4b: M-L bonding

Question 1

Name and define the greek letters for labelling ligands.

Answer 1

η = hapacity - adjacent linked atoms coordinating to a metal

κ = denticity - non-adjacent linked atoms attached to a metal

μ = number of metal atoms bridged by a ligand

Question 2

What is the orbital order for metals?

Answer 2

3d is lower than 4s due to orbital shielding.

Question 3

What is the equation for oxidation state for a metal?

Name common ligand charges with examples.

Answer 3

Complex charge - ligand charge

+1 = NO (linear)

0 = CO, NR3, PR3, N2, O2, H2, C2H4, H2O, C6H6

• 1 = F, Cl, Br, I, H, CN, NR2, NO (bent), C5H5
• 2 = O, S, CO3, NR
• 3 = N, P

Question 4

What is the equation for d electron count and total valence electron count (TVEC)?

Name the common numbers of electrons donated by a ligand and give examples.

Answer 4

d-electron count = group no. - oxidation state

TVEC = d-electrons + electrons from ligands

2e = CO, NR3, PR3, N2, O2, H2, C2H4, H2O, H^-, CH3^-, X^-

4e = en, η⁴-dienes, NR2^-, O2^-

6e = (η⁵-C5H5)^-, η⁶-C6H6, NR^2- (linear), O^2-

Question 5

What rule defines the most stable complexes? What are exceptions to this?

Answer 5

The 18 electron rule decide how stable a molecule is. The metal electrons plus the donated electrons.

Exceptions are: 1st row complexes with mainly ionic bonding

Square planar d⁸ complexes (16e^-)

Early metal (low d electrons) with π-donor atoms

Paramagnetic complexes

Question 6

What are the 3 types of ligands and some examples of each?

Answer 6

σ donor: H^-, R^-, NH₃, H₂O

σ donor, π acceptor: CO, CN, NO, H₂, N₂, O₂, C₂H₄, PR₃

σ donor, π donor: X^-, O^2-, S^2-, RO^-, N^3-, NR^2-, NR₂^-

Question 7

Describe COs σ donor and π acceptor interactions. How does this affect CO bonding?

Answer 7

The CO homo, 5σ, donates into the metal d orbitals and the lumo, 2π (antibonding), accepts the d orbital electron density. The two reinforce each other, called synergic bonding.

As the density from the metal increases and the anti-bonding π orbital is filled and the σ donation increases. M-C≡O → M=C=O

Question 8

Describe the trends in (CO)ν, where a lower ν means a weaker CO.

Answer 8

(CO)ν decreases as the oxidation state decreases as complexes are more electron rich.

(CO)ν decreases with fewer ligands as the electron back donation is less shared.

(CO)ν decreases as other ligands donate more electron density into the metal.

(CO)ν decreases as other ligands accept less electron density.

In summery - CO gets weaker when more electron density is donated to it.

Question 9

Why are isoelectronic ligands such as CN^- and NO⁺ are good ligands but N₂ is a poor ligand?

Answer 9

The nitrogen atoms have the same energy so the orbitals are the same size. On the other ligands the donating orbital has a lower energy orbitals so they have a larger orbital density where the density is being donated from.

Question 10

Describe and draw the 2 different σ donor, π acceptor interactions that O₂ can have.

Answer 10

O2 can have η1 or η2 bonding but in both cases the homo and lumo are the half filled π* orbital.

Question 11

What extreme states can oxygen as a ligand exist in and how does this come about?

Answer 11

The oxygen is very oxidising so it can take the electrons being back donated to it and form the superoxide O₂^- or the peroxide O₂^2-.

Question 12

How are the O^2- and N^3- ligands formed?

Answer 12

O2 and N2 coordinate to a powerful reducing agent such as Ta(OSi(^tBu)₃)₃ which occupy the π* and σ* orbitals of the diatomics, breaking the diatomic bond.

Question 13

How many electrons does NO donate in its bent and linear form?

Answer 13

Linear NO donates 3 electrons - 2 electrons from the homo and and one free electron given to the metal.

Bent NO donates 1 electron - 2 electrons from the homo but takes one electron to form NO^-.

Question 14

How do you find the TVEC and oxidation state for metals with complex ligands such as NO?

Answer 14

Ignore the complex ligands and find the ox. and TVEC.

Add 1 or 3 electrons to the TVEC to get as close as possible to 18, the closest without going over is the state the NO takes.

Adjust the ox. state according to NO (+ if bent, - if linear).

Question 15

What is the proccess of forming 2 hydride molecules from H₂ coordinating to a metal called and how does it occur?

Answer 15

Oxidative addition. The π back donation fills the σ* orbital and breaks the H₂ bond.

Question 16

Describe how alkenes act as ligands. How can this be monitered?

Answer 16

The homo is the π orbital so with low back donation the alkene remains intact and donates from one orbital. As back donation increases into the π* orbital the alkene bond weakens and the carbon atoms change from sp² to sp³.

The alkene can rotate easily with no back donation and at a high energy cost when the back donation is large however sometimes other orbitals can form which stablise the rotation.

Question 17

Why is PR₃ a good ligand?

Answer 17

It is a π acceptor ligand with strong σ donation. The back donation can stablise low oxidation states and the strong σ donation stablises high oxidation states.

Question 18

What ligands are π donors? What metals are best suited to this?

Answer 18

Ligands that can form multiple bonds to the same atom (without back donation) such as oxide and nitride.

The metals best suited are high oxidation state/low d electron count as there must be an empty d orbital.

Question 19

How many electrons does oxide donate? Draw out this interaction. How does it compare for NR₂^-?

Answer 19

Oxide is a 4-6 electron donor.

NR₂^- is the same except 2-4.

Question 20

How does the nature of the t_2g orbital change with different types of ligands? Draw a diagram to show how Δ_oct changes.

Answer 20

Question 21

How do each type of ligand affect the 18 electron rule?

Answer 21

With π donors the t_2g orbital is antibonding so it is often more stable with less than 18 electrons.

Question 22

In the spectrochemical series, where are the different types of ligands?

Answer 22

π-acceptor are stong field, σ-only are in the middle and π-donor are weak field.

Question 23

What is the trans effect and what are the major influences of this?

Answer 23

The trans effect is where a non-labile ligand influences the rate of substitution on the ligand trans to it. The ligands with the greatest labilising effect are CO and CN^-.

Question 24

What are the 5 types of inorganic mechanism?

Answer 24

Ligand substitution, association, dissociation, redox reactions and reactions of coordinated ligands.

Question 25

What are the different types of ligand substitution reactions?

Answer 25

They range from dissociative to associative with most inbetween, called interchange.

Question 26

Give examples of labile and inert complexes.

Answer 26

Labile: d¹⁰ complexes (no CFSE)

3d M(II) ions

Inert: d³ and low spin d⁶ complexes

4d and 5d complexes

cheleating ligands

Question 27

Name the 3 activation parameters.

Answer 27

ΔV^‡ is the volume of activation, the difference between the activation state and the ground state

ΔH^‡ is the enthalpy of activation

ΔS^‡ is the entropy of activation

Question 28

How do square planar complexes (normally d⁸) undergo ligand substitution?

What is the rate of the reaction?

Are the reactions stereospecific?

Answer 28

By an associative mechanism with either the solvent or the ligand before the leaving group leaves.

This is evidenced by negative activations of entropy and volume.

Rate = k₁[ML₃X] + k₂[ML₃X][Y] where [solvent] = constant

Berry psuedorotation can occur in the 5-coordinate intermediate.

Question 29

What 5 factors affect the rate of substitution of ligand?

Answer 29

1. The entering group - nucleophilicity and well matched hardness
2. The leaving group - hardness and stability
3. The trans effect - competition over the same orbital, strong σ donors are strong influences of this: H^-, R^-, CO, PR₃, I^-
4. Stabilising the transition state - π-acceptors stablise charge build-up. The combination of stablisation and the trans influence means the best ligands for a trans substitution are CN^-, CO, NO⁺, C₂H₄
5. The metal centre - 3d reacts quicker than 4 and 5d due to worse M-L overlap and therefore lower lower activation energy

Question 30

What are the substitution reactions for octahedral complexes and how can they be differentiated?

Answer 30

I_a and I_d represent interchange initiated by either an associative or a dissociative process.

They can’t be distinguished by the rate law as the solvent masks the associative reaction as first order.

Evidence for I_d:

• The entering group has small effect on the rate
• There is a linear relationship between logK (reflects bond strength) and logk (rate of addition) hence the RDS slows when the M-LG bond gets stronger
• Steric effects, if L is large, are much more prominent
• Electron count favours 16e intermediates over 20e.

Overall: square planar = associative, octahedral = dissociative

Question 31

What types of electron transfer are there? Define them.

Answer 31

Outer sphere - no bonds broken or formed

Inner sphere - bridging ligand links the complexes which may transfer

Question 32

What are the three steps of outer sphere transfer?

What three factors affect the rate of OS?

Answer 32

Forming the precursor complex, then transfering an electron, then dissociating.

ΔG^‡ = ΔG_t^‡ + ΔG_O^‡ + ΔG_i^‡

ΔG_t^‡ = the energy to overcome the coulombic potential energy and bring the complexes together

ΔG_O^‡ = the energy to reorganise the solvent molecules around the complexes to form the precursor complex

ΔG_i^‡ = the energy to adjust the M-L bond lengths so the orbitals have the same energy

Question 33

What are the requirements which limit the rate of OS electron transfer?

Between which orbitals is OS fast?

Answer 33

It requires orbital overlap and orbitals of the same symmetry with orbitals of the same energies.

As e_g is a σ* orbital in octahedral complexes the ligands sterically interfere with it so a large change in bond length is required. Therefore e_g to e_g transfers are very slow.

t_2g orbitals are π or π* so the change in bond length required is much smaller and there is better overlap. This makes the transfer fast.

Question 34

What may slow OS electron transfer down?

Answer 34

Small orbitals on higher up metals. Activation energy to transfer electrons into a configuration able to transfer electrons.

Question 35

Describe how bond lengths change during OS electron transfers.

Answer 35

The bond lengths average between the reactant and product, then the electronic-vibrational coupling, ΔE_c, determines the probability(they are proportional), κ_el, the electron will transfer.

The bond length difference, Δr, is related to ΔG^‡ (activation energy) by r² ∝ G.

Question 36

What is the Marcus inverted region and how did it come about?

Answer 36

Plotting ΔG^‡ (free energy of activation) against ΔGº (free energy of reaction) is an x²=y plot.

The rate of transfer increases as the energy of the reaction increases until there is no longer an energy barrier. Then as the energy of the reaction continues to increase, the reaction barrier increases again so the rate slows down.

Question 37

How can you predict the rate of an electron transfer react?

Account for the differences between the predicted rates and the observed.

Answer 37

Using the Marcus cross-relation. k_AB = (k_AAk_BBK_AB)^½ where K_AB can be caluclated from constants.

Differences: No differences in attractions from self exchange assumed

Some reactions may not procced due to the electronic-vibrational coupling

Not all cross reactions go by outer sphere

Question 38

What steps have to be taken for octahedral atoms to under IS electron transfer?

When is it faster than OS?

Answer 38

Ligand dissociation from one atom, bridging complex forms, transfer, breaks, ligand assocaition.

IS is faster when OS requires a large reorganisation as the e_g orbitals overlap with the ligands. However large ligands heavily influence the ability to form bridging ligands.

Question 39

How do you distinguish between a IS or OS transfer?

Answer 39

Is there a vacant coordination site?

Is there a labile reactant for substitution? (d0,1,2,4 HS, 5 HS, 7 HS)

Has ligand transfer occured?

Does the rate change when a bridging ligand is added/substituted?

Is the rate with N₃^- faster than with NCS^-?