Metal-diolefin complexes

Question 1

Stability of non-conjugated diolefins

Answer 1

Approx. the same as mono-olefin complexes

Question 2

Why do conjugated dienes give stable complexes?

Answer 2

Due to the larger number of pi MO diene-metal orbital interactions possible

Question 3

Psi1

Answer 3

0 nodes

Sigma bond to metal s, pz and dz2 orbitals

Question 4

Psi2

Answer 4

HOMO
1 node
Pi bond to metal py and dxy orbitals (empty)

Question 5

Psi3

Answer 5

LUMO
2 nodes
Pi-backbonding from metal px and dxz into pi* orbitals (population of Psi3)

Question 6

Psi4

Answer 6

3 nodes
Delta-backbonding from metal dyz into Psi4
Overlap less efficient, weak interaction

Question 7

Effect of pi-interactions on C-C bond lengths

Answer 7

Lead to terminal C-C lengthening (less electron density, less ‘bonding character’, lower bond order)
Lead to internal C-C shortening (more electron density, more ‘bonding character’, higher bond order)

Question 8

2 canonical forms for describing the bonding in conjugated dienes

Answer 8

1. ~metallocyclopentene
2. Classic eta4 ‘butadiene’ (2 ways of drawing)

Which one depends on identity of metal and other ligands present

Question 9

Bonding in conjugated diene complexes of early TMs

Answer 9

Generally take the ‘metallocyclopentene’ form

Question 10

Bonding in conjugated diene complexes of late TMs

Answer 10

Generally the classic eta4 ‘butadiene’ form

Question 11

Fe(CO)3(butadiene)

Answer 11

Fe = mid to late TM, classic eta4 bonding
Conformationally labile (COs exchange at RT)
Configurationally stable (maintains same geometry overall, approx. tetrahedral)

Question 12

Zr(Cp)2(subs-butadiene)

Answer 12

Zr = early TM
Configurationally labile (non-rigid) - chemical bonding mode can change
Can form metallocyclopentene which can undergo a ring flip, resulting in racemisation

Question 13

Reactivity of conjugated olefins

Answer 13

Unreactive compared to the free olefin (i.e. non-coordinated olefin) i.e. no hydrogenation reactivity, no Diels-Alder, not susceptible to nucleophilic attack

Question 14

Reactions of conjugated olefins

Answer 14

Can react with strong electrophiles e.g. strong acids with really low pKa e.g. HBF4 (pKa = -10)
e.g. Fe(CO)3(butadiene) + HBF4 (in CO atm [or any atm that is coordinating]) —> diene is protonated, extra CO added to fill valence shell/stabilise otherwise low-coordinate complex (because coordination mode decreased from eta4 to eta3)
This electron deficit at the metal can also be compensated for by agostic interactions
NOTE: Metal will only exist in the pi-allyl complex form if it is electron rich, if metal is electron poor it needs more electron density so is more likely to form an agostic interaction

Question 15

Agostic interaction

Answer 15

Requires a coordinatively unsaturated TM with a ligand containing a C-H bond
The 2 electrons in the C-H bond interact with the empty d-orbital on the TM, resulting in a 3c-2e bond
If metal is really electron poor, oxidative addition can occur and metal hydride forms

Question 16

Non-conjugated olefins

Answer 16

Substituents on the olefin influence the stability of the complex (EWG increase stability through increased backbonding, EDG decrease stability by pushing electron density towards metal and weakening M-L bond)
If the 2 ends of the tether can isomerise, the most stable complex is formed by the cis isomer (rather than the double bonds being trans to one another)
Ring strained complexes are surprisingly stable (stability also increased by their ability to form stable chelate complexes)

Question 17

Reactions of non-conjugated olefins

Answer 17

Basically the same as mono-olefins i.e. substitutions, nucleophilic attack
Draw