Transition Metal Reaction Mechanisms

Front 1

Kinetics deals with the RATE of a reaction. What does the rate depend on?

Back 1

Overcoming the energy barrier (Activation energy) for the reactants to be transformed into products.

Front 2

What does a large/small activation energy imply about a reactant?

Back 2

Large - Inert (Unreactive)

Small - Labile (Reactive)

Front 3

Thermodynamics deals with the EQULIBRIUM position of a reaction. What does the equilibrium depend on?

Back 3

Equilibrium is dependent on the relative energies of reactants and products. It does not depend on activation energy.

Front 4

What can be deduced about the equilibrium of a reaction using ΔG from the equation:

ΔG = -RTlnK

Back 4

If Δ G is negative, then K must be a positive value larger than 1 as ln(fraction) gives a negative number which would make ΔG positive.

If K>1 then the reaction shifts right in favour of the products

Front 5

Define the mechanism of a reaction:

Back 5

The pathway (or pathways) that reactants are converted into products

Front 6

In a typical reaction, aside from products and reactants what are likely to be the energy maxima and minima?

Back 6

At the maxima energy there is likely to be a transition state which is not directly observable

At the minima will be an intermediate which is sometimes possible to observe although only briefly as highly reactive

Front 7

What is the principle of microscopic reversibility?

Back 7

Lowest energy pathway must be the same for bot forward and backwards reaction

Front 8

Name 3 product studies can be used for studying mechanisms:

Back 8

Stoichiometric analysis

Isotopic labelling

Product structure

Front 9

How can rate of reaction be used to study mechanisms as a function of:

Temperature:

Reactant Concentration:

Pressure:

Solvent:

Back 9

Temperature: Activation parameters (Ea, ΔH#, ΔS#)

Reactant Concentration: Rate Law

Pressure: Activation Volume ΔV#

Solvent: Solvation of reactants, intermediates and transition states

Front 10

Name 6 reaction monitoring techniques:

Back 10

Mass spectrometry

Gas uptake - or gas formation

Magnetic Resonance - NMR

Absorption Spectrometry - UV/vis(beer-Lambert) and IR (absorbance proportional to concentration)

Titration - slow

Conductivity - when there are ionic reactants/products

Front 11

Name 3 special techniques for fast reactions and when they're used:

Back 11

Stopped flow: rapid mixing of solutions

Flash photolysis: Reactive products from photochemical reactions

NMR line shape analysis: Solvent exchange kinetics

Front 12

Distinguish between associative and dissociative ligand substitution reactions.

Give examples using LnM-X + Y

Back 12

Dissociative: Intermediate with a lower coordination number. LnM-X --> LnM + X

LnM + Y --> LnM-Y

Associative: Intermediate with a higher coordination number. LnM-X + Y --> LnM-XY

LnM-XY --> LnM-Y + X

Front 13

Define Intergange (I)

Back 13

Concerted (coordinated) M-X bond cleavage and M-Y bond formation making a transition state of an increased coordination number

Front 14

Define dissociative interchange (Id)

Back 14

Transition state involves substantial M-X bond extension (weakens) and weak M-Y bond interaction leading to a 'loose' transition state.

Front 15

Define associative interchange (Ia)

Back 15

Transition state involves strong bonding of metal to both X and Y leading to a 'tight' transition state.

Front 16

How does a pseudo first order reaction work when the rate law is:

Rate = k [LnMX] [Y]

Back 16

Make on of the reactants in excess so that it's concentration does not change significantly during the reaction.

kobs = k[Y]

Rate = kobs[LnMX]

Front 17

What is the Arrhenius Equation?

Back 17

k=A e^-Ea/RT

Where Ea is the activation energy

A is Arrhenius Constant

R is the gas constant

T is temperature

k is rate

e is exponential

Front 18

What is the Eyring equation?

Back 18

Relates Gibbs energy of activation ΔG# to rate constant k:

k= (kbT/h)e^-ΔG#/RT

Where kb is Boltzman constant

h is Planck's constant

Front 19

How can the Eyring equation be manipulated to be in terms of the activation parameters ΔH# and ΔS#?

And what are they?

Back 19

As ΔG# = ΔH# -TΔS#

k= (kbT/h) e^-ΔH#/RT * e^ΔS#/R

ln(k/T) = ln(kb/h) - ΔH#/RT + ΔS#/R

ΔS# = Entropy of activation

ΔS#H# = enthalpy of activation

Front 20

How can the sign of ΔS# help us determine whether a mechanism is associative or dissociatve?

Back 20

Dissociative reaction have a transition state with a higher entropy so ΔS# is positive

Associative reactions form a transition state with a lower entropy so ΔS# will be negative

Front 21

What is the effect of changing the pressure on the activation parameters?

Back 21

Effects ΔV# - the volume of activation

dlnk/dp = -ΔV#/RT

ln(k/ko) = -PΔV#/RT

Front 22

Using the equation ln(k/ko) = -PΔV#/RT

What happens to k with increasing p for dissociative and associative mechanisms respectively?

Back 22

Dissociative: The transition state occupies more volume (long weak bond to departing ligand) so ΔV# is positive.

Increasing P therefore decreases k

Associative: Transition state occupies less volume as short tight bond ΔV# is negative. Increasing p causes increase in k

Front 23

What is the advantage and disadvantage of using ΔV# in place of ΔS#?

Back 23

Advantage: ΔV# is easier to visualise

Disadvantage: High pressures required to notice change

Front 24

What happens to the stereochemistry of a square planar substitution?

Back 24

Stereochemistry is retained. I.e. cis replaces cis, trans replaces trans

Front 25

When py is the incoming ligand, and it is in excess, it can be seen from a linear plot of kobs vs [py] that:

kobs = k1 + k2[py] (y=mx + c)

What do the two k terms indicate?

Back 25

There are two possible pathways

Front 26

What can be varied in the reaction to alter k1? (i.e. the dissociative mechanism)

Back 26

The solvent.

In some cases polar coordinating solvents cause a larger value for k1

Front 27

What can be varied in the reaction to alter k2?

(i.e. the associative mechanism)

Give an example for the reaction Pt(Cl2)(py)2 + Y

Back 27

Vary incoming ligand Y

e.g. for Pt(Cl2)(py)2

Pt is a relatively soft electrophile so soft nucleophiles such as PPh3 will have a much larger k value than hard nucleophiles such as Cl-

Front 28

If the kinetics of a reaction is consistent with two pathways e.g:

Pt(L)3X --> (ks +sol -X) P(L)3sol -->(+Y - sol) PtL3Y

Pt(L)3X --> (k2 +Y -X) --> Pt(L)3Y

Derive the rate equation

Back 28

Rate = ks[sol][PtL3X] + k2[Y][PtL3X]

ks[sol] = constant = k1

Rate = k1[PtL3X] + k2[Y][PtL3X]

Rate = kobs[PtL3X]

Where kobs = k1 + k2[Y]

Front 29

What effect can spectator ligands have?

Back 29

If they are bulky they will slow down the rate of associative pathways. Can be used as evidence of an associative pathway

Front 30

How does the position of the R group affect the rate?

Back 30

Bulky R groups cis to the leaving ligand will have a greater effect

Front 31

How can two associative pathways exist in a square planar complex ligand substitution?

Back 31

Through a coordinating solvent (k1 route)

or an added ligand Y (k2 route)

Front 32

What can be noted about the intermediate in an associative square-planar ligand substitution reaction?

Back 32

5-coordinate intermediate can change geometry (e.g. from square pyramidal to trigonal pyramid and bak again)

Front 33

What effect does the leaving group X have on rate of M-X substitutions?

Back 33

The weaker the M-X bond the faster the rate

Front 34

What is the effect of changing the metal on the k2 of a ligand substitution reaction?

Back 34

M-L bond strength increases as you go down the group of transition metals, decreasing the reactivity.

E.g. Ni>Pd>Pt as reactivity decreases 3d>4d>5d

Therefore decreasing k2

Front 35

Why is k2 for [Au(CN)4]- so much larger than for [Pt(CN)4]2-?

Back 35

Pt(II) complex has +1 charge

Au(III) complex has +2 charge

Au is more electrophillic

The complex has a -1 charge as opposed to -2, this causes less repulsion of CN-

Front 36

Distinguish between the electronic and steric effects of trans and cis spectator ligands

Back 36

Cis: Small electronic effect - large steric effect

Trans: Large electronic effect - small steric effect

Front 37

What is the trans effect?

Back 37

The effect of a coordinated ligand on the rate of substitution of a ligand trans to itself in a square planar or octahedral complex.

I.e. a ligand with a high trans effect accelerates the substitution of the ligand trans to itself

Front 38

Which compounds have the largest trans effect?

Back 38

Electron donors e.g. CN- is a good trans spectator ligand, H2O is bad.

Front 39

What is important to note about the best trans spectator ligands CN-, CO, NO, C2H4?

Back 39

They are all good pi acceptors

Front 40

Define trans influence

Back 40

The extent to which a coordinated ligand weakens the metal-ligand bond trans to itself

Front 41

Explain the 'ground state' (bond weakening) effects (sigma bonding) of trans spectator ligands in the complex:

ML2TX

Back 41

If T is a strong sigma donor (e.g. H-), it has a strong interaction with M and weakens the M-X bond resulting in a lowering of activation energy and a faster rate for M-X substitution.

Front 42

Explain the 'transition state' effects (pi bonding) of trans spectator ligands in the complex:

ML2TX

Back 42

Transition state is stabilised by pi backbonding to ligand T.

Formation of 5-coordinate intermediate increases electron density on M, electron density goes into vacant Pi* orbital on ligand T stabilising intermediate

Front 43

Explain why:

[PtCl4]2- + NH3 --> [PtCl3NH3]- + NH3 --> [PtCl2NH3] where NH3 are cis to each other

[PtNH3]2+ + 2Cl- --> [PtCl2(NH3)2] where Cl are trans to each other

Back 43

Cl has a higher trans effect than NH3 so the second in coming ligand will go trans to Cl in both scenarios.

Front 44

Why does PR3 have a higher trans effect than Cl?

Back 44

It is a strong sigma donor and also a pi acceptor

Front 45

How can there be a dissociative mechanism for a square planar complex?

Back 45

Pt(Me)2(SMe2)2 ---> M(Me)2(SMe2)

labile ligands due to strong trans effect of methyl ligands (very strong sigma donors?)

Involves chelating ligand, allows the substitution of the other SMe2 group

Front 46

What is the chelate effect?

Back 46

A chelate ligand is essentially a two attached ligands. Once one of the ligands is bound to a central metal atom the other is more likely to also bond to it, i.e. may cause a substitution reaction.

Front 47

What about a nucleophile can determine whether an associative or dissociative substitution reaction takes place for e.g. Pt complexes

Back 47

Whether it is a hard or soft ligand (nucleophile). Pt is a soft electrophile, so for an associative mechanism requires a soft ligand, it may do both mechanisms. A hard ligand would generally only be able to do a dissociative mechanism.

Front 48

How can cis-trans isomerisation occur in square-planar complexes?

Back 48

2 possible routes:

1) Intramolecular route, changes from cis square planar to tetrahedral intermediate and back to trans square planar. Disfavoured as the tet. intermediate changes spin state.

2) 2 catalysed substitution reactions by traces of free ligand. Cl- substituted by NH3, NH3 substituted by Cl- but goes trans due to Cl- having a stronger trans effect than NH3

Front 49

What can be noted about the ligands of an Fe(CO)5 complex?

What effect does this have on their bond angles?

Back 49

They can undergo berry-pseudo rotation. 2 of the equitorial CO ligands will change positions with the axial CO ligands (with a square pyramidal intermediate in between). Only 1 C NMR peak as fast.

Change in bond angles from 180 to 120 degrees for axial and visa versa

Front 50

Give three examples of ligand substitution reactions for octahedral complexes

Back 50

Solvent exchange: (e.g. swap H2O for H2O*)

Anation: (e.g. swap H2O for X-) n.b. changes charge of complex

Aquation: (e.g. substitution of Cl- with H2O) May affect charge

Front 51

For an [M(OH2)6] complex surrounded by bulk water a primary and secondary coordination shell exist respectively. What determines the rate and mechanism at which these H2O ligands exchange?

Back 51

The charge on the metal

M n+

Front 52

Which metals cause the fastest solvent exchange?

Back 52

Aqua ions of group 1,2,12 and 13 metal ions are all very labile.

Solvent exchange rates are determined by charge and - large cations with small charge most labile

Front 53

Why do Al and Ga cations result in dissociative solvent exchange mechanisms whilst In undergoes an associative mechanism?

Back 53

In is much larger so can increase coordination number

Front 54

Explain the signs of ΔS# and ΔV# for dissociative and associative mechanisms

Back 54

Dissociative: ΔS# is negative, as increased species so increased disorder. ΔV# is negative as long loose weak bonds in transition state.

Assosicative: ΔS# positive as decrease in number of species so deceased disorder. ΔV# positive as short tight strong bonds in transition state

Front 55

Are dissociative (Id) mechanisms associated with small or large metal ions?

Back 55

Dissociative (Id) associated with small metal ions

Associative (Ia) with large metal ions

Front 56

Explain why for a high spin d4 octahedral complex an dissociative mechanism is favoured

Back 56

Goes from an octahedral complex to a square pyramidal complex with the loss of a ligand. There is a different crystal field splitting pattern.

dz^2 is lowered in energy (stabilised) resulting in a higher CFSE.

Front 57

Compare the effect of ligand dissociation for d3 and d4 octahedral complexes.

Back 57

d3: -1.2Δoct - (-1.0Δoct)= CFSE/Δoct -0.20

d4: -0.6Δoct - (-0.914Δoct) = CFSE/Δoct +0.314

Loss of CFSE inhibits ligand dissociation - d3 is inert

Gain of CFSE encourages ligand dissociation - d4 is labile

Also note that high spin d4 (and d9) complexes will experience Jahn-teller distortion which will weaken 2M-L bonds

Front 58

Which of the following will be inert/labile (i.e. will not or will undergo ligand dissociation):

d3

d4 high spin

d6 low spin

d7 low spin

d8

d9

Back 58

Inert: d3, d6 low spin, d8

Labile: d4 high spin, d7 low spin, d9

Due to gain in CFSE for labile complexes due to stabilisation of dz^2 and also J.T. distortions weakening M-L bonds

Front 59

Explain why V2+ has a greater H2O exchange rate than Cr3+, but lower than V3+

Back 59

Lower charge makes V2+ more labile than Cr3+ as they are both d3 complexes. V3+ is a d2 complex and has a lower CFSE than d3 so is more labile

Front 60

Why do mechanisms become more dissociative as you go from left to right of the d-block?

Back 60

Smaller ionic radius

Full t2g leads to repulsion, incoming ligand can't associate easily

Occupied eg (weakens M-L bond)

Favours dissociation of H2O from M

Front 61

When is a pre-equilibrium formed?

Back 61

Pre-equilibrium assumes reactants and intermediates of a multi-step reaction exist in a dynamic equilibrium. This is the fast step

Front 62

If kobs = k2Ke[Y] / 1+ Ke[Y]

What can be deduced at high and low concentrations of Y in terms of the kobs and the order of [Y]?

Back 62

At high [Y]: Ke>>1 so kobs ≈ k2

- i.e. approximately zero order for [Y]

At low [Y]: Ke<<1 so kobs ≈ k2Ke[Y]

-i.e. approximately first order for [Y]

Front 63

Distinguish between anation and aquation in terms of their dependence on the identity of X-

Back 63

Anation involves the substitution of a ligand by Xn-. There is a small dependence on the identity of X

Aquation involves the replacement of an H2O ligand by Xn- and depends largely on the identity of X

Front 64

What does the lack of dependence on the identity of incoming ligand X- imply about anation reactions?

Back 64

They are dissociative, the ligand leaves first

Front 65

For an aquation reaction:

[Co(NH3)X]2+ + H2O --> [Co(NH3)5(OH2)]3+ + X-

For when identity X is changed the strength of the M-X bond changes resulting in a change in equilibrium constant K. When K is plotted against rate constant k there is a positive linear correlation. What does this suggest about aquation reactions?

Back 65

That it is a dissociative mechanism as there is a faster rate when the M-X bond is weaker.

Front 66

Why does I- dissociate fastest from the complex [Co(NH3)5X]2+ but F- dissociates fastest from the complex [Co(CN)5X]3- ?

Back 66

When Co is N-bound it is a hard metal centre and I- is a soft nucleophile.

When Co is S-bound it is a soft centre so the hard F- nucleophile dissociates fastest

Front 67

Why does reactivity of a metal decrease down a group 3d>4d>5d?

Back 67

M-L bonds get stronger as you go down the group, crystal field splitting increases

Front 68

What is the effect of charge in octahedral substitution?

Back 68

Smaller positive charge increases lability

Front 69

What are two mechanisms for the hydrolysis of [Cr(OH2)5F]2+ ?

Back 69

1) aquation (i.e. -F- + H2O)

2) Acid-catalysed hydrolysis (convert F- into better leaving group using H+) Faster

Front 70

What is the evidence for deprotonation in the base-catalysed hydrolysis of [Co(NH3)5Cl]2+ ?

Back 70

Can show H/D exchange with the addition of D2O

No base-catalysis possible for [Co(CN)5Br]3- as there are no acidic protons

Front 71

How does deprotonation of [Co(NH3)5Cl]2+ assist the dissociation of Cl- ?

Back 71

Lowers the positive charge on the complex

Converts one of the ligands to NH2 which is a good pi donor

Front 72

Explain how inidividual cis and trans isomers of octahedral complexes can give both isomers as products, and why the product ratio depends on spectator Y and not leaving X

Back 72

Dissociative reaction so 5-coordinate intermediate is formed that can isomerise. This therefore depends on the attached Y as the x has already dissociated

Front 73

What are the bond breaking mechanisms of octahedral complex isomerisation?

Back 73

1) Ligand dissociation (via a non-chiral intermediate). Ligand replaced by solvent, then substituted back again by ligand resulting in a different stereochemistry.

2) Ring opening - One of a pair of chelating ligands detaches, then reattaches resulting in a different stereochemistry

Front 74

What are the bond breaking (twist) mechanisms of octahedral complex isomerisation?

Back 74

1) Bailar twist - rotates triangular face of octahedron (D3h symmetry).

2) Ray-Dutt twist - rhombic twist (C2v symmetry)

Front 75

State a similarity and difference between Bailar and Ray-Dutt twists

Back 75

Both go through trigonal prismatic transition state.

They differ in the arrangement of bridging groups

Front 76

Comment on the values of ΔS# and ΔV# for:

Ligand dissociation racemisation

Ring-opening racemisation

Twist racemisation

Back 76

Ligand dissociation: Large positive ΔS# and ΔV#

Ring-opening: Smaller positive ΔS# and ΔV#

Twist racemisation: ΔS# and ΔV# values close to 0

Front 77

What kind of substitution reactions do neutral binary 18 electron metal carbonyl complexes do, and what can be deduced about the rate determining and step and the values for ΔS# and ΔH#?

e.g. [Ni(CO)4] + L --> [Ni(CO)3L] + CO

Back 77

Dissociative mechanism. Rate determining step therefore Ni-CO bond breakage and is independent of L. Large positive values for ΔS# and ΔH#

Front 78

What are the effects of metal/charge and number of substitutions on rate of substitution?

Back 78

Metal/charge: more negative charge causes stronger backbonding with CO resulting in slower substitution. due to more e- density on metal.

More substitutions: more sigma donation from ligand more e- density stronger backbond so slower

Front 79

What is the effect of position of metal in the group on rate of CO substitution?

Back 79

Varies, weakest backbonding for 2nd row metals so fastest substitution, 3rd row slow than 1st as associative mechanism can compete for larger metals

Front 80

How can dissoicative Co substitution be promoted?

Back 80

Photochemical reactions (16 e- can be observed at low T or by flash photolysis)

Chemical activation by oxygen transfer (nucleophillic attack on CO ligand - leaves as CO2)

Front 81

For an octahedral complex Mn(CO)5Br where the Br is axial; explain why the CO is lost from an equitorial position

CHECK

Back 81

The halide Br has a lone pair in a p orbital that pushes electron density into the filled M orbitals, this pushes more electron density into the trans CO pi* orbital making it less labile resulting in the loss of a cis equitorial CO group

Front 82

What are going to be the values for ΔS# and ΔH# for the substitution of a 17-electron metal carbonyl?

E.g. [V(CO)6

Back 82

Electron deficient so will be an associative mechanism with positive ΔH# and large negative ΔS#

Front 83

How can CO substitution of an 18-electron metal carbonyl be accelerated?

e.g. [Fe(CO)5]

Back 83

1 electron oxidation/reduction

e.g. 17e [Fe(CO)5]+' (radical)

Front 84

If the rate of CO substitution changes for [Ni(CO)3NO] changes depending on the incoming ligand, what can be deduced? Why is this important?

Back 84

It follows an associative mechanism. NO+ is isolectronic with CO and it is an 18 electron complex so would expect dissociative.

Front 85

How does NO+ allow 18 electron complexes to undergo associative CO substitution (e.g. in [Ni(CO)3NO])?

Back 85

NO+ is linear M-N≡O

NO- is bent M-N=O

Conversion of NO+ to NO- requires formal transfer of 2e from M to NO, allows another ligand to come in and coordinate to M.

Oxidation state of metal changes and charge of complex, but stays 18 electron. Can convert back after loss of CO

Front 86

What is the evidence for dissociative mechanisms for dinuclear metal carbonyl as opposed to M-M bond hydrolysis?

E.g. Re2(CO)10

Back 86

Isotopic labelling using Re(185) and Re(187) show no Re(185)-Re(187) bond in the products, indicating that the original Re-Re bonds do not break

Front 87

What are the two types of electron transfer mechanisms between metal centres?

Back 87

Inner sphere: Involves the sharing of at least one bridging ligand between metal centres

Outer sphere: involves the transfer of an electron from one metal ion to another without a change in the primary coordination sphere of either metal ion

Front 88

What does inner sphere electron transfer between two metal centres involve?

Back 88

Bonds being broken and reformed

One of the reactant complexes has to be labile

Front 89

What evidence is there that inner sphere electron transfer between two metal centres occurs using a bridging ligand?

Use complex [Co(NH3 )5Cl] 2+ + [Cr(OH2 )6 ] 2+ ---> [Co(OH2 )6 ] 2+ + [Cr(OH2 )5Cl] 2+ as an example

Back 89

If Cl-(36) is added to the solution it does not show up in the product. This shows it is the same chlorine atom being passed from one complex to the other

Front 90

Why is electron transfer slower for V(II) complexes than Cr(II)?

Back 90

Cr(II) is labile d4, V(II) is less labile d3 so bridging step is rate determining step as opposed to electron transfer

Front 91

Under what conditions can H2O and NH3 take part in inner sphere electron transfer reactions?

Back 91

In high pH as they can be deprotonated

Front 92

What must be available for bridging ligands for inner sphere electron transfer processes?

Back 92

A lone pair of electrons

Front 93

Why are unsaturated organic with conjugated pi systems good bridging ligands for inner sphere electron transfer?

Back 93

Delocalisation of pi electrons helps transfer electron across bridge

Front 94

What affects the rate of outer sphere electron transfer?

Back 94

Rate is slower if eg levels involved

Rate is faster for ligands with a conjugated pi system

Front 95

What are the three necessary steps for outer sphere electron transfer?

Back 95

1) Reactants move to within required distance

2) Reorganisation of surrounding solvent molecules

3) Distortion of primary coordination spheres (i.e. M-L bond elongation/compression) through bond vibrations

Front 96

Why is the distortion of the primary coordination sphere necessary?

Back 96

Typically M-L bonds are a shorter length for more highly charged complexes

Front 97

Why would instantaneous e- transfer not work for outer sphere electron transfer?

Back 97

Would give products of wrong geometry and require instantaneous energy input

Front 98

Why do similar M-L bond distances Δd lead to faster electron transfer for outer sphere electron transfers and visa versa?

Back 98

Less distortion needed therefore small ΔG# needed and a fast electron transfer

Front 99

What about the d-electron configuration may slow down rate of outer sphere electron transfer?

Back 99

Movement of electrons too or from eg* orbitals as requires greater reorganisation (e.g. Cr(III) t2g3 --> Cr(II) t2g3 eg1

Especially if spin state is changed e.g. Co(III) t2g6 ---> Co(II) t2g5 eg2

Front 100

What is Marcus's theory:

Back 100

Self-exchange rates can be used to predict rates of other outer sphere electron transfer reactions

Front 101

What is the Marcus cross relation?

Back 101

Of two separate self-exchange reactions for M and M' where rate = k11 and k22 respectively, can work out the rate of exchange between M and M' k12.

k12 = (k11*k22*K12*f12)^1/2

Where K12 is the equilibrium constant of M and M' electron exchange and f12 is a function of other parameters which normally = approx 1

Front 102

What is the reverse of an oxidative addition reaction?

Back 102

Reductive elimination

Front 103

What happens in an oxidative addition reaction?

Back 103

A coordinately unsaturated metal (i.e. has vacant sites) donates two electrons to X-Y to give X- and Y- ligands. Coordination and oxidation number of M increase by 2

Front 104

What does the tendency for an oxidative addition depend on?

Back 104

Stability of higher oxidation states (preferred for heavier metals)

Strength of M-X, M-Y bonds relative to X-Y

Ligand electronic and steric properties - oxidation favoured by strong sigma donor ligands and inhibited by bulky ligands

Front 105

Describe nucleophillic oxidative addition mechanisms

Back 105

Metal acts as nucleophile, end on approach of X-Y. Forms linear transition state, then ionic intermediate with X, before Y- is coordinated to M to form product. Can be cis or trans.

Front 106

Describe concerted oxidative addition mechanisms

Back 106

Side on approach of X-Y, forms triangular 3-centre transition state. X-Y bond breaking concerted with M-X and M-Y bond formation. Specific cis-addition

Front 107

Which polents will be most effective for nucleophillic oxidative addition?

Back 107

Polar solvents as they can help stabilise ionic transition state and intermediate

Front 108

Why is there a large decrease in ΔS# for nucleophillic oxidative addition?

Back 108

Increased solvation of dipolar transition state

Front 109

What are the electronic and steric ligand effects on oxidative addition

Back 109

Electronic effect: strongly donating ligands promote oxidative addition by increasing nucleophilicity of the metal centre e.g. phosphines

Steric effect: Bulky ligands inhibit oxidative addition by crowding the Sn2 transition state e.g. PPr3 strong donor but slow rate due to large cone angle

Front 110

Why are solvent effects relatively small for concerted oxidative addition mechanisms?

Back 110

Non-polar transition state

Front 111

Which ligands are most likely to favour concerted oxidative addition mechanisms?

Back 111

Electronegative ligands e.g. X-

Front 112

Why does M-L sigma bond strength increases down a group of transition metals, i.e. Pd-X is a stronger bond than Pt-X, and why does reactivity decreases 3d>4d>5d

Back 112

Due to stabilisation of the complex from crystal field splitting, which increases from 3d<4d<5d

Front 113

Why do V2+ and Ni2+ aqua ions behave differently in solvent exchange?

Back 113

V2+ has a larger ionic radius and is d3 so the t2g orbitals are only half-full, and there is not too much repulsion for an incoming H2O. So a more associative (Ia) subs mechanism is favoured (evidenced by negative volume of activation)

Ni2+ has a smaller ionic radius so there is less room to associate an extra H2O. It is d8 so the t2g orbitals are full, resulting in greater repulsion for an incoming H2O. Also there are 2 electrons in the eg orbitals which have M-L anti bonding character so M-L bonding is weakened and a more dissociative (Id) subs mechanism is favoured (evidenced by positive volume of activation).