Metallocenes

Question 1

Metallocene

Answer 1

Organometallic compound with a sandwich-like spatial arrangement of ligands, consisting of a transition metal situated between 2 cyclic organic compounds

Question 2

5 common types of sandwich complex

Answer 2

1. Sandwich complexes
2. Half-sandwich complexes
3. Tilted sandwich complexes (ligands out of the plane)
4. More than 2 CnHn ligands per metal (different ligands ligated in different modes)
5. Multidecker-sandwich complexes

Question 3

C3H3+ complexes

Answer 3

Limited number of examples due to the limited number of cyclopropyl ligands
Kettle (1965) example

Question 4

C4H4 complexes

Answer 4

4 different synthesis methods reported:
1. Dehalogenation of a dihalocyclobutene
2. Dimerisation of alkynes
3. Ligand transfer
4. Ring-opening of metallocyclopentenes
Draw all of these

Question 5

Reactions of C4H4 complexes

Answer 5

Very similar reactivity to free aromatic molecules
i.e. C4H4 readily undergoes EAS
Can also do halide abstraction reactions with strong-enough Lewis acids e.g. SbCl5 (wants to be SbCl6)

Question 6

Methods of synthesis of binary Cp complexes (i.e. only Cp ligands)

Answer 6

Cp = C5H5-
1. Metal salt + Cp reagent
MCl2 + (C5H5)2Mg —> Cp2M + MgCl2
(number of Cp ligands around the metal depends on the identity of the metal and its oxidation state)
2. Metal salt + cyclopentadiene (i.e. protonated Cp, may require a base or reducing agent to form Cp)
FeCl2 + 2C5H6 + 2Et2NH —> Cp2Fe + 2[Et2NH2]Cl
(Et3P)Cu(tBuO) + C5H6 —> CpCu(PEt3) + tBuOH

Question 7

Methods of synthesis of Cp metal carbonyl complexes

Answer 7

(can then remove carbonyl ligand after and force further reactivity)
1. Metal carbonyl halide + Cp reagent
[mu-ClRh(CO)2]2 + TlCp —> 2CpRh(CO)2 + TlCl
2. Metal carbonyl + cyclopentadiene
(draw)
3. Install carbonyls onto Cp metal halides
Cp2TiCl2 + Zn + CO + NH3 —> Cp2Ti(CO)2 + [Zn(NH3)4]Cl2
4. Metallocene + CO
5. Metallocene + metal carbonyl
NiCp2 + Ni(CO)4 —> [CpNi(CO)]2 + 2CO (conproportionation reaction)

Question 8

Properties of Cp complexes

Answer 8

C5R5- ligands are not particularly effective in enforcing the 18 electron rule
e.g. VCp2 = 15 VE, CrCp2 = 16 VE, MnCp2 = 17 VE, FeCp2 = 18 VE, CoCp2 = 19 VE, NiCp2 = 20 VE

Question 9

CoCp2

Answer 9

19VE
Powerful reducing agent: CoCp2 —> [CoCp2]+ + e-
[CoCp2]+ is now 18 VE

Question 10

2nd and 3rd row Cp complexes

Answer 10

RuCp2 and OsCp2 are known but most other 2nd and 3rd row analogues are unstable

Question 11

MOs in bent sandwich complexes

Answer 11

Reduced symmetry leads to loss of orbital degeneracy
SALCs are no longer the same energy
Non-bonding (/partially bonding) a’1g is destabilised
e* anti-bonding orbital reduces in energy (therefore complex as a whole is more reactive)
Loss of linearity/symmetry requires energy - can be compensated for by the presence of other ligands to form complexes of the yep Cp2MLn

Question 12

Imide ligand

Answer 12

Isolobal with Cp
Both are 6 electron donors (treating the imide as 2-)
Both have sigma and pi donor interactions
Cp: sigma = a1, pi = e1
Imide: sigma = pz, pi = px, py
Draw as R-N=M

Question 13

Reactions of binary Cp (C5H5-) complexes

Answer 13

Ferrocene, ruthenocene and osmocene undergo electrophilic substitution and Friedel-Crafts acylation

Question 14

Reactions of Cp metal carbonyl complexes

Answer 14

Reduction at the metal using Na/Hg or KC8
Reaction with with halogens at the metal
Substitution at Cp (i.e. cf ferrocene)
Substitution of CO with L
Ziegler-Natta polymerisation

Question 15

Preparation of bis-C6H6 complexes

Answer 15

1. Alkyne trimerisation (cf C4H4 but with slightly different conditions)
2. Metal atom/ligand vapour co-condensation i.e. Ti(g) + C6H6(g) —> Ti(C6H6)2
3. Reductive complexation e.g. TiCl4(THF)2 + 5KC12H10 —> K[Ti(C12H10)2] + C12H10 + 4 KCl
   Can then add 0.5 eq. I2 to form the binary complex Ti(C12H10)2

Question 16

C12H10

Answer 16

Biphenyl

Question 17

Preparation of C6H6 carbonyl complexes

Answer 17

Similar to preparation of Cp carbonyl complexes
1. Metal carbonyl halide + arene + Lewis acid
Mn(CO)5Cl + arene + AlCl3 —> ]Mo(eta6-arene)(CO)3]AlCl4- + 2CO
2. Metal carbonyl + arene
Mo(CO)6 + arene –(UV)–> Mo(eta6-arene)(CO)3 + 3CO
3. Ligand exchange
Cr(arene)(CO)3 + arene’ —> Cr(arene’)(CO)3 + arene

Question 18

Reactivity of neutral binary M(C6H6)2 complexes

Answer 18

1. Sensitive to oxidation because there is no stabilisation of M, M is unlikely to be in a favoured oxidation state
   Replacing one H on the arene with an EWG can allow air-stable complexes to be prepared because electron density is removed from the metal centre, preventing oxidation
   e.g. (eta6-C6H5Cl)2Cr
2. EAS does not occur at the ligand because the metal is more reactive than the ligand - the electrophile E+ readily reacts with and oxidises the metal centre
3. Metallation by strong organometallic reagents occurs more readily than with free benzene (draw scheme)

Question 19

Reactivity of C6H6 carbonyl complexes

Answer 19

Similar reactivity to

1. EAS does occur due to the inductive effect of M(CO)n
2. Nucleophilic attack directly at the ring - ring is more prone to nucleophilic attack because the backbonding COs remove electron density
3. Enhanced acidity of protons on the ring and on any organic ring substituents - prone to being kicked out by electrophiles

Question 20

C4H4 metal-ligand interactions

Answer 20

Draw