Metal-Ligand Bonding and Inorganic Reaction Mechanisms Flashcards
What is hapticity (η)?
The number of contiguous atoms of a ligand attached to a metal
What is the difference between η1-O2 and η2-O2?
For η1 the metal atom is bound only to 1 oxygen atom whereas in η2 the metal is bound to both atoms
What is denticity (κ)?
The number of donor groups in a given ligand that bind to the central metal atom (from latin meaning tooth)
What is μ?
The number of metal atoms a ligand is bound to
What is the equation for oxidation state?
Oxidation state = charge on complex - sum of the charges of all the ligands
How can you calculate the charge on a ligand?
Ask yourself how many H are needed to make a neutral molecule. (Except NO (linear) which is +1)
By convention, how are formal charges written and why?
Written as Roman Numerals, in reality atoms only have ±1 electron - the formal charge is not the actual charge distribution
What is the equation for d-electron count?
d-electron count = group number - oxidation state
What is the equation for total valence electron count (TVEC)?
TVEC = d-electron count + electrons donated by the ligands + number of M-M bonds
What are the 3 facts that determine if a ligand bonds to a metal and with how much energy in terms of orbitals?
a) Orbitals must have appropriate symmetry
b) Orbitals must overlap
c) Orbitals must be of similar energy
For an anti-bonding orbital, will it look more similar to the highest energy AO or lowest energy AO?
The highest energy AO as the antibonding orbital is high in energy. The same is true for the lowest energy AO and the bonding orbital
What is an indication that a molecule isnt stable in an MO diagram?
If electrons occupy an antibonding orbital
If there are 9 AO how many MO will there be?
Also 9.
n. of AOs = n. of MOs
What are the 3 types of orbitals in an MO diagram?
1) Bonding
2) Non-bonding
3) Anti-bonding
What are the 3 different types of ligand?
1) σ-donor
2) σ-donor, π-acceptor
3) σ-donor, π-donor
What do σ-donor ligands have? List some examples
A ‘lone pair’ of electrons to donate to the metal
- H, CH3, H2O, NH3, NR2(bent), R
What do π-acceptor ligands have? List some examples
A ‘lone pair’ of electrons and an empty orbital to accept electron density from the metal
- CO, CN, NO, H2, Alkenes, N2, O2, PR3, BR2
What do π-donator ligands have? List some examples
Multiple ‘lone pairs’ to donate to the metal
- Halides, O, OR, S, SR, N, NR2(linear), NR2(bent & linear), P, delocalised rings
In an MO diagram for a metal and its ligands, which orbitals determine bonding?
The frontier orbitals - the HOMO and LUMO
Synergic bonding occurs during σ-donor, π-acceptor ligand bonding. Explain what synergic bonding is and what happens to electron density on the metal centre during this ligand bonding.
Synergic bonding - The bond is self strengthening (both effects reinforce each other)
σ-donor interaction increases electron density on the metal centre and away from the ligand
π-acceptor interaction decreases electron density on the metal and increases electron density on the ligand
How do π-acceptor ligands stabilise low oxidation state metals?
They relieve high electron density around the metal centre
Experiments show decreasing the oxidation state of a metal results in a decrease in the bond strength within a π-acceptor ligand (e.g. C-O bond in CO). Why?
π-acceptor interaction increases as electron density is accepted by the C atom weakening the C-O bond. (Essentially resulting in a change in bonding from M-C≡O to M=C=O)
Experiments show decreasing the number of ligands around a metal decreases the bond strength within a ligand for any remaining π-acceptor ligands. Why?
Decreasing the number of ligands increases the electron density around the metal centre which is then distributed to the rest of the π-acceptor ligands. This increases the π-acceptor interaction which lowers and bonding within the ligand
Experiments show that the bond strength of C-O within a CO ligand changes when using different π-acceptor ligands for a given metal. What does this tell us about the other ligands being used?
If the C-O bond strength is decreasing then more electron density must be around the C and so there must be a stronger π-acceptor interaction. This suggests that the other π-acceptor ligand must be a weaker π-acceptor. The opposite is also true, if the C-O bond strength increases then it is becoming a worse π-acceptor and the other ligand must be a better acceptor
For CO, what does increasing the number of metal centres do to the C-O bond strength?
Increasing the metal centres increases the electron density and increases the π-acceptor interaction and therefore decreases the bond strength
What two other ligands bond similarly to the π-acceptor ligand of CO?
CN- and NO+
In terms of orbital size, why is the CO ligand more reactive than the N2 ligand?
C has a larger orbital resulting in greater overlap and bond strength than M-N. As a π-acceptor, the C-O bond weakens more than the N-N bond so it is more reactive
What are the three possible ‘forms’ of the O2 molecule binding to a metal centre?
- Neutral M - O2
- Superoxide M(+) - O2(-)
- Peroxide M(2+) - O2(2-)
What does populating the anti-bonding orbitals in O2 do to the O-O bond?
It weakens the O-O bond. Neutral O2 has a stronger bond than superoxide which is stronger than peroxide
What happens if the anti-bonding LUMO for O2 is completely occupied?
The bond order = 0 - the O-O is completely broken
What are the two ways in which covalent bonds are broken? (In relation to MO theory)
1) Increase electron density in anti-bonding orbitals
2) Decrease electron density in bonding orbitals
What are the two terminal coordination modes that NO can bind by?
Bent (double bond) and Linear (triple bond)
How many electrons does linear NO donate and what is its formal charge?
3e NO+
How many electrons does bent NO donate and what is its formal charge?
1e NO-
How do we calculate what terminal mode (bent or linear) NO is in?
1) Remove NO (neutral) from the molecule and calculate electron count and oxidation number of fragment
2) Add 1e or 3e to get to 18e (or as close as possible)
3) Determine the metal oxidation state using either NO+ or NO-
How does oxidative addition occur?
Sufficient electron density is transferred from the metal centre to the σ* orbital of H2 resulting in bond rupture and formation of a dihydride complex. (increasing oxidation state)
What do NMR spectra show occurs to alkene complexes?
At high temperatures the spectra have low resolution and at low temperatures the spectra have high resolution - suggesting the alkenes are spinning
What increases the activation energy of alkene rotation?
The π-bond changes during spinning so the stronger the π-acceptor interaction the higher the activation energy for rotation
For a π-donor interaction what is needed?
A full ligand orbitals AND an empty metal orbitals
What is interesting about the oxide (O2 (-)) ligand?
It can exhibit double or triple bond character - donating either 4e or 6e
How many electrons can an amido (NR2 (-)) donate?
2e or 4e
How many electrons can an imido (NR2-) donate?
4e or 6e
How do we electron count for with π-donor interactions?
1) Calculate the charge on the ligands for the Oxidation state
2) Group number - oxidation for d-electron count
3) d-electron + electrons donated which could be a range depending on amount of π-donation
How many electrons does N(3-) donate?
6e
What happens to Δoct for π-acceptor and π-donor ligands compared to just σ-donor ligands?
For π-acceptor ligands, the t2g orbital decreases in energy from the eg orbital resulting in an increase in Δoct
For π-donor ligands, the t2g orbital increases in energy towards the eg orbital resulting in a decrease in Δoct
What is the spectrochemical series?
A list of ligands in order of increasing field strength
π-acceptor > σ-donor > π-donor
The larger Δoct the stronger the field strength
For π-donor ligands the t2g orbital becomes π*. Why does this mean that complexes with π-donor ligands are less likely to follow the 18e rule?
It is less energetically favourable to fill the anti-bonding orbitals
Are high spin or low spin complexes inert?
Low spin complexes are inert, high spin complexes are labile as t2g and eg are similar in energy and so an electron can be moved
What are the two inorganic reaction mechanisms that we study?
1) Ligand substitution
2) Electron transfer (redox)
Which complexes are labile?
1st row and d10 complexes
What is the difference between a dissociative and associative ligand substitution? And what is the RDS for each?
Dissociative - loss of a ligand. RDS is breaking of M-L bond
Associative - gain of a ling. RDS is forming M-L bond
What are ΔS‡ and ΔV‡ and why if both of theses are negative indicate that an associative mechanism has occurred?
ΔS‡ - entropy difference between initial state and transition state
ΔV‡ - volume difference between initial state and transition state
If both a negative this suggests lower disorder and smaller volume in the transition state
For square planar molecules during ligand substitution stereochemistry is usually preserved. Why is it sometimes not preserved?
If the 5-coordination transition state complex is long lived then Berry pseudorotation can occur
What are the 4 thing that effect the rate of ligand substitution?
1) The entering group
2) The leaving group
3) The nature of the other ligands in the complex
4) The metal centre
What is the trans-effect?
A kinetic phenomenon that describes the influence on a non-labile group on the rate of substitution of a ligand trans to it.
π-acceptor > π-donor > σ-donor
For a “soft” metal like Pt is the rate of substitution by an entering group faster for “soft” or “hard” nucleophiles?
The rate is greater for “soft” nucleophiles
For a “soft” metal like Pt is the rate of substitution by a leaving group faster for “soft” or “hard” nucleophiles?
The rate is greater for “hard” nucleophiles
What is ΔG‡?
The free energy of activation - the difference between the reactant ground state and the first transition state
What are the two ways to decrease ΔG‡?
1) Destabilise the ground state by weakening the bond trans- to the ligand leaving
2) Lower the energy of the transition state with a ligand with π-acceptor character to accept electron density from the nucleophile
As you go down a group are the M-L bonds in a complex more or less strong
More strong as orbitals are larger and have better overlap
For octahedral ligand substitutions which of the 3 mechanisms is like SN1, SN2 and rare?
Associative - must go via a 7 coordinate intermediate and is rare
Dissociative - SN1
Interchange - SN2 and most common (simultaneous bond breaking and making)
What suggests that there is a dissociate RDS in octahedral ligand substitution reactions?
1) Varying the entering group makes little difference to the rate
2) Plotting bond strength vs rate gives a linear free energy relationship
3) Large spectator ligands means that the transition state is preferred and the rate increases
4) If the mechanism was associative then 20e intermediate complexes would be formed which is unfavourable compared to 16e complexes for the dissociative mechanism
What are the two electron transfer mechanisms that we consider?
- Outer sphere mechanisms
- Inner sphere mechanisms
What occurs in outer sphere mechanisms?
Electrons are transferred between complexes without substitution - no bonds a formed or broken
What occurs in inner sphere mechanisms?
Electrons are transferred between complexes via a bridging ligand - bonds are broken and formed
What are the 3 steps in outer sphere mechanisms?
1) Formation of precursor complex
2) Activation/reorganisation of precursor complex. Electron transfer. Relaxation of successor complex
3) Dissociation of successor complex
Why do the M-L bonds change length?
So that the electron can jump between orbitals of similar energy
Why must the M-L bond change first?
The change in coordination number of the complex will cause the bonds to change lengths and release heat - we need to put in energy to ensure the conservation of energy
Does a large change in bond length mean the electron transfer is fast or slow, and is it faster for eg or t2g?
Small bond length change is faster
eg –> eg (σ) is a large bond length change and therefore a slow electron transfer (+poor orbital overlap)
t2g –> t2g (π/π/nb) is a small bond length change and therefore a fast electron transfer
Why do 2nd and 3rd row complexes have faster outer sphere electron transfers than 1st row?
Their orbitals are bigger and have larger orbital overlaps
During inner sphere electron transfer a bridging ligand must be formed, what must happen in an octahedral complex for this to occur?
Ligand dissociation must occur to allow a bridging ligand to bond
How can we prove that when a ligand is transferred from one complex to another in an inner sphere transfer, that it DIRECTLY comes from the other complex and not from solution?
We can isotopically label that ligand before the transfer and find the same ligand on the other complex after the electron transfer
Is ligand transfer a requirement for an inner sphere mechanism?
NO - It depends on the relevant bond strengths of both complexes and all their ligands
What factors affect the rate of inner sphere electron transfer mechanisms?
- Electron transfer is usually the RDS but the formation of the bridging complex (or dissociation of the complex) could be the RDS
- The bridging ligand
- Which orbitals are involved
Why is inner sphere electron transfer so fast for eg orbitals?
There is good orbital overlap of the eg orbitals in the molecular bonding orbital
What are two factors that affect how good a bridging ligand is for inner sphere electron transfer?
1) The size of R groups - larger R groups cause steric hinderance
2) The ligand might be able to mediate the electron transfer which circumvents the need for reorganisation of complexes in outer sphere transfer
How do we distinguish if electron transfer is outer or inner sphere?
- Is there a vacant coordination site?
- Is there a labile reactant?
- Is there a potential bridging ligand?
- Has a ligand been transferred in the reaction?
- Are there large differences in rate on addition or substitution of a bridging ligand?
How can we tell if an electron transfer is outer or inner when using the rates for N3- and NCS-?
If kN3-/kNCS- ~ 1 then it is outer sphere
If kN3-/kNCS-»_space; 1 then it is inner sphere
