CH267: Transition Metal Coordination Complexes: Stability and Redox Reactions

What must a ligand have in order to allow inner sphere-electron transfer?

A spare lone pair of electrons must be available to allow a ligand to span two metal centres (eg. halide). It also works with aromatic ligands with two N-donor termini.

How do we calculate beta?

In general, it is the product of all the K values up to that point.

What is ∆S‡ in terms of transition states and what important information does it give us?

It is the change in entropy to attain the transition state. It gives us useful mechanistic information.

What is 'K' and what does it tell us?

It is the equilibrium constant (thermodynamics) and tells us about the position of equilibrium. (Not rate)

What is ∆H‡ in terms of transition states and bonds.

It is the increase in enthalpy to attain a transition state. It is a measure of the extent of bond breaking. It is large if a strong bond is broken in the rate-determining step. Basically bond strength!

What is 'k' and what does it tell us?

It is the rate constant (kinetics) and tells us about how fast the reaction goes. (Not position).

What is beta?

It is the stability constant. It is cumulative (all the steps put together up to that point).

What is ∆V‡?

It is the volume of activation. This is the change in volume to attain the transition state.

What is associative ligand substitution?

It is two step process, where the ligand binds in the first step to form an intermediate. Then in the second step, the leaving group leaves.

Why is ΔH for tren coordination unfavourable in competition reaction? (Substituting for en ligands)

It is unfavourable because one Ni-N bond is weak and folding up of the tren ligand creates unfavourable ligand-ligand reactions. However, entropy is still large, so this compensates for ΔH.

What three things have to happen for an electron to transfer in outer-sphere electron transfer?

1. Approach of reactant complexes to required distance. 2. Reorganisation of secondary solvation shells. 3. Distortion of primary coordination spheres (M-L distance changes - as without this after electron transfer the M-L distances would be too long. Eg. M3+ would have long bonds that should be short and vice versa for M2+).

What are the two steric issues associated with chelating ligands?

1. Bite angle (can lead to small loss of CFSE, ΔH slightly unfavourable). 2. Chelate ring size (5- and 6- membered rings generally most stable).

What is the equilibrium constant in terms of kinetic quantities?

K = kf/kr (rate of forward reaction/ rate of backwards reaction).

What happens in base catalysed hydrolysis (-OH), say with an ammonia ligand?

The -OH deprotonates an ammonia ligand, leaving it as NH2-. Then Cl- leaves as it is a good leaving group, leaving us with an amido ligand. (=NH2). Water is then added, substituting an -OH ligand and the amido ligand protonated to give back ammonia.

What happens when chelating ligands bind?

There is a large increase in entropy.

What are class 'a' metal ions also called?

They are called 'hard' they are small and highly charged. They have a high charge density and are highly polarising (pull electron density towards themselves).

What are class 'b' metal ions also called?

They are called 'soft' and they are large with a smaller charge. They have a low charge density and are weakly polarising.

Why do soft cations and soft ligands combine well?

They are large cations and anions with low charge density and large distances between them. So there is weak ionic/electrostatic bonding. However, they have weak electronegativity, so extended orbitals which means good covalent bonding dominates.

What are soft ligands?

They are larger, less electronegative donors. They are more polarisable. Eg. H-, C2H4, CN-, CO, PR3, AsR3, R2S, RSH, RS-, I-.

What are siderophores?

They are molecules which chelate Fe(III) extremely strongly.

When are rates faster and slower in outer-sphere electron transfer?

They are slower when eg electrons are involved. They are faster with aromatic ligands.

What are hard ligands?

They are small, electronegative atoms with high negative charge density. They are not polarisable. Eg. NH3, H2O, -OH, ROH, RO-, CH3CO2-, F-

Why do hard metals and hard ligands combine well?

They are small, electronegative cations and anions with high charge density. So there is strong ionic/ electrostatic bonding which dominates. This is because there is poor orbital overlap as the orbitals are small and close to the nucleus so there is little orbital overlap.

Are more electronegative atoms better or worse sigma-donors?

They are worse sigma-donors as they hold onto their electrons more.

How does Co(III) stabilise an NH2- ligand?

Via pi-donation. There is a sigma-donor dative bond as usual, but there is also a pi-bond from two electrons in a p-orbital. The high positive charge of Co(III) stabilises deprotonated NH3.

What can we use to synthesise inert complexes?

We can use Zn as a reducing agent and then oxidise to the desired complex.

How do we get the right bond lengths in outer-sphere electron transfer?

We wait for random vibrations to bring the starting material to an intermediate geometry that the product can also tolerate. The bond lengths of the two complexes then become the same, so the change in energy = 0, and therefore there is no barrier to electron transfer. Finally, the two complexes will then relax to optimal geometry.

How do we induce the loss of X in the trans effect?

We want T (our trans ligand) to be a good pi-acceptor. This means that the electron density from the incoming ligand Y can be delocalised onto T, stabilising the intermediate. This induces a loss of the leaving group, X.

How do increase reaction rate using only ∆S‡ and ∆H‡?

We want to decrease ∆G‡, so we can increase ∆S‡ or decrease ∆H‡.

What signs do we have to put before delta G, H and S to make them favourable?

-G, -H, +S

What are the four things that outer-sphere electron transfer self-exchange rates depend on?

1. Change in M-L bond distances (small = fast) 2. Ligand properties - conjugated pi-systems delocalise electrons and speed up ET to/from the metal ion at the centre of the complex. Provides 'conducting bridge' to help ET. 3. d-electron configuration - eg electrons are antibonding so cause significant change to M-L distances (slow) 4. Spin-state changes - make ET slow as collapse to ground state is spin-forbidden.

What is the inertness of a complex in water substitution related to?

1. Charge density on metal ion (M-O bond) 2. High CFSE for TM ions - ions with high CFSE create activation energy barrier in 5 coordinate intermediate. (dxy electrons temporarily raised in energy and dz2 stabilised). The least labile will have the strongest CFSE. 3. (Jahn Teller)

How do we go from a trans octahedral complex to a cis one?

1. Dissociation of ligand to give a 5-coordinate intermediate. 2. Berry-pseudo rotation: conversion between trigonal bipyramid and square pyramid. 3. Re-formation of M-Y bond: leads to mixture of cis and trans isomers.

What are the different ways an octahedral chelate complex can isomerise?

1. Dissociation, Berry pseudo-rotation, reformation. 2. For rigid chelating ligands (e.g. bidentate), the same can happen as above, but only one side of the ligand has to dissociate, whilst the other side remains attached. 3. Twisting - where no bonds are broken (Bailar twist or Ray-dutt twist).

What can a ligand substitution mechanism vary with?

1. Ion size (smaller ions most likely to be associative) 2. Charge 3. Configuration

What are the 4 reasons for the steadily decreasing K values pattern to be broken?

1. Irregular coordination geometry around metal (Eg. Cu(H20)6, Cu2+ Jahn Teller - forms weaker bonds due to elongation of axial bonds). 2. Change in coordinate geometry at one step. 3. Steric hinderance. 4. Change in electronic structure (change in spin state).

What are three types of ligand substitution reactions? (Not mechanisms).

1. Solvent exchange 2. Anation (replacement with anion) 3. Aquation (replacement with water) - dissociative

Give a brief outline of the inner-sphere electron transfer mechanism.

1. The labile complex loses a ligand, and attaches to the bridging ligand of the other complex to form a precursor complex. 2. Electron transfer occurs to form a successor complex. 3. The bond is broken between the bridging ligand and it's original complex (so the bridging ligand now ends up on a different complex to where it began). The original ligand is added to the empty ligand space. V2+ is used as a reductant (eg. for Co and Cr). ET = RDS (usually, but can sometimes be bridge formation).

What is the difference between a transition state and an intermediate?

A transition state is at a maximum and cannot be observed or isolated as it is too short-lived. An intermediate is a local minimum and has a finite lifetime. It may be observable spectroscopically or even isolated. It only takes a small activation energy to further the reaction in either direction.

Why is complex formation with bidentate ligands faster than with monodentate ligands?

As one side of the ligand binds and then the second M-L bond can form easily because it is already close to the metal.

Why does the log K of lanthanide 3+ cations increase as we go across the series?

As there is a steady decrease in ionic radius (lanthanide contraction) which causes a steady increase in charge density across the series.

Which type of substitution are larger ions more likely to undergo?

Associative substitution, as they can form 7-coordinate transition states.

How do spectator ligands have an effect on the rate of substitution?

Bulkier R groups slow down the reaction, especially if they are cis to the reaction site (leaving group).

How do we get pure copper metal? What are the four steps?

By using salicyladoxime. 1. Leaching (CuO + H2SO4) 2. Extraction (2HL + CuSO4) 3. Stripping (CuL2 + H2SO4) 4. Reduce (Electrochemically)

What is a large contribution of delta H dependant on?

Bond strengths.

What are intermediate cations and ligands?

Cations of medium charge/ size (most transition metals) - Co2+, Fe2+, Rh3+ Ligands of medium electronegativity/ size - pyridine, Br-.

What is the volume of activation for a dissociative and associative ligand substitution? What happens to the rate constant with pressure?

Dissociative - ∆V‡ > 0, as the transition state occupies more volume when one ligand is leaving. The rate constant decreases with pressure. Associative - ∆V‡ < 0, as the transition state occupies less volume when one ligand is simultaneously leaving and adding. The rate constant increases with pressure.

What metal can we use to get excited 1H nuclei to relax faster?

Gd3+ complexes (highly paramagnetic). They have molecular vibrations which produce oscillating magnetic fields which stimulate excited 1H nuclei to relax faster. However, Gd3+ is highly toxic! So it must be wrapped up in strongly binding ligand.

Which metals are considered class 'a' metal ions and what ligands do they form complexes with?

Hard metals. Form stronger complexes with F- > Cl- > Br- > I-. Ti4+, Cr3+, Na+, Ca2+. They form strong complexes with ligands containing first-row donors (more electronegative).

What determines the rate of outer-sphere electron transfer?

How long it takes the starting material and product to match sizes by random vibrations, which depends on how different they are. Bigger differences in geometry = more initial distortion needed = higher activation energy = slower rate.

Why is it much harder to remove a bidentate ligand?

If one M-L bond breaks, the other end of the ligand is still attached to the metal so the ligand is not lost. Two donor sites reinforce each other, so you need to break both bonds at the same time to lose, for example, an en ligand.

What can affect whether a mechanism is purely associative or not?

If the Pt-X bond is weakened in the transition state. Dependent on rate of leaving group and whether the Pt-X bond is strong or weak.

What type of substitution do we have if ∆S‡ is positive or negative?

If ∆S is positive, then we have a dissociative (breaking first) mechanism as the transition state has a higher entropy than the reactant(s). If ∆S is negative, then we have an associative (making) mechanism as the transition state has a lower entropy than the reactant(s).

How does solvent participate in a ligand substitution mechanism? Use an associative mechanism as an example.

In an associative mechanism, we would expect to see a y intercept of 0 if we create a pseudo first order reaction with an excess of ligand. However, we don't! This shows us that coordinating solvents can make the background reaction faster. k1 depends on solvent! It suggests that we see two substitution mechanisms running in parallel; one associative mechanism dependent on the ligand, and one in which the solvent is involved in the RDS.

What is dissociative substitution?

It is a 2 step substitution, conceptually similar to organic Sn1. There is an initial loss of a leaving group, X, to give an intermediate MLn. The intermediate has a coordination number decreased by 1. Then in the next step, the entering group, Y, is added.

What is Ia?

It is associative interchange - where the transition state is 'tight' as the bonds are short. There is a strong M-Y interaction without much M-X dissociation, so it is more like associative. Normal I is when you are between Ia and Id - 50% loss of X/binding of Y

What is Id?

It is dissociative Interchange - where the transition state is 'loose' as the bonds are long. There is a substantial M-X extension and weak M-Y interaction, so it is more like dissociative.

What is Inner-Sphere electron transfer?

It is electron transfer that occurs between metal ions via a shared bridging ligand. M-L bonds are broken and formed. It requires one reactant to be labile.

For square planar complexes, is ligand substitution normally associative or dissociative?

It is normally associative (Ia or A) as it is possible for there to be a temporary expansion of coordination number to 5 using vacant coordination sites.

What is the trans effect in substitution in square planar complexes?

It is the ability of a coordinated ligand to direct substitution trans to it.

What is a labile and inert complex?

Labile - Complexes that undergo fast ligand substitution reactions. (Most first row transition metals). Inert - Complexes that undergo slow substitution reactions. (Cr3+ and Co3+ with water).

What are chalcophiles?

Metals found in combination with sulfide, S2- (soft ligand).

What are lithophiles?

Metals found in the earth's crust in association with oxide, O2- (hard ligand).

What is the ratio of the probability of the forward reaction to the back reaction in ligand substitution?

N-n/n+1

What type of complexes is dissociative substitution common in? How can we stabilise 5-coordinate intermediates in this case?

Octahedral complexes, as they cannot easily expand their coordination sphere. By donation of electrons from the solvent.

What is an interchange ligand substitution mechanism?

One in which there is concerted M-X bond cleavage and M-Y bond formation in a single process. It is mid-way between A and D with no intermediate. However, it does have a transition state with a coordination number increased by 1 (one bond half formed, one bond half broken). It is conceptually similar to an organic Sn2 reaction.

What are the two main mechanisms for electron transfer between metal complexes?

Outer sphere and inner sphere.

What is the rate of a dissociative substitution?

Rate = k[MLnX]. It is independent on the nature of the incoming ligand Y (second step). It is first-order overall.

What is the rate of an associative substitution?

Rate = k[MLnX][Y]. It depends on the nature of the incoming ligand, Y. It is second order overall.

Which metals are considered class 'b' metal ions and what ligands do they form complexes with?

Soft metals.s Forms stronger complexes with I- > Br- > Cl- > F-. Cu+, Ag+, Hg+, Hg2+, Pd2+, Pt2+. They form strong complexes with ligands containing second or third row donor atoms (less electronegative).

In what type of complexes is associative ligand substitution common?

Square-planar complexes as they can expand their coordination spheres. This can create a 5-coordinate intermediate that is sufficiently stable to be isolated and characterised by X-ray crystallography.

If you have a hard donor set and a strong chelate effect, what does this mean for binding even in water?

Strong binding!

What is the principle of microscopic reversibility?

That the lowest energy pathway (i.e. mechanism) must be the same for forward and reverse reactions.

Why does Cu2+ form the most stable ligand complexes in the Irving Williams Series?

The electrostatic effects are best as it has the smallest ionic radius/ strongest bonds. The Jahn-Teller effect often gives extra stability to Cu2+ complexes (unique to Cu2+). However, the CFSE effects are the greatest for Ni2+.

What is the Chelate Effect?

The enhanced stability of a complex containing polydentate ligands compared to a complex containing similar monodentate ligands. For example, one bidentate ligand will bind more strongly than two monodentate ligands.

What is the rate limiting step in a dissociative substitution?

The first step to give the intermediate is slow and the RDS (small k1). The second step to give the final product is then fast (large k2)

For chelating ligands, is the forward or backwards reaction faster?

The forward reaction is faster (kf increased) and the backwards reaction is slower (kr decreased).

What does the rate of exchange of solvent from the 2nd coordination sphere to the primary coordination sphere depend on?

The properties of the metal.

What does the intensity of a signal depend on in NMR?

The relaxation time of a proton after excitation. A nucleus cannot absorb another photon until it has relaxed.

What is the rate limiting step in associative substitution?

The rate-limiting step is the first step in which the new ligand binds (small k1). The second step to give the final product is then fast (large k2). The intermediate has a coordination number increased by 1.

Which ligands are better at trans directing in square planar complexes? Why?

Those that are good pi-acceptors and good sigma-donor power, e.g CN-, CO, NO, C2H4. (strong field ligands). The weakening of a bond by a good sigma-donor ligand trans to it makes it more likely to break and be replaced. This is because the lone pair on the leaving group will be repelled by the trans ligan and so the M-X bond is weakened.

What is Outer-Sphere electron transfer?

Transfer of an electron from one metal ion to the other without any change in the primary coordination sphere of either metal ion. No ligands are shared between the two metal centres during the process. No bonds are made/broken but there is a reorganisation of secondary solvation shells. Need an oxidant and a reductant - two different oxidation states.

What is a 9-coordinate structure called?

Tri-capped trigonal prism.

How many transition states are there in dissociative substitution?

Two - one for bond breaking and one for bond formation.

How many transition states are there in associative substitution?

Two - one for bond forming and the second for bond breaking.

What structural and spectroscopic evidence do we see for the trans effect?

X-ray crystallography we can see that the M-X bond is significantly longer.

Is substitution of Pt(II) complexes, is stereochemistry retained?

Yes!

Can fully dissociative mechanisms operate for square planar complexes?

Yes, if we use ligands such as SMe2 which are labile. This is due to the trans influence of strong donor Me groups (relatively weakly bound).

What d electron configuration do we normally find metals in for square planar complexes?

d8