more reactive (c.f. Cp which is less reactive because it is aromatic and wants to preserve this)

Question 18

DGM rules

Answer 18

Can predict the direction of kinetically controlled nucleophilic attack at 18 VE cationic complexes

Question 19

Applying DGM rules

Answer 19

1. Even coordinated polyenes react first (i.e. eta2/4/6)
2. Open polyenes react preferentially over closed polyenes
   i. e. so the order is Even Open, Even Closed, Odd Open, Odd Closed
3. For even and open polyenes, attack takes place at the terminal carbon
4. For odd and open polyenes, attack only takes place at the terminal carbon if MLn+ is a strongly electron-withdrawing fragment

Question 20

C3H5 radical nucleophilic attack

Answer 20

C3H5 radical = ‘odd & open’
Ligand HOMO is singly occupied and low lying so can easily accept electron density from other ligands on the metal (to become C3H5-) or can have electron density removed from the HOMO by other ligands on the metal (to become C3H5+)

Question 21

C4H6 nucleophilic attack

Answer 21

C4H6 = ‘even & open’
LUMO is high energy so cannot accept electron density
Therefore, the only choice is to remove electron density from the HOMO, meaning the terminal carbon becomes delta+

Question 22

DGM points to note

Answer 22

Regio- and chemoselectivity of attack is determined by the unequal positive charge on ligand carbon atoms as a result of metal coordination
(In free alkyls/polyenes/cyclic systems, the charge distribution is approx. equal)
Open polyenes are attacked preferentially over closed polyenes because the charge distribution is less even