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Flashcards in Paul Webb - catalysts Deck (114):
1

homogeneous catalyst

metal complex in solution
high active
highly selective - can tailor

2

heterogenous

solid
low selectivity
easy to separate products and cataylsts
used in industry

3

key design components of homogeneous catalysts

ligands, transition metal, substrate, solvent

4

key design components of homogeneous catalysts - ligands

1. donor/acceptor properties
2. chelate effect
3. bite angle

5

key design components of homogeneous catalysts - transition metal

1. e- configuration (d0 - d10)
2. orb symmetry
3. number of coord. sites

6

examples of L-type ligands

H2O, CO, NH3

7

examples of x-type ligands

Cl, Br, I

8

X-type ligands

ionic interactions between metal and ligand

9

L-type ligands

covalent interactions, eg lewis acid base interactions give dative bonds with increasing covalent contribution

10

lewis acid

substance acting as e- pair acceptor

11

lewis base

substance acting as e- pair donor

12

hard acid - base interactions

largely electrostatic

13

soft acid - base interactions

largely covalent

14

Hard acids

high +ve charge
small size
low electronegativity
low polarisablity
high energy LUMO

15

examples of hard acids

H+, Mg2+, Cr2+, Fe3+

16

hard bases

high electronegativity
weakly polarisable
high energy LUMO
difficult to oxidise

17

examples of hard bases

F-, H2O, NH3, NO3-

18

soft acids

low +ve charge
large size
intermediate electronegativity
highly polarisable
low energy LUMO

19

examples of soft acids

Cu+, Hg+, Pd2+, Pt2+

20

soft base

interediate electronegativity
highly polarisable
easy to oxidise
low energy HOMO

21

examples of soft bases

H-, CN- CO

22

σ-donor ligands

increase e- density on metal

23

π- back donation

low valent metals have tendency to return e- density to ligands with empty π-symmetric orbs

24

high spin e- configuration

eg level can be filled, fill all orbs before paring up

25

low spin configuration

fill all t2g level before filling eg level

26

spectrochemical series - ligands

NO+ > CO- > PF3 > CN- > NO2 > NH3 > H2O > OH- > F- > S2- > Cl- > Br-
CO - low spin, strong field splitting ligand, large delta
I- high spin, weak field splitting ligand, low delta

27

spectrochemical series - metals

Co 3+ > V3+ > Cr 3+ > Fe3+ > V2+ > Fe2+ > Co 2+ > Ni 2+ > Mn2+

28

Δo - ox state

increases with increasing oxidation state

29

Δo - group

increases down a group

30

overlap integral, s

s = ∫ φ A. φB .dt

31

overlap integral, s> 0

bonding interaction

32

overlap integral, s< 0

anti-bonding interaction

33

overlap integral, s =0

non-bonding interaction

34

Δo - d orb

Δo increases with lower energy d orbs eg 5 lower than 4

35

Δo - ligand orbs

Δo increases with higher energy ligand orbs

36

examples of π -acceptor ligands

CO, CN-, PF3

37

examples of π - donor ligands

Cl-, F-, Br-, OH-

38

π -acceptor ligands

strong field
stabilise t2g - goes lower down in energy- large Δ

39

examples of σ-donor

NO2, NH3, H2O

40

σ-donor

t2g is non-bonding
intermediate Δ

41

π-donor

weak field
destabilises t2g- goes higher in energy- small Δ

42

efficient bonding requires

orbs of correct symmetry
orbs of similar energy
orbs of correct spatial orientation

43

Jahn Teller Effect

non-linear molecules in degenerate electronic state will distort so as to remove the degeneracy- when eg level is filled symmetrically will distort to stablise the orb that has more electrons in it

44

what e- configurations does Jahn teller effect?

high spin d4
low spin d7
d9

45

square planar jahn teller

all orbs with Z components are lowered in energy. generates 4 low lying d orbs and one inaccessible d orb

46

electronegativity affecting Δ

Δ increases as electronegativity decreases

47

Oh and Td complexes - number of bonding orbs

9 so can accomodate 18 e-

48

square planar complexes - number of bonding orbs

8 so can accomodate 16 e-

49

valence electrons of complexes class 1

12-22 VE
weak field ligands
3d metals
small Δ

50

valence electrons of complexes class 2

12-18 VE
stronger field ligands
Δ large for 4d and 5 d

51

valence electrons of complexes class 3

18 VE
Δ largets
good π acceptors

52

Hapticity, η

number of atoms directly bonded to metal

53

k

how many donor atoms bonded to metal

54

μ

how many metals atoms is bridging to

55

when does the 16 e rule come into play?

with square planar complexes, the highest dx2-y2 orb is inaccessible

56

what happens to the energy difference between the metal orb and CO 2pi orb as M has a higher oxidation state?

the energy difference increases and so get less efficient overlap and so less back donation

57

what happens if the ligand increases e density M what happens to the CO bond?

it will weaken due to more electron density on the metal and so electron will be donated into CO pi* orb which decreases the bond strength and so the CO stretch will appear at lower wavelength

58

what happens if the ligand competes for e density onM what happens to CO bond?

CO bond will strengthen, there will be less e- density on the metal and so will have less back donation onto the MC bond and so the CO bond will strengthen and the stretch will appear at a higher wave number

59

why is CO more susceptible to nucleophilic attack when attached to a metal centre?

get polarisation of bond, C becomes more positive

60

how many e- does the linear NO donate? what is its charge>

2 e donor
+ charge

61

how many e- does bent NO donate? what is its charge?

2 e donor
- charge

62

how is the acidity of metal hydrides measured?

stability of conjugate base - stabilised by electron withdrawing ligands

63

lower pKa =

more acidic

64

order of ew power of ligands P(OR)3, CO, PR3

CO > P(OR)3 > PR3

65

what does the effect of more back donation do to the length of C=C bond?

makes it longer and so weakens it

66

how do you chnage the steric profile of phophine ligands?

changing size of R group

67

how does the electronegativity of R group affect π acid character of PR3 -

as R gets more electronegative, the energy of σ* orb decreases (it is stabilised), becomes closer in energy to metal orb and so more efficient donation of e- from metal occurs. Becomes a better π-acceptor

68

what orb in PR3 can act as π-accpetor orb?

empty σ* orb

69

alkyl phosphines

good σ donors - carbon based increase electron density on metal

70

phosphites

O based phosphorus ligand
O is electronegative and so withdraws the electron pair that can be used as dative bond to itself, is a poor σ donor and better π-acceptor.

71

cone and bite angles

measure of steric influence of ligand

72

what reaction does steric crowding favour?

dissociative mechanism and facilitates migratory insertion reactions due to alleviation of steric strain

73

larger the cone angle

more steric profile

74

can steric and electronic control of phosphine ligands be controled independently or dependently

independent of each other

75

natural bite angle

preferred chelation angle determined only by the ligand backbone constraints - not by metal valence angles
size of chelate ring influences B angle which influences hydridisation of bonded atom

76

Phosphines as ligands - large Tolman's cone angle

promotes dissociative elementary step
stabilised low coordinate species
influences regioselectivity
promotes reductive elimination

77

Phosphines as ligands - σ donor

have higher electron density on metal
promote oxidative addition
stabilises higher oxidation states

78

Phosphines as ligands - π-acceptor

lower electron density on metal
promote reductive elimination
stabilise lower oxidation states
promote CO dissociation and alkene coordination

79

chelating large bite angle ligands

reduce e- density
promote CO dissociation and alkene corrdination
influence regioselectivity
promote reductive elimination

80

chelating small bite angle ligands

promote oxidative addition
stabilise higher oxidation states ]fix cis configuration

81

dissociative mechanism

X leaves first and leaves metal unsaturated
y then adds on to metal

82

associative mechanism

y adds in to metal complex and then x leaves

83

trends for associative mechanism

occurs with 16/17 e
favoured for sterically accessible metal centres

84

trends for dissociative mechanism

occurs with 18 e-
favoured for sterically hindered centres

85

what type of mechanism is dissociative?

Sn1

86

labile complexes

substitution occurs quickly
mostly octrahedral
d1-2, HS d 4-6, d7,9,10
small LFSE

87

inert complexes

substitution occurs slowly
d3, LS d4-6
large LFSE

88

trans effect

strong π-acceptor or σ donors accelerates substitution of a ligand in trans position

89

Trans influencce

σ-effect
thermodynamic - influences ground state
If M-T bond is strong, M-X bond will be weak (x will be more easily substituted)

90

trans effect (π)

π- effect
kinetic - stabilisation of transition state (charge) by pi- acceptor T ligand

91

what is the reverse reaction of oxidative addition?

reductive elimination

92

what is oxidative addition promoted by?

high e- density on metal
want e- rich ligands
want coordinately unsaturated systems

93

oxidative addition electrophilic substrates

electronegatve atoms
good oxidising agents
don't require vacant orb on meta

94

oxidative addition non-electrophilic substrates

no electronegative atoms
poor oxidising agents
need vant orb on metal

95

what orientation of oxidative addition of H2 is allowed?

only cis, trans is symmetry forbidden

96

what metals can't undergo oxidative addition?

d0 metals

97

what arrangement is needed for migratory insertion?

cis arrangement of ligands that are migrating

98

is insertion and migration an inner or outer sphere process?

innersphere - ligand that is migrating comes from the metal complex (outer sphere would require ligand from solvent)

99

why do bulky ligands cause faster rate of reductive elimination

due to alleviation of strain ie going from octrhedral complex to square planar

100

solvent effects on rate of migratory insertion

bulky solvents (ie methyl -THF) will cause migratory insertion to go slower due to steric hinderance

101

what is reverse reaction of migratory insertion?>

migratory CO de-insertion (CO goes back to orginal place)

102

what is required for migratory de-insertion?

vacant site athat is cis to ligand that is being de-serted

103

what is the reverse reaction of hydride migratory insertion?

B-elimination and de-insertion - regenerates alkene

104

what is required for B-hydride elimination

vacant site cis to alkyl and B-hydrogens

105

how to stop B-H elimination

maintain coordinate saturation
stabilisation of metal alkyl through choice of M
steric hindrance - can stop geomtry required for B-elimination

106

is syn addition inner or outer sphere?

inner

107

is anti addition inner or outer sphere

outer - solvent attacking alkene so get addition to other end to metal

108

what is reductive elimination favoured by?

electron deficient metal
presence of bulky groups
thermodynamically stable product

109

reductive elimination of CH3-X - X effects

X = halogen - no elimination
x= H elimination v favourable
x = CH3 favourable but slower

110

reductive elimination mechanisms for square planar

dissociative - one ligand leaves and then cis ligands ligands leave
Non-dissociative - cis ligands leave stright away
Associative - addition of ligand makes T-shape and two equatorial cis ligands leave

111

hydroformylation

alkene turned into carbonyl group

112

what metals cataylse hydrofromylation

Rh and Co

113

what metals are used to cataylse the carbonylation of methanol?

Rh and Ir

114

what are the reactions called that have carbonylation of methanol?

Cativa and Monsanto