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Transition Metal Chemistry & Homogeneous Catalysis > Transition Metal Chemistry & Homogeneous Catalysis > Flashcards

Transition Metal Chemistry & Homogeneous Catalysis Flashcards

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1
Q

Are tetrahedral transition metal complexes always high spin or low spin?

A

High Spin- the delta T is less than the pairing energy

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2
Q

What are the three requirements for AOs to become MOs

A
  • Similar energy
  • Good overlap
  • Symmetry
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3
Q

What is the symmetry of a 4s orbital on a metal ion?

A

a1g

Spherically symmetrical; really good overlap therefore the most stabilised

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4
Q

For filled ligands with pi(p)-orbitals which type of ligands reduce delta O?

A

pi donor ligands (electron rich)
e.g: Cl-/ Br-/ H2O
(SMALL delta O)

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5
Q

For empty ligands with pi(p)-orbitals which type of ligands increase delta O?

A

pi acceptor ligands
e.g: CO, N2 , CN-
(LARGE delta O)

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6
Q

What are the 4 electronic transitions for a T.M complex ?

A
  • 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)

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7
Q

What are the selection rules for electronic spectra?

A
  • delta S=0

Laporte selection rule: delta l= +/- 1

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8
Q

Why do we see the t2

A

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
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9
Q

Name three effects of molecular vibrations (exclude the break down in selection rules)

A
  • Changes in M-L
  • Changes in M-L interaction
  • Changes in energy differences between orbitals
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10
Q

What can vibrational spectroscopy be used for?

A
  • Ligand vibrations
  • M-L interactions
  • Structures of complexes
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11
Q

What is the difference between the structure in IR and NMR give?

A
  • IR: instantaneous structure

- NMR: time-averaged structure

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12
Q

What is the selection rule for IR?

A

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
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13
Q

What is the selection rule for Raman?

A

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

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14
Q

What is an associative mechanism?

A

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

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15
Q

What is a dissociative mechanism?

A

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

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16
Q

What an an interchange mechanism?

A

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

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17
Q

What are the 2 types of interchange mechanisms?

A

1) Bond formation dominates - Ia

2) Bond breaking dominates - Id

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18
Q

What is a liable complex?

A

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

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19
Q

What is an inert complex?

A

Complexes with long reaction times

A kinetic term- to do with how quickly eqm is reached

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20
Q

What is a stable complex?

A

Large K and large negative delta G

Thermodynamic term- relates to the position of eqm

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21
Q

What is an unstable complex?

A

Positive or small negative delta G

Thermodynamic term- relates to the position of eqm

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22
Q

In isotopic labelling studies, what is a very fast exchange characterised by?

A
  • 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
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23
Q

In isotopic labelling studies, what is a fast exchange characterised by?

A

Ions such as Ti3+ and M2+ where M= Mn / Fe / Co / Zn

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24
Q

In isotopic labelling studies, what is a relatively slow exchange characterised by?

A

Some CFSE and/ or moderate charge radius ratios

e.g M2+ where M= Pd, V, Be; M3+ where M= Fe/ V/ Al / Ga

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25
Q

In isotopic labelling studies, what is a very slow exchange characterised by?

A
  • Large charge radius ratios
  • Large CFSE; big loss of CFSE
  • M2+ (M= Ru, Pt); M3+ (Cr, Ru, Co)
  • d3, LS d5 and d6
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26
Q

What does changes in geometry affect?

A
  • CFSE
  • delta G
  • Rate constant, k(f)
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27
Q

What is the rate equation for a dissociative mechanism?

A

Rate = (kf ky [M(X)] [Y] )/ (kb[X] + ky[Y])

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28
Q

What is the observed rate equation for a dissociative mechanism?

A

(kf ky [Y] )/ (kb[X] + ky[Y])

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29
Q

What is the rate equation for an associative mechanism?

A

Rate = (k1[M(X)][Y])/ ((k-1/k2) +1)

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30
Q

What is the observed rate equation for an associative mechanism?

A

k1[Y]/ ((k-1/k2) +1)

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31
Q

What is the rate equation for an interchange mechanism?

A

Rate = (kf ky [M]T [Y])/ (1+ ky[Y])

32
Q

What is the observed rate equation for an interchange mechanism?

A

(kf ky [Y])/ (1+ ky[Y])

33
Q

What can be said about the limiting rates for an associative mechanism?

A

No limiting rate

Bimolecular; rate depends on [Y]

34
Q

What can be said about the limiting rates for Ia?

A

Wide range of limiting rates

35
Q

What can be said about the limiting rates for Id?

A

Limiting rates are slower than solvent exchange

All limiting rates are similar as bond breaking dominates; independent of Y

36
Q

What can be said about the limiting rates for a dissociative mechanism?

A

No Y- dependence at high [Y]
Some limiting rates faster than solvent exchange
Little variation in limiting rates for different Y

37
Q

In an associative mechanism’s transition state, what can be said about the entropy and volume of activation?

A

Fewer particles in the transition state therefore negative entropy of activation and negative volume of activation

38
Q

In a dissociative mechanism’s transition state, what can be said about the entropy and volume of activation?

A

Positive entropy of activation and positive volume of activation

39
Q

How can we obtain a value for the entropy of activation?

A

Plot ln(kf) vs 1/T

slope: -deltaH/R
intercept: 1/T=0 of lnC from which delta S can be found

40
Q

What is a more accurate method to determine the entropy of activation?

A

Plot ln(kf) vs P

slope: -deltaV/RT

From deltaV we can calculate delta S

41
Q

What is the trans influence?

A

In a square planar complex, the substitution of X is influenced by the nature of the ligand trans to it

42
Q

In the trans influence, how does the ligand trans to X affect the bonding between M-X?

A

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)

43
Q

What is the kinetic influence of the trans effect?

A

Variation in substitution rate of X with different trans ligands

44
Q

How does the trans ligand affect the energy barrier to the reaction, delta G, in the trans effect?

A

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
45
Q

What is the outer sphere electron transfer mechanism?

A

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+

46
Q

What does the Frank-Condon principle require for an outer sphere electron transfer?

A

Requires that the nuclei are fixed during electron transfer, so complexes must first adopt the same structure

47
Q

What is the inner sphere electron transfer mechanism?

A

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+

48
Q

What is a homogenous catalyst?

A

All constituents of the reaction, including the catalyst, are present in the same phase (usually in solution)

49
Q

Why are transition metals used as catalysts?

A
  • 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)

50
Q

Why is the bonding ability important for transition metals used as catalysts?

A 
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)
51
Q

What are the two ways ligands can be classified?

A

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

52
Q

What does the presence of participative and non-participative ligands control?

A

Allows control of steric and electronic factors at TM centre

e.g Ph ligands and the Tolman angle

53
Q

True or false: pi-acceptor ligands prefer M in high oxidation states

A

False, pi-acceptor ligands prefer M in low oxidation states

54
Q

True or false: sigma- bonding ligands prefer M in high oxidation states

A

True

55
Q

What are the 3 stages of a catalytic cycle?

A

1) Assembly
2) Modification
3) Expulsion

56
Q

What are the ways assembly can occur?

A

1) Transmetallation
2) Substitution/ Addition (example of addition: Wilkinson’s catalyst)
3) Oxidative addition reactions (oxidation of a TM by a substrate X-Y)

57
Q

What are the factors that affect oxidative addition?

A

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)

58
Q

What type of insertion does n1-ligands undergo?

A

1,1 insertion

59
Q

What type of insertion does n2-ligands undergo?

A

1,2 insertion

60
Q

Name the two types of insertion in the modification process:

A

1,1- migratory insertion and 1,2-migratory insertion

61
Q

What are the important features of migratory insertion?

A

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-)

62
Q

What is the reverse of a 1,1 migratory insertion process?

A

Extrusion

63
Q

What is the reverse of a 1,2 migratory insertion process?

A

b-hydride elimination

64
Q

When can nucleophiles attack alkenes?

A
  • 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
65
Q

What are the two mechanisms for nucleophilic attack on alkenes?

A

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)

66
Q

What is the Wacker process?

A

Oxidation of an alkene bond to Pd by H2O resulting in cis addition

67
Q

What are the two types of expulsion processes?

A

1) 1,1- reductive elimination

2) Hydride abstraction

68
Q

In the 1,1-reductive elimination process, what can be said for the change in VE, OS and CN?

A

delta VE: -2
delta OS: -2
delta CN: -2

69
Q

What is a requirement for 1,1-reductive elimination?

A

Groups being coupled must be cis to each other

70
Q

What are the two types of hydride abstraction processes?

A

1) b-Hydride elimination

2) a- hydride elimination

71
Q

What is the requirement for b-hydride elimination?

A

M-C-C-H needs to be co-planar

72
Q

True or false: b-hydride elimination is harder than a-hydride elimination

A

False, b-hydride elimination is easier

73
Q

What does b-hydride elimination result in?

A

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

74
Q

Which catalyst is used for the hydrogenation of alkenes?

A

Wilkinson’s catalyst [RhCl(PPh3)3]

75
Q

Which catalysts are used for the hydroformylation process?

A

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
76
Q

What is the catalyst for the BP acetic acid process?

A

[RhI2(CO2)]-

Transition Metal Chemistry & Homogeneous Catalysis (1 decks)

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