Transition metals Flashcards
Properties of transition metals
- form compounds which are often paramagnetic
- shows variable oxidation states
- formed coloured ions and compounds
- form compounds with profound catalytic activity
- form stable complexes
- have high melting and boiling points
What’s the chemistry of transition metals dominated by?
the d-orbitals (valence electrons are always 3d, not 4s) which are lower in energy than the s-orbitals
The Lanthanide Contraction
- The ionic radii of the lanthanide ions decrease with increasing atomic number (they get heavier but smaller)
- f electrons do not shield very well but more protons are added
What are ligands treated as?
point negative charges
What is Δ o ?
crystal field splitting parameter
What do the symmetry labels e and t mean?
e = doubly degenerate
t = triply degenerate
What sort of Δ do tetrahedral structures have?
Δ is always much smaller for tetrahedral complexes than square planar therefore they are almost always high spin
How does crystal field splittings vary with oxidation state?
Δ are usually larger for metals in higher oxidation states because the higher charge on the metal exerts more attraction to the ligands resulting in shorter bond lengths and more interaction between d-orbitals and ligand charge
Factors affecting Δ o
- oxidation state of metal
- position on periodic table
- type of ligand
How does crystal field splitting vary with position on periodic table?
Δ are usually smaller for the 1st row and so first row transition metals have a tendency of being high spin
Δ link to wavelength
small splitting at higher wavelengths (red)
large splitting at smaller wavelengths (purple)
What is Δ o measured between in a MO diagram?
t 2g and e g *
General trend of the spectrochemical series for crystal field splittings
⫪- donor < weak ⫪- donor < no ⫪ effects < ⫪- acceptor
(⫪- donor make Δ smaller)
What are the different light absorption and electronic transitions?
- Intra-atomic (localised) excitations
- Interatomic (charger transfer) excitations
- Molecular Orbital (HOMO to LUMO) excitations
- Band to band transitions
- Intraband excitations
- Defects and colour centres
What are the 2 key light absorption and electronic transitions for transition metals?
- Intra-atomic (localised) excitations
- Interatomic (charger transfer) excitations
What are the selection rules for transitions?
Spin selection rule, ΔS = 0
Laporte selection rule, Δl = +/- 1 because a change in parity is required for complexes with an inversion centre, i
When can the selection rules for transitions be relaxed?
- Spin forbidden transitions are ‘partially allowed’ because of spin-orbit coupling
- Laporte rule is relaxed if there’s no centre of symmetry because of orbital mixing
What is ε in the beer lambert law?
a molecules ability to absorb light at a given wavelength
What type of transition is Fe(II) → Fe(III)?
electron transition is a low energy transition therefore absorbs in the red region
The Jahn- Teller effect
any non-linear molecule with a spatially degenerate electronic ground state will undergo a geometrical distortion that removes that degeneracy because the distortion lowers the overall energy of the species
(the z repulsions are reduced)
Dynamic Jahn-Teller effect
exceptions to jahn-teller effect occurs when the time frame of the measurement does not allow the distortion to be seen because the molecule randomly undergoes movement
What can x-ray be used to see in structures?
X-ray structure obtained at varying temperatures is possible to “freeze” a molecule into a static position showing the distortions
How do the orbitals differ when undergoing z elongation and z compression in Jahn-Teller?
Z elongation
- z components go down in energy
Z compression
- z components go up in energy
What can magnetic moments be used to determine?
the number of unpaired electrons
How is magnetic moment measured?
using a Gouy Balance
What are assumptions to be made with the spin-only formula?
- Only electron spin contributes to magnetism
- Orbital contribution is negligible (valid for many first-row TM but not for heavier)
- Assumes high-spin configurations unless stated otherwise
Fundamentals of how the Gouy method works
Makes use of the interaction between unpaired electrons and a magnetic field
- A diamagnetic material is repelled by a magnetic field
- A paramagnetic material is attracted into it
What is an alternative to using a Gouy balance to measure magnetic moments?
SQUID
(Superconducting quantum interference device) for measuring magnetic susceptibilities.
It is extremely sensitive.
What is a quantum phenomenon that can occur with orbital contribution when using magnetic susceptibility to determine geometry?
Orbital contribution is often not entirely quenched when there is a partially filled t 2 or t 2g set.
The t 2 or t 2g orbitals are related by rotational symmetry and a non-zero orbital contribution is expected when these rotations move an unpaired electron from one orbital to another.
What is spin crossover?
It occurs when the spin pairing energy (P) is approximately equal to the crystal field splitting energy (Δ).
Therefore compounds can flip between high and low spin magnetism.
How does spin crossover differ with T?
- LOW T: adopt low-spin
- HIGH T: adopt high-spin
What is the EDTA 4- ion important for?
Stabilising compounds
Factors affecting complex stability
- Statistical factors
- Charge/ radius ratio
- Steric factors
- Electronic factors: Hard/ soft acid/ base
Characteristics of hard acids
- High positive charge
- Low polarisability
- Small size (H + , Al 3+
Characteristics of hard bases
- High electronegativity
- Difficult to oxidise
- Low polarisability (F - )
Characteristics of soft acids
- Low positive charge
- High polarisability
- Larger size (Rh + )
Characteristics of soft bases
- Low electronegativity
- Easily oxidised
- High polarisability
- High negative charge (H - )
HOMO-LUMO gap of hard molecules vs soft molecules
Hard molecules have a large gap
Soft molecules have a small gap
What compounds experience no crystal field stabilisation energy (CFSE)?
d 5 and d 10 configurations
Why does oxide and fluoride stabilise high oxidation states?
- They’re highly electronegative ligands (high oxidation states = more positive → need strongly electronegative ligands to counter it)
- M-O and M-F are very strong bonds (provide thermodynamic stability)
- High oxidation state metals are often good oxidising agents
- ⫪-donation to the metal d-orbitals (this delocalises electron density and helps stabilise the +ve charge on the metal
- Low polarisability (esp for fluoride) thus they’re small and hard, and so a perfect match for hard, high-charge-density metal ions
How does the CO ligand stabilise low oxidation states?
Lone pair on carbon → σ-donation to the metal
Empty ⫪* orbital → accepts d-electrons via ⫪-back bonding
How does the PR 3 stabilise low oxidation states?
⫪-acceptor ability comes from empty d-orbitals on P or from ⫪* orbitals on aryl groups (if R = Ph)
They stabilise low oxidation states through ⫪-back bonding but less strongly than CO
How do alkenes stabilise low oxidation states?
Forward donation from the olefin ⫪-bonding orbital to an empty metal p-orbital.
Back donation from filled metal d-orbital into olefin antibonding orbital
What is the 18 electron rule?
In general, the most stable complexes of the transition metals are those in which the metal centre accepts enough electrons from the ligands to attain the electron configuration of the next Noble gas
When are M-M bonds favoured?
Low oxidation state (relatively soft) metal ions in which there’s plenty of e - density to form overlap between the metal centres
Which d orbitals are not involved in M-M bonding?
d x 2 - y 2 as they are involved only in metal-ligand bonding thus don’t show in MO diagram for M-M bonds
σ-bond in M-M bonding
Two d z 2 orbitals form a σ-bonding and antibonding molecular orbital combination from overlap of the two large lobes of atomic orbitals
⫪-bond in M-M bonding
Doubly degenerate MO set is formed from the ⫪-overlap of the d xz and d yz
δ-bond in M-M bonding
pair of d xy orbitals form a new type of bond arising from the face to face overlap of all 4 lobes of the orbitals
M-M bonding down a group
Heavier transition metals tend to have more M-M bonding - as metals get bigger, the bond length decreases due to M-M bonding
How is free energy related to the standard electrode potential, Eº?
ΔGº = -nFEº
where F = Faraday constant (96,486.7 C mol -1 )
Magnitudes of Eº
- If the electrode potential is large and positive then the oxidised form is a powerful oxidising agents
- Low or negative Eº mean that the reduced form is a reducing agent
What is a reason for transition metals showing a variety of oxidation states?
Due to the closeness of 3d and 4s energy states
What are Frost diagrams?
Plots of nEº against oxidation number (N)
Key features of frost diagrams
- LOWEST POINTS: most thermodynamically stable oxidation states
- SLOPE BETWEEN 2 POINTS: gives the standard reduction potential for that redox couple
- POINTS ABOVE THE LINE CONNECTING TWO OTHERS: disproprtionation-prone
- POINTS BELOW THE LINE CONNECTING TWO OTHERS: comproportionation-prone
What do Pourbaix Diagrams show you?
Plots potential (Eº) against pH
- It shows the thermodynamic stability of different species of an element in aqueous solution
What is the case below the H 2 O/ H 2 line?
Any redox couple with a more -ve potential can theoretically reduce protons to hydrogen gas (H 2 gas is thermodynamically favoured to form)
What is the case above the O 2 / H 2 O line?
Above this line, oxygen gas evolution is thermodynamically favoured
Why may boundaries in the Pourbaix diagram have different slopes?
due to changes in proton-coupled electron transfer (PCET)
What are kinetically inert electron configurations and why?
d 3
d 6 (low spin)
d 8 (square planar)
esp 2nd and 3rd row
They have large CFSE so have much slower rates of ligand exchange
Crystal Field Activation Energy (CFAE)
the change in CFSE that occurs during a reaction, specifically when the reacting complex transforms into a transition state or intermediate
(+ve value means slow reaction)
How can you get an inert substance to react?
Use a catalytic amount of reducing agent
Dissociatively activated reaction
an intermediate with an decreased coordination number is involved
Associatively activated reaction
an intermediate with an increased coordination number is involved
Interchange mechanism
there is no detectable intermediate; bond breaking and bond making are both involved in the transition state
Associative interchange (I a ) reaction
- The rate is sensitive to the entering group (Y)
- The entering group and leaving group are strongly bound in the transition state
Dissociative interchange (I d reaction
- The rate is insensitive to entering group (Y)
- Both the entering and leaving group are weakly bound in the transition state
What are the two kinetic measurements
- Rate Law
- Intimate Mechanism
Kinetic measurements: Rate Law
- The measurement of the rate law gives the composition of the transition state species (i.e. the stoichiometric mechanism)
Kinetic measurements: Intimate Mechanism
- Study of the variation of rate constant with structure and substituent
The trans effect
the kinetic effect where some ligands increase the rate at which the ligand trans to them is replaced during a substitution reaction in a square planar complex
How does the trans effect work?
- Polarisation effect
- 𝛑-component
How does polarisability affect the trans effect?
Strongly polarisable ligands can induce a dipole moment in the metal-ligand bond which lowers the e- density at the metal centre, weakening the metal-ligand bond weaker and less stable thus creating a strong trans effect
How does 𝛑-contribution affect the trans effect?
𝛑-accepting ligands can accept e- density back from the metal into their 𝛑* orbitals so when they coordinate to the metal, they reduce e- density on the metal, making the metal-ligand bond weaker and less stable thus creating a strong trans effect
Trans effect v Trans influence
Trans effect
- Kinetic
- Focus on how fast a ligand opposite a given ligand is replaced in a substitution reaction
Trans influence
- Thermodynamic
- Focus on bond strength and bond length, not reaction rate
2 classes of electron transfer mechanisms
- Inner sphere mechanism
- Outer sphere mechanism
Inner sphere mechanism
the reductant and oxidant share a ligand in their inner coordination sphere with the electron being transferred across the bridging group
Outer sphere mechanism
electron transfer from reductant to oxidant with the co-ordination spheres remaining intact
What is the structure of [PMo 12 O 40 ] 3-
the “Keggin” structure
- PO 4 tetrahedron surrounded by 4 groups of three MoO 6 octahedra
What is a lacunary ion?
Removal of one of the metal (Mo or W) ions by base degradation from a polyoxometalate which creates a gap
How can lacunary ions be used?
The missing metal can be replaced by another reactive metal ion (e.g. Zr 4+ ) and these can be used as protease catalysts
What can isopoly-molybdenum blue be used for?
it’s intense blue and can be used as a sensitive test for reducing reagents
Formula for tungsten bronzes?
NaxWO3
Why are Mo complexes containing N2 important?
- Mo occurs naturally in the nitrogenase enzyme
- First inorganic complex to successfully reduce N2 to ammonia under ambient conditions
What are the structure types of first row transition metals?
Rutile or CdI2
Structure of haemoglobin
- Tetrameric protein containing 4 myoglobin units.
- The axial site of the FeII centre binds to the N atom from the proximal protein histidine residue
- The remaining axial site on the iron atom is available for oxygen binding
What is a problem with oxygen binding?
Oxygen is very reactive and has a tendency to react irreversibly with oxidation of the metal centre
What is “doming”?
Molecular deformation where the heme iron atom moves out of the plane of the porphyrin ring and towards the proximal histidine residue of the surrounding protein
Toxicity of CO to haemoglobin
- CO binds to haemoglobin with a greater affinity than O2
- In haemoglobin, the protein structure forces CO to bind at an angle (~120º), making it less ideal and reducing CO’s binding strength compared to the free haem.
- O2 binds at a bent angle too (120º) but this angled binding has evolved to favour O2 more
