Organometallics Flashcards
What is an L and X type ligand
L - neutral
X - anionic (-)
How many valence electrons does a transition metal tend to have?
18
What does μx mean
Bridging - x designates the number of metal centers bridged by the ligand
What is ηx
Hapticity
the x superscript indicates the number of ‘points of contact’ between the ligand and metal in a continuous sequence.
What is κx
Denticity (kx)
the x superscript indicates the number of ‘points of contact’ between the ligand and metal.
Total valence electron count =
dn count + electrons donated by the ligand + number of M-M bonds
Which is stronger M-H or M-Me
M-H
better overlap with spherical 1s orbital no nonbonding electron repulsions and minimal steric repulsion for H
What is the trend of M–Me bond strength DOWN a group
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
Describe:
σ bonds
π bonds
δ bonds
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
What are pi donor ligands
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
What are pi acceptor ligands
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.
When is t2g raised, lowered in energy
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.
What is back bonding
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
What are the consequences of back bonding
- 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.
Give the consequences of back donation on cyclobutadiene
- planar C4R4
- equal C-C bond lengths
- reduction to the 6π aromatic dianion (LX2)
What are ‘Bent’ metallocenes and why do they form
- 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
what type of ligand are arenes
6 electron donors - L3 type
Explain M-H position in 1H NMR spec
- 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
What is a Fluxional molecule
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.
How do we know a process is operative
- broadened lines
- temperature dependance
- field dependence
- spectra that are too simple/ complicated for expected structure
What are carbenes
A molecule containing a neutral carbon atom with a valence of 2 and 2 unshared valence electrons
Describe Fischer carbenes
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
Describe Schrock alkylidenes
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
What do X-ray crystallographic structures show
- triganal planar sp2 hybridised C centre
- M=C shorter than a single bond but not as short as a M-CO bond
What stabilisation is seen in NHCs
Inductive effects through σ framework
Mesomeric effects through π framework
describe the Metathesis reactions
Metal salt + organic nucleophile
anionic metal complex + organic electrophile
Describe Protonolysis and hydrogenolysis
Essentially an acid-base reaction
Describe reductive routes
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
Describe insertion reactions
Common for the synthesis of alkyl, vinyl and allyl complexes
reversible (opposite of β-elimination)
Describe Oxidative addition
Common synthetic route to s-bound carbon and hydrogen ligands (alkyl, aryl, hydrides, etc)
Gringard synthesis
common for square planar complexes
Describe ligand subsitutions
Common synthetic route for neutral ligands (arenes, olefins and alkynes)
entropically driven
thermodynamics plays a role based on bond strength differences
Associative (A) substitution
Common for sq. planar 16 electron complexes
Rate = k1[Complex][Ligand] SN2 reaction
Ligand directing effects
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
Dissociative (D) substitution
Common for octahedral 18 electron complexes
Rate = k1[Complex] c.f. SN1 reaction
Oxidative Addition (O.A.)
What factors favour?
§ 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
Reductive Elimination (R.E.)
What factors favour
§ 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)
What are the Key features for b-hydride eliminations to occur
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
What factors disfavour β-hydride elimination
-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*
What factors favour Nucleophilic attack to coordinated ligands
Factors favouring nucleophilic attack
§ Coordinatively saturated metal centre
§ Electron poor metal centers / cationic metal
centers
§ Soft nucleophiles (hard nucleophiles usually attack the metal first)
