Transition Metal Chemistry & Homogeneous Catalysis Flashcards
Are tetrahedral transition metal complexes always high spin or low spin?
High Spin- the delta T is less than the pairing energy
What are the three requirements for AOs to become MOs
- Similar energy
- Good overlap
- Symmetry
What is the symmetry of a 4s orbital on a metal ion?
a1g
Spherically symmetrical; really good overlap therefore the most stabilised
For filled ligands with pi(p)-orbitals which type of ligands reduce delta O?
pi donor ligands (electron rich)
e.g: Cl-/ Br-/ H2O
(SMALL delta O)
For empty ligands with pi(p)-orbitals which type of ligands increase delta O?
pi acceptor ligands
e.g: CO, N2 , CN-
(LARGE delta O)
What are the 4 electronic transitions for a T.M complex ?
- Ligand Chromophore transitions
- d-d transitions
- LMCT
- MLCT
(last 3 are the ones we study; they involve a metal centre so we can relate these to MO diagrams)
What are the selection rules for electronic spectra?
- delta S=0
Laporte selection rule: delta l= +/- 1
Why do we see the t2
In a tetrahedral complex there is no centre of symmetry.
- g and u parities do not apply
- some p-d mixing occurs resulting in much stronger d-d transitions
Name three effects of molecular vibrations (exclude the break down in selection rules)
- Changes in M-L
- Changes in M-L interaction
- Changes in energy differences between orbitals
What can vibrational spectroscopy be used for?
- Ligand vibrations
- M-L interactions
- Structures of complexes
What is the difference between the structure in IR and NMR give?
- IR: instantaneous structure
- NMR: time-averaged structure
What is the selection rule for IR?
Dipole moment of the molecule must change during vibration:
- Molecule must change shape
- Irreducible representation of the vibration must transform as a cartesian axis in the character table of the point group involved
What is the selection rule for Raman?
Polarisability tensor of the molecule must change during vibration
- Irreducible representation of the vibration must transform as a product of the cartesian axes in the character table of the point group involved
What is an associative mechanism?
ML(n)X + Y –> ML(n)Y + X
The ligand, Y, reacts with the complex to form a detectable 7-coordinate (pentagonal bipyramid D5h) intermediate
What is a dissociative mechanism?
ML(n)X + Y –> ML(n)Y + X
The ligand, X dissociates from the complex to form a detectable 5-coordinate ( square pyramidal c4v) intermediate
What an an interchange mechanism?
ML(n)X + Y –> ML(n)Y + X
Both X and Y are associated with the metal ion at the same time but there is no detectable 5/7 coordinates intermediate
What are the 2 types of interchange mechanisms?
1) Bond formation dominates - Ia
2) Bond breaking dominates - Id
What is a liable complex?
Complexes which react rapidly, with reaction 1/2 way to completion in under 30 seconds
A kinetic term- to do with how quickly eqm is reached
What is an inert complex?
Complexes with long reaction times
A kinetic term- to do with how quickly eqm is reached
What is a stable complex?
Large K and large negative delta G
Thermodynamic term- relates to the position of eqm
What is an unstable complex?
Positive or small negative delta G
Thermodynamic term- relates to the position of eqm
In isotopic labelling studies, what is a very fast exchange characterised by?
- Low charge radius ratios and absence of significant CFSE
- E.g: Group 1 and 2 metal ions and larger lanthanide trications
- d0, d10, HS d5
In isotopic labelling studies, what is a fast exchange characterised by?
Ions such as Ti3+ and M2+ where M= Mn / Fe / Co / Zn
In isotopic labelling studies, what is a relatively slow exchange characterised by?
Some CFSE and/ or moderate charge radius ratios
e.g M2+ where M= Pd, V, Be; M3+ where M= Fe/ V/ Al / Ga
In isotopic labelling studies, what is a very slow exchange characterised by?
- Large charge radius ratios
- Large CFSE; big loss of CFSE
- M2+ (M= Ru, Pt); M3+ (Cr, Ru, Co)
- d3, LS d5 and d6
What does changes in geometry affect?
- CFSE
- delta G
- Rate constant, k(f)
What is the rate equation for a dissociative mechanism?
Rate = (kf ky [M(X)] [Y] )/ (kb[X] + ky[Y])
What is the observed rate equation for a dissociative mechanism?
(kf ky [Y] )/ (kb[X] + ky[Y])
What is the rate equation for an associative mechanism?
Rate = (k1[M(X)][Y])/ ((k-1/k2) +1)
What is the observed rate equation for an associative mechanism?
k1[Y]/ ((k-1/k2) +1)
What is the rate equation for an interchange mechanism?
Rate = (kf ky [M]T [Y])/ (1+ ky[Y])
What is the observed rate equation for an interchange mechanism?
(kf ky [Y])/ (1+ ky[Y])
What can be said about the limiting rates for an associative mechanism?
No limiting rate
Bimolecular; rate depends on [Y]
What can be said about the limiting rates for Ia?
Wide range of limiting rates
What can be said about the limiting rates for Id?
Limiting rates are slower than solvent exchange
All limiting rates are similar as bond breaking dominates; independent of Y
What can be said about the limiting rates for a dissociative mechanism?
No Y- dependence at high [Y]
Some limiting rates faster than solvent exchange
Little variation in limiting rates for different Y
In an associative mechanism’s transition state, what can be said about the entropy and volume of activation?
Fewer particles in the transition state therefore negative entropy of activation and negative volume of activation
In a dissociative mechanism’s transition state, what can be said about the entropy and volume of activation?
Positive entropy of activation and positive volume of activation
How can we obtain a value for the entropy of activation?
Plot ln(kf) vs 1/T
slope: -deltaH/R
intercept: 1/T=0 of lnC from which delta S can be found
What is a more accurate method to determine the entropy of activation?
Plot ln(kf) vs P
slope: -deltaV/RT
From deltaV we can calculate delta S
What is the trans influence?
In a square planar complex, the substitution of X is influenced by the nature of the ligand trans to it
In the trans influence, how does the ligand trans to X affect the bonding between M-X?
If the ligand trans to X is more polarisable than X, or a strong sigma donor:
- The trans ligand more effectively donates electrons to M
- exerts repulsion on X
- weakens the M-X bond
(Thermodynamic effect)
What is the kinetic influence of the trans effect?
Variation in substitution rate of X with different trans ligands
How does the trans ligand affect the energy barrier to the reaction, delta G, in the trans effect?
If the ligand trans is a strong pi-acceptor:
- the trans ligand effectively stabilised a 5-coordinate intermediate
- lowers delta G
- accelerates the substitution reaction
What is the outer sphere electron transfer mechanism?
Electron transfer occurs between 2 colliding molecules without any bond breaking
e.g the change of electrons between [Fe(H2O)6]2+ and [Fe(H2O)6]3+
What does the Frank-Condon principle require for an outer sphere electron transfer?
Requires that the nuclei are fixed during electron transfer, so complexes must first adopt the same structure
What is the inner sphere electron transfer mechanism?
A single ligand links the 2 metal ions during electron transfer and bond breaking occurs
e.g [Co(NH3)5Cl]2+ and [Cr(H2O)6]2+
What is a homogenous catalyst?
All constituents of the reaction, including the catalyst, are present in the same phase (usually in solution)
Why are transition metals used as catalysts?
- Bonding ability
- Ligand variety
- Ligand effects
- Variability of oxidation states
- Variability of coordination number
(All factors can be attributed to availability of d-orbitals for TMs)
Why is the bonding ability important for transition metals used as catalysts?
TMs have 9 valence shell orbitals These allow sigma and pi bonding The metal can adopt: - an electron accepting and - an electron donating role (e.g TM carbonyls)
What are the two ways ligands can be classified?
1) Charge (ionic/ neutral)
2) Participative role ( participative ligands take an active role in the catalytic process, while non-participative do not take part directly)
Usually, participative ligands end up in the products: e.g in 1,1’ reductive elimination
What does the presence of participative and non-participative ligands control?
Allows control of steric and electronic factors at TM centre
e.g Ph ligands and the Tolman angle
True or false: pi-acceptor ligands prefer M in high oxidation states
False, pi-acceptor ligands prefer M in low oxidation states
True or false: sigma- bonding ligands prefer M in high oxidation states
True
What are the 3 stages of a catalytic cycle?
1) Assembly
2) Modification
3) Expulsion
What are the ways assembly can occur?
1) Transmetallation
2) Substitution/ Addition (example of addition: Wilkinson’s catalyst)
3) Oxidative addition reactions (oxidation of a TM by a substrate X-Y)
What are the factors that affect oxidative addition?
1) Oxidation state and coordination number both need to be low
2) TM needs to be in atomic (single metal atom) complex
3) non participative ligands (electronic/ steric)
What type of insertion does n1-ligands undergo?
1,1 insertion
What type of insertion does n2-ligands undergo?
1,2 insertion
Name the two types of insertion in the modification process:
1,1- migratory insertion and 1,2-migratory insertion
What are the important features of migratory insertion?
1) No change in oxidation state
2) Mutually cis sites are involved and a vacant 2 electron donor site is created
3) The transition state is 3 coordination (1,1-) or 4 coordinate (1,2-)
What is the reverse of a 1,1 migratory insertion process?
Extrusion
What is the reverse of a 1,2 migratory insertion process?
b-hydride elimination
When can nucleophiles attack alkenes?
- When the TM is in a high oxidation state (+2 to +4)
- TM possesses a formal positive charge
- TM has an electron withdrawing ligand coordinated
What are the two mechanisms for nucleophilic attack on alkenes?
1) Trans attack at the more substituted end of the alkene to produce a sigma-alkyl metal complex
2) Attack at M followed by alkene insertion into the M-Nu bond - results in cis product (e.g WACKER PROCESS)
What is the Wacker process?
Oxidation of an alkene bond to Pd by H2O resulting in cis addition
What are the two types of expulsion processes?
1) 1,1- reductive elimination
2) Hydride abstraction
In the 1,1-reductive elimination process, what can be said for the change in VE, OS and CN?
delta VE: -2
delta OS: -2
delta CN: -2
What is a requirement for 1,1-reductive elimination?
Groups being coupled must be cis to each other
What are the two types of hydride abstraction processes?
1) b-Hydride elimination
2) a- hydride elimination
What is the requirement for b-hydride elimination?
M-C-C-H needs to be co-planar
True or false: b-hydride elimination is harder than a-hydride elimination
False, b-hydride elimination is easier
What does b-hydride elimination result in?
Production of alkenes, which can easily dissociate as products . This also creates a source of M-H which can undergo 1,1-reductive elimination with other organo-bound species
Which catalyst is used for the hydrogenation of alkenes?
Wilkinson’s catalyst [RhCl(PPh3)3]
Which catalysts are used for the hydroformylation process?
Cobalt and rhodium carbonyl based systems
- Co2(CO)8 (pre-catalyst which is transformed into an active catalyst)
- HCo(CO)3L where L=phosphines
- HRh(CO)2L2 where L=phosphines
What is the catalyst for the BP acetic acid process?
[RhI2(CO2)]-
