Download A Summary of Organometallic Chemistry

A Summary of Organometallic Chemistry
Counting valence electrons (v.e.) with the ionic model
1. Look at the total charge of the complex
Ph3P Cl
Rh
Ph3P PPh3
charge:0
+
CO 2–
OC
Fe
OC
Co
CO
charge: -2
charge: +1
2. Look at the charge of the ligands (see table in next
page) and calculate the formal oxidation state of the
metal and therefore the d electrons at the metal
center
Ph3P Cl
Rh
Ph3P PPh3
Rh+
d8
Fe
OC
+
CO 2–
OC
CO
Fe2d10
Co
Co3+
d6
3. Add the electrons coming from all ligands
Ph3P Cl
Rh
Ph3P PPh3
1 x 2e- (Cl)
3 x 2e- (PPh3)
TOT 8 e-
OC
Fe
OC
+
CO 2–
CO
Co
4 x 2e- (CO)
2 x 6e- (Cp)
TOT 8 e-
TOT 12 e-
4. Add the electrons of both metal and ligands to get
the valence electrons (v.e.)
Ph3P Cl
Rh
Ph3P PPh3
16 v.e.
CO 2–
OC
Fe
OC
+
CO
18 v.e.
Co
18 v.e.
The 18-electron rule
The rule helps us to decide whether a complex is likely
to be stable.
To have a stable transition metal we need to fill all s, p,
and d orbitals. These are a total of 9 orbitals, which
means 18 electrons.
The rule is similar to the 8-electron rule of main group
elements.
Exceptions to the 18-electrons rule
- Early transition metal complexes and/or complexes
with bulky ligands are stable with less than 18 v.e.
mainly for steric reasons.
- Complexes with many π-donor ligands (see below)
are stable even with less than 18 v.e. because the πdonation adds electron density to the metal,
stabilizing it.
- d8 complexes with square planar geometry are stable
with 16 v.e. because the dx2-y2 orbital is very high in
energy and therefore usually empty.
Electron density at the metal center
The Pauling’s principle of electroneutrality says that
molecules arrange themselves so that their net charge
falls between a narrow limit close to neutrality.
This means that:
1) Electropositive and high oxidation state elements
tend to balance their net charge by binding highly
electronegative elements.
2) Elements with intermediate electronegativity prefer
to bind to each other
Trends in electron density
- The higher the oxidation state, the more stable the d
orbitals. This means that the high oxidation state
metal prefers to bind with ligands donating electron
density (see also below).
- d-orbitals in 2nd and 3rd row transition metals are
more available to be donated. Higher
electronegativity and higher electron density is
observed going down in the periodic table.
- The spectrochemical series of ligands below indicates
of how ligands have an active role in modifying the
electron density at the metal center. π-donor ligands
donate more electron density at the metal than pure σdonor, whereas π-acceptors withdraw electron density
from the metal.
I– > Br– > Cl– > F– > H2O > NH3 > PMe3 > H, PPh3 > CO
π-donors
σ-donors
π-acceptor
donate electron density ————withdraw electron density
New Concepts
– Oxidative Addition
– Insertion
/
/ Elimination
– Nucleophilic attack onto a Ligand
– H2-Activation
Source: Huheey, Crabtree
Reductive Elimination
Oxidative Addition
Reductive Elimination
Examples
Oxidative Addition:
Reductive Elimination:
Which Factors determine the tendency of a complex to give oxidative additions?
– Tendency to oxidation
(electronic effects)
→ The electron richer the metal, the easier the oxidation: Good donors (e. g., trialkylphosphines)
favor oxidative addition.
→The tendency to give oxidative addition increases upon descending in a triad:
Co(I) < Rh(I) < Ir(I) (mainly because the M–L-bonding energy increases in this order)
– Relative stability of the coordination numbers (steric effects)
→The tendency to higher coordination numbers within a transition period decreases from left to
right.
Os(0) > Ir(I) > Pt(II) (all d8 systems): Os(0) has a greater tendency to oxidative addition than Pt(II).
→Bulky ligands favor low coordination numbers, and therefore disfavor oxidative addition
reactions (if they do not dissociate!)
–Strength of the newly formed M–X- and M–Y-bonds relative to X–Y
(see CO-insertion below)
Mechanism
X–Y apolar (H2, O2): Concerted reaction with a three-center transition state:
X–Y polar, electrophilic molecule (CH3I, HCl): SN2-Mechanism
Insertion (Einschiebung) / Elimination
Althought the oxidative addition of X–Y to M gives a complex that formally results from the insertion of a metal
atom into the covalent X–Y-bond, the term “insertion” (Einschiebung) is reserved for reactions in which a
molecule (which must possess a multiple bond) is inserted into a metal-ligand bond.
The oxidation state of the metal remains unchanged:
Both reactions have fundamental significance in homogeneous catalysis!
Olefin-Insertion into a M–H-Bond:
Mechanism:
–Ligands involved in an insertion reaction are mutually
cis.
–The insertion reaction produces a free coordination
site.
–An elimination from an 18-electron complex can only
occur upon dissociation of a ligand.
–Also elimination reactions from square planar
complexes require a free coordination site:
CO-Insertion into an M–C-Bond
Mechanism
–
–
No 13C is incorporated into the acetyl group.
No 13CO trans to the acetyl group.
Mechanism: insertion (Einschiebung) or migration
(Wanderung)?
Methyl-Migration, CO-Elimination
Unambiguously confirmed by the inverse reaction:
25%
25%
50%
Predicted for Me-migration
observed product distribution
25%
75%
Predicted for elimination
not observed
(Principle of microscopic reversibility: The direct and
inverse reaction follow the same elementary steps)
Thermodynamics of CO-Insertion
Calculated Reaction Enthalpies:
J. A. Connor, M. T. Zafarani-Moattar, J. Bickerton, N. I. El Saied, S. Suradi, R. Carson, G. Al Takhin, H. A.
Skinner, Organometallics 1982, 1, 1166.
Nucleophilic Attack onto a Ligand
A nucleophile (Nu–) can attack either the metal (→ substitution) or a coordinated molecule.
For instance:
The coordination to a metal activates olefins (and related compounds, such as allyls) toward
nucleophilic attack (why?).
The highest degree of activation occurs in metal complexes that either contain strong π-accepting
ligands or are positively charged.
Relative Reactivity
Nucleophiles
of
Coordinated
Unsaturated
Ligands Toward
In cationic 18-electron complexes, the reactivity decreases according to:
Adapted form: S. G. Davies, M. L. H. Green, D. M. P. Mingos, Tetrahedron 1978, 34, 3047.
Note that nucleophilic attack onto an allyl complex is accompanied by 2 e– -reduction of the metal:
H2-Activation
Some of the catalytic reactions discussed in this course involve the activation of dihydrogen by
transition metal complexes. This is a short overview of the possible activation pathways.
–Oxidative Addition
(catalyst: dihydride complex)
–Hydrogenolysis (catalyst:monohydride complex)
–Heterolytic H2-Activation (catalyst: monohydride complex)
–Homolytic H2-Activation (catalyst: monohydride complex)