Organometallics

Question 1

What is an L and X type ligand

Answer 1

L - neutral
X - anionic (-)

Question 2

How many valence electrons does a transition metal tend to have?

Answer 2

18

Question 3

What does μx mean

Answer 3

Bridging - x designates the number of metal centers bridged by the ligand

Question 4

What is ηx

Answer 4

Hapticity
the x superscript indicates the number of ‘points of contact’ between the ligand and metal in a continuous sequence.

Question 5

What is κx

Answer 5

Denticity (kx)
the x superscript indicates the number of ‘points of contact’ between the ligand and metal.

Question 6

Total valence electron count =

Answer 6

dn count + electrons donated by the ligand + number of M-M bonds

Question 7

Which is stronger M-H or M-Me

Answer 7

M-H
better overlap with spherical 1s orbital no nonbonding electron repulsions and minimal steric repulsion for H

Question 8

What is the trend of M–Me bond strength DOWN a group

Answer 8

The M–Me bond strength INCREASES DOWN a group
Overlap between the C(sp3) hybrids and TM d orbitals improves with increasing principal quantum number

Question 9

Describe:
σ bonds
π bonds
δ bonds

Answer 9

σ bonds
head-on overlapping between atomic orbitals

π bonds
lateral overlapping of two lobes of an atomic orbital with two lobes of another atomic orbital

δ bonds
covalent chemical bonds, where four lobes of one involved atomic orbital overlap four lobes of the other involved atomic orbital

Question 10

What are pi donor ligands

Answer 10

Ligands capable of π-donation typically have lone pairs of electrons that can be shared with the metal center, engaging in back-donation from the metal. These ligands generally have atoms with lone pairs that can overlap with empty or partially filled d-orbitals on the metal

Question 11

What are pi acceptor ligands

Answer 11

are ligands that can accept electron density from a metal center through back-donation. This involves the metal center donating electron density from its filled d-orbitals into the empty π* (antibonding) orbitals of the ligand. This interaction stabilizes the metal-ligand complex, especially for metals in lower oxidation states or with high electron density.

Question 12

When is t2g raised, lowered in energy

Answer 12

p-donor DECREASES Δ0 (electron density transferred from ligand, so t2g set RAISED in energy).

p-acceptor INCREASES Δ0 (electron density transferred to ligand p* orbital so t2g set LOWERED in energy.

Question 13

What is back bonding

Answer 13

the transfer of electron density from a filled metal d-orbital to an empty or partially filled π* (antibonding) orbital of the ligand. This can stabilize both the metal center and the ligand, resulting in a more stable complex

Question 14

What are the consequences of back bonding

Answer 14

• The metal-ligand bond is often strengthened due to the synergistic interaction between σ-donation from the ligand to the metal and π-back donation from the metal to the ligand

-internal bonds within the ligand becomes longer and weaker because there’s more electron density in their anti-bonding orbitals

• changes seen in IR e.g the C-O stretching frequency in IR spectra shifts to lower wavenumbers in metal carbonyl complexes due to weakened C-O bonds.

Question 15

Give the consequences of back donation on cyclobutadiene

Answer 15

• planar C4R4
• equal C-C bond lengths
• reduction to the 6π aromatic dianion (LX2)

Question 16

What are ‘Bent’ metallocenes and why do they form

Answer 16

• the geometry around the central metal atom is not linear but instead adopts a bent or non-linear configuration

mostly seen in d0 and d10 because there are no d-electrons to stabilize a linear arrangement through π-bonding with the cyclopentadienyl (Cp) rings

Question 17

what type of ligand are arenes

Answer 17

6 electron donors - L3 type

Question 18

Explain M-H position in 1H NMR spec

Answer 18

• hydrides generally experience a strong shielding effect: resonances far upfield of 0 ppm are diagnostic for M-H

more electron rich metal centere = more upfield (shielded) hydride resonance

bridging hydrides are more shielded

Question 19

What is a Fluxional molecule

Answer 19

A fluxional molecule is one that undergoes a dynamic molecular process that
interchanges two or more chemically and/or magnetically different groups in a molecule.

Question 20

How do we know a process is operative

Answer 20

• broadened lines
• temperature dependance
• field dependence
• spectra that are too simple/ complicated for expected structure

Question 21

What are carbenes

Answer 21

A molecule containing a neutral carbon atom with a valence of 2 and 2 unshared valence electrons

Question 22

Describe Fischer carbenes

Answer 22

σ-donation: from the HOMO of carbene to an
empty metal orbital of correct symmetry

π-back donation: from a filled metal orbital to the empty π-orbital of Carbene

Lone pair on E can stabilize the empty π orbital on Carbene resulting in a 3c4e bond -> partial M=C bond

Question 23

Describe Schrock alkylidenes

Answer 23

σ-bond: interaction of the electron in the sp2
hybrid Ccarbene and an unpaired electron on metal

π-bond: from the electron in the p orbital with an unpaired electron on the metal centre.

Bonding is analogous to formation of ethylene from two triplet methylene (CH2) fragments -> True M=C double bond

Question 24

What do X-ray crystallographic structures show

Answer 24

• triganal planar sp2 hybridised C centre
• M=C shorter than a single bond but not as short as a M-CO bond

Question 25

What stabilisation is seen in NHCs

Answer 25

Inductive effects through σ framework
Mesomeric effects through π framework

Question 26

describe the Metathesis reactions

Answer 26

Metal salt + organic nucleophile

anionic metal complex + organic electrophile

Question 27

Describe Protonolysis and hydrogenolysis

Answer 27

Essentially an acid-base reaction

Question 28

Describe reductive routes

Answer 28

Useful for complexes with neutral hydrocarbon ligands (alkenes, alkynes, dienes and arenes) as well as for preparation of related phosphine and carbonyl complexes

Metal precursor + reducing agent -> Mx(L)Y

Question 29

Describe insertion reactions

Answer 29

Common for the synthesis of alkyl, vinyl and allyl complexes

reversible (opposite of β-elimination)

Question 30

Describe Oxidative addition

Answer 30

Common synthetic route to s-bound carbon and hydrogen ligands (alkyl, aryl, hydrides, etc)

Gringard synthesis

common for square planar complexes

Question 31

Describe ligand subsitutions

Answer 31

Common synthetic route for neutral ligands (arenes, olefins and alkynes)

entropically driven

thermodynamics plays a role based on bond strength differences

Question 32

Associative (A) substitution

Answer 32

Common for sq. planar 16 electron complexes
Rate = k1[Complex][Ligand] SN2 reaction

Question 33

Ligand directing effects

Answer 33

Ligand directing effects: some ligands direct substitutions trans to themselves

strong σ- donors destabilised the trans M-L bond

Strong π acceptors remove electron density in the equatorial plane of 5-coordinate tbp transition state - stabilisation of transition state

Question 34

Dissociative (D) substitution

Answer 34

Common for octahedral 18 electron complexes
Rate = k1[Complex] c.f. SN1 reaction

Question 35

Oxidative Addition (O.A.)
What factors favour?

Answer 35

§ Low-valent 16VE complexes (Pd(0), Rh(I))
§ Electron rich complexes (those containing strong s-donor) § 5d metals react faster than 4d which react faster than 3d
§ Sterically unhindered metal centres
§ Weak Y-X bond compared to M-X and M-Y bonds

Question 36

Reductive Elimination (R.E.)
What factors favour

Answer 36

§ The two groups need to be cis
§ 3d metals react faster than 4d which react faster than 5d
§ Electron deficient complexes (those containing strong p-acceptors)
§ Sterically bulky ligands
§ Complexes with odd C.N. (i.e. 1, 3) react faster then those with even C.N. (i.e. 2 or 4)

Question 37

What are the Key features for b-hydride eliminations to occur

Answer 37

Key features for b-hydride eliminations to occur:
1. The b-C must contain a hydrogen
2. M-CandC-Hmustbesyncoplanar
3. The metal must possess a vacant coordination site
and an accessible empty orbital (coordinatively
unsaturated)
4. The metal must be electronically unsaturated

Question 38

What factors disfavour β-hydride elimination

Answer 38

-Ligands with no b-hydrogens
-Inability to achieve a syn-coplanar transition state
-Coordinatively and electronically saturated – no vacant sites for agostic H interaction
- Unstable products – C=Si bonds, Bredt’s rule
- Metal with a d0 electron count – no electron density to donate into the C-H s*

Question 39

What factors favour Nucleophilic attack to coordinated ligands

Answer 39

Factors favouring nucleophilic attack
§ Coordinatively saturated metal centre
§ Electron poor metal centers / cationic metal
centers
§ Soft nucleophiles (hard nucleophiles usually attack the metal first)