organometallics

Question 1

explain the bonding interaction in metal carbonyls

Answer 1

1) sigma donation from the C lone pair into vacant metal orbital
2) pi back donation from occupied metal d-orbital into vacant CO antibonding pi orbital
3) strengthens the M-C bond but weaken the C=O bond

Question 2

electron counting. explain the donor pair method

Answer 2

find the number of electrons (d^n) group number minus ox state
add electrons donated by ligands
total metal valence electron count

Question 3

explain the neutral ligand method (e counting)

Answer 3

group number of metals and add the electrons donated by ligands (x or l type)

Question 4

explain bonding interaction in metal-alkene

Answer 4

1) sigma donation from alkene pi bonding orbital (HOMO) to vacant metal orbital (LUMO)
2) back donation from occupied metal d-orbital into vacant alkene antibonding pi orbital
3) strengthens the M-alkene bond but weakens the C=C bond

Question 5

metal-alkene applications

Answer 5

catalysis
gas separation, alkene binds to metal but alkane doesn’t
fruit ripening

Question 6

orbital interaction of alkyne complexes

Answer 6

alkyne has two pi-bonding pairs
pi parallel goes to the vacant metal orbital (s/pz/dz2)
pi perpendicular goes to the vacant metal orbital (dyz/py)
back donation from M to alkyn pi antibonding
pi parallel has good overlap with dxz orbital

Question 7

how are cyclic alkynes stabilised by coordination

Answer 7

The coordination of the alkyne to a metal centre can stabilize the triple bond by reducing the bond order of the carbon-carbon triple bond.

Question 8

how does metal-carbon bond polarity change across the periodic table

Answer 8

the bond becomes more covalent across and down the groups. because the difference in electronegativity

Question 9

explain the trends of the stability of metal alkyls

Answer 9

in transition metals, bond strength increases down the group due to better d orbital overlap (5d/4d are bigger and can get involved in the ligand orbitals easier)

Question 10

explain alkylation (addition of CH3) of anionic metal complex

Answer 10

the complex is reduced by Na to create anionic complex
then undergoes an Sn2 reaction with CH3-X
oxidation state increases by 2

Question 11

insertion of alkene into a metal hydride (migratory insertion)

Answer 11

alkene coordinates with the metal
H migrates to the alkene end
the ligand can coordinate to the vacant site

Question 12

explain how the insertion of an alkene can be branched or linear

Answer 12

due to the rotation of the alkene, the H can migrate to either end of the alkene

Question 13

explain Beta-elimination

Answer 13

1. the hydrogen on the beta position of the ligand requires heat
2. transfers to the metal centre followed by the
3. elimination of the alkene

Question 14

how can beta-elimination be avoided

Answer 14

1. alkyl groups have no B-H
2. steric crowding
3. small metallacycles
4. energetically unfavourable products

Question 15

for migratory CO insertion, explain some conditions that affect the rate of reaction

Answer 15

M-CH3 bond strength increases down the group = slower rate
rate increases as the length of R group increases
M-C bond weakens by steric for longer R

Question 16

explain the bonding in schrock carbene complex

Answer 16

Typically high ox state M with sigma/pi donor ligands
1. sigma donation from C sp2 lone pair to M
2. pi donation from C pz to M d orbital

Question 17

explain the bonding in Fischer carbene complexes

Answer 17

1. sigma donation from C sp2 lone pair to M
2. pi back-donation from M to C pz
3. pi donation from X lone pair to C pz

Question 18

the reactivity of Fischer carbene complexes

Answer 18

Carbon is electrophile
allows OR group to be substituted

Question 19

alkene insertion into M-H

Answer 19

the double bond coordinates with the metal and the H can migrate into either end of the C=C which forms an alkane chain off the metal

Question 20

explain agostic interaction

Answer 20

When metal centre is electron deficient it can form a weak intramolecular interaction with the electron pair from a ligand C-H bond
donation from C-H sigma bonding orbital into vacant metal orbital.
backdonation from filled metal d-orbital to vacant C-H antibonding sigma orbital

Question 21

explain the steps in alkene hydrogenation

Answer 21

1. dissociation happens to remove a ligand (14e)
2. oxidative addition (add hydrogen atoms) (16e)
3. alkene coordination (alkene binds to metal centre) (18e)
4. alkene insertion (H binds to one end of the double bond)
5. alkane chain dissociates

Question 21

explain the steps in alkene hydroformylation

Answer 21

1. dissociation
2. alkene coordination (alkene binds to metal centre)
3. alkene insertion (hydrogen has migrated to form alkane)
4. carbonyl coordinates on metal centre
5. CO insertion (CO has migrated to the alkane) (16e)
6. oxidative addition (to make complex 18e)
7. reductive elimination (removes the chain)

Question 22

what is η1-allyl

Answer 22

metal binded to sp3 carbon M-C-C=C
2 electron donor

Question 23

synthesis of allyl (η1 and η3) complexes

Answer 23

1. Sn2 reaction to make η1 and then hv to make η3
2. transmetallation
3. deprotonation of propene ligand

Question 24

how many electrons are donated by η3, η4, η5, η6, η7, η8 (donor pair method)

and donations in the neutral method

Answer 24

η3 = 2e (+ ligand)
η4 = 6e (2- ligand)
η5 = 6e (- ligand)
η6 = 6e (neutral)
η7 = 6e (+ ligand)
η8 = 10e (2- ligand)

CyHy is neutral in a neutral method so electrons donated corresponds to the number on η
η3 = 3 η4 = 4

Question 25

Explain an oxidative addition reaction

Answer 25

Oxidative addition is a reaction when metal complex nucleophile attack CH3-X to coordinate to CH3
X- can coordinate to metal centre