Advanced Organometallic and Coordination Chemistry

What is the UV-vis reigons?

10 000 - 50 000 cm^-1

→ Reigon where electronic excitations usually occur

Why is UV-vis wavelength reigon the reigon where electronic excitations occur?

• Ligand spectra

• Counter-ion spectra

• Charge-transfer spectra

• Ligand-field spectra → (d-d transitions)

What are the selection rules?

• Δ S = 0

• Δ L = ± 1

• Laporte rule - In a centrosymmetric system, transitions are forbidden ( g → g or u → u)

→ e.g = s → p and p → d aren’t allowed

Why is it complicated to use electronic spectra to determine the number of transitions for more than one electrons?

Ignores the effect that other electrons have

What do you use Russell-Saunders coupling for?

Derive terms arising from dⁿ configurations → Ground state term (and maybe first excited state term)

How do you work out Russell-Saunders coupling to work out ground state terms?

How do you draw out orgel diagrams to determine the number of transitions (use d¹ as an example)

What is hole formalism?

Figuration with one hole = Opposite to configuration with one electron

→ Or d¹ is the opposite to d⁹

• dⁿ = dⁿ⁺⁵

How and why does energy level diagram for a d²,d³,d⁷,d⁸? configuration lead to a semiquantitative Tanabe Sugano diagram?

Energy now depends on:

• Crystal field splitting parameter (Δ)

• Separation between ³F and ³P (and how they can vary with Δ

• Magnitude of interaction between ³T_1g(F) and ³T_1g(P)

What are the energies of the 4 levels for d²,d³,d⁷,d⁸? (DONT NEED TO REMEMBER)

What are Racah Parameters?

In the free ion, they are parameters that describes electron-electron repulsion

How do you work out the energy gaps between states for d electrons?

Use sums of multiples of B and C parameters

How do you work out the energy gap between states of the same multiplicity?

Only use multiple of B

What is the Nephelauxetic effect?

Greater the reduction in the Racah B parameter, the greater the covalency in the M-L bond

How do you calculate the Nephelauxetic ratio?

What is the order of the Nephelauxetic series?

Very different to the spectrochemical series

How do you calculate the Racah B parameter and crystal field splitting parameter (Δ) starting from transition energy formulas using d² and d⁷ as an example?

How does the splitting of tetrahedral complexes compare with octahedral complexes?

Smaller splitting

∆_t = 4/9 ∆_oh

why is one of three transitions not observed in tetrahedral complexes in UV-vis?

• Transition occurs in NIR (Near infra-red spectroscopy)

• Smaller crystal field splitting so transition outside uv-vis range

Why is ε more intense in tetrahedral complexes than octahedral complexes?

• All d orbitals in octahedral complexes are Laporte-forbidden

• Tetrahedral complexes have no inversion centre

  

→ Tetrahedral complexes have d-p mixing, relaxing the rule

What are the ε values of octahedral and tetrahedral complexes?

• Tetrahedral ε = 100’s

• Octahedral ε = 10’s

Why does a high spin d⁵ complex gives a 6A_1g state in its ground state and therefore why do we observe several very weak transitions?

• Has one sextet term

• Laporte forbidden

• Spin forbidden

What causes the broadness of UV-vis/ electronic transitions for a high spin d⁵ complex?

• Electronic transitions are much quicker than nuclear motion → Frank-Condon principle

• Crystal field splitting energy varies over course of vibration when m_l is short, crystal field splitting energy is large, when m_l is large, crystal field splitting energy is short → We see range of delta values contributing to signal

• But if transition is independent of delta, vibration broadening will not be seen

→ ASSIGN PEAKS TO ORGEL DIAGRAMS IN EXAM

Why is there a very weak transition at low energy for a low spin d⁶ complex?

How do you derive d⁸ configuration complex from octahedral to square planar?

• dz² orbital is now lower in energy

How do you know if a d⁸ complex is octahedral/tetrahedral or square planar?

• If P (pairing energy) is relative constant when crystal field splitting energy is large, -2.4 + P » -1.2 = SQUARE PLANAR

• If P is relatively constant when crystal field splitting energy is small, -1.2 » -2.4 + P = OCTAHDEDRAL OR TETRAHEDRAL

Why might you be expected to see 6 transitions in a d⁸ square planar complex in UV-vis?

May see 1 sharp transition typically between 450-600nm

→ Tend to label as dxy → dx²-dy² transition

How do you calculate the effective magnetic moment?

• Directly related to spin and orbital quantum numbers

• Don’t have to obey full equation

How does orbital moment of an electron moving from orbit to orbit related to Lenz’s law?

• Electrons in atoms develop an induced magnetic moment that opposes the external field

• This is the basis of diamagnetism — all materials have this to some extent

• So we need orbital degeneracy where 2 or more degenerate orbitals exist which can be interconverted by rotation about an axis and these orbitals must be asymmetrically filed → T,A,E degeneracy terms

Can complexes with T ground states be quenched?

Yes → For Oh or Td complexes

Can complexes with A and E ground states be quenched?

No → For Oh or Td complexes

Show if complexes from d¹-d⁵ show orbital contribution (include low / high spins)

• Complexes with a T term ground state have a direct 1st orbital order contribution to their magnetic moment

• A & E terms do not have a first order orbital contribution

What is the formula for complexes that possess no first order orbital contribution in the ground state to find the magnetic moment spin only formula?

Mu is expected to be temperature independent since spin only formula contains no spin only term

What can spin-orbit coupling do instead if it’s unable to occur by a first order effect?

Can mix in some orbital contribution from high T terms of the same multiplicity (as the ground term)

• So in the case of the A_1g term (h.s d⁵) there are no relevant higher T terms and magnetic moment spin only is exactly as predicted

What is the formula if A₂ and E terms have higher T terms of the same multiplicity as the ground state

Gamma is the spin-orbit coupling constant has has positive sign for d^1-4 (less than half filled orbitals) and is negative for d^6-9 (greater than half filled)

→ REMEMBER EQUATION

What method do you use to determine and calculate the effect of spin-orbit coupling on the orbitally degenerate ground term?

KOTANI plots

→ Plot μ_eff against kT/λ

What does a KOTANI plot look like?

What are the disadvantages of a KOTANI plot?

• Covalent bonding = Unpaired electrons do not remain entirely on metal of MLCT where it can no longer contribute to orbital angular momentum around metal

• Jahn-Teller distortions = After distortion, ground state will split and be A or E

→ Size of splitting can be small but can be detected by magnetic measurements. Size of distortion can be estimated by fitting μ vs T with δ/λ. Best fit gives value of δ. 0 = Oh symmetry

• δ = Separation between singlet and doublet levels

What does the study of magnetic properties over temperature allow?

Allows distinction between T or A/E ground states, and deduction of stereochemistry

What are the other causes for variations in magnetic moment?

High spin vs low spin in O_h complexes

d⁴ , d⁵ , d⁶ and d⁷ can exist in one or two spin states depending on whether pairing energy > Δ or pairing energy < Δ

• However, some complexes have pairing energy ~ Δ and depending on temperature, different spin states exist

What are the two types of spin cross-over complexes?

1) Simple equibrillia

2) More complex transitions

What are spin cross-over complexes?

When the internal energy difference between the two spin states is small for a transition

→ Happens as temperature varies

• High spin state is invariable that of higher energy and transition is observed on lowering the temperature

What is simple equibrillia for spin cross-over complexes? (Use Fe(S₂CNR₂)₃ as an example)

• Complex has Δ_o ~ 16,500 cm^-1

• At low temperatures, magnetic moment of low spin Fe(III) = 2.25 BM

• As temp increases, magnetic moment increases as more high spin species are formed

When does spin cross-over occur?

When Δ_o ~ 16, 500 cm^-1 in the high spin complex

What are more complex transitions? (Type of spin cross-over)

• Occurs (especially in solid state) when there are interactions between molecules that may trigger phase transition

• Bulk sample changes from high to low spin on cooling

• Sample may change from one ‘frozen mixture’ of spin states to another

Observed for Fe(phen)₂XY and Fe(bipy)₂XY complexes

→ X and Y are anionic ligands of moderate field strength

How does Jahn-Teller distortion effect e_g occupancy?

E/A to E/A term: No change

How does Jahn-Teller distortion effect t₂g occupancy?

Leads to trigonal rather than tetragonal distortions which preserve one of 3-fold axes

How are orbitals split/ ranked in trigonal bipyramidal complexes

dz² > d_x-y² > d_xz d_yz

Do trigonal bypyramidal complexes show any temperature dependence in their magnetic moment

No unless there is a change of spin state

How are trigonal bipyramidal M(II) formed?

By using ren

What is the significace of trigonal bipyramidal M(II) complexes?

Gives invariable high spin species

→ However, QP (phosphorus analogue of tren) gives low spin species

What is the difference in the magnetic moments between H.S trigonal bipyramidal and H.S O_h?

• In some cases, H.S O_h may be a T term and therefore temperature dependent → all H.S trigonal bipyramidal complexes are temperature dependent

• QP ligands lead to low spin complexes but difference will be seen

How are the frontier orbitals (d-orbitals) split in organometallic compounds (e.g = metallocenes and bis-arene compounds)

How do you show the relative ordering for organometallic complexes (e.g = metallocenes and bis-arenes)

Why is e₂ low in energy?

Strong δ aspect of bonding in complexes

What is diamagnetic?

Material that is repelled from magnetic field

What is paramagnetic?

Material that is attracted into magnetic field

Does paramagnetism or diamagnetism have a stronger resultant effect?

Paramagnetism

→ Effects are additive

• Two effects are opposed

How do you measure the magnetic induction of flux, B?

• H = Magnetic field of strength

• I = Intensity of magnetisation

How do you calculate the volume of susceptibility? (K)

• H = Magnetic field of strength

• I = Intensity of magnetisation

How do you calculate the gram susceptibility? (ϗ)

• K = Volume of susceptibility

• P = Density

How do you calculate the molar susceptibility? (ϗ_M?

• M = Molecular weight

• ϗ = Gram of susceptibility

How can you tell if a compound is diamagnetic (paired electrons)?

Compound is repelled by field

• B < H

• Susceptiblity is -ve

How can you tell if a compound is paramagnetic (unpaired electrons)?

Compound is attracted by field

• B > H

• Susceptibility is +ve

What law do paramagnetic substances obey?

Curie’s law

What is Curie’s law?

Susceptibility depends inversely on temperature

• Thus a plot of 1/ϗ vs T should give a straight line of slope 1/C passing through origin

How is Curie constant (C) given?

Langevin expression

What is Langevin expression?

Relates susceptibility, ϗM , to magnetic moment,µ, in Bohr Magnetons

What is the expression for Langevin expression and how can you use it to obtain Curie constant?

What is the Curie-Weiss law?

When many substances give straight lines that do not intersect the temperature axis at 0K

• Associated with breakdown of Langevin assumption (B=H) and indicative of magnetic interaction between discrete molecules → i.e = Magnetically non-dilute materials

What is the formula for Curie-Weiss law?

• -ve θ = Antifero

• +ve θ = Feromagnetic

When is a compound considred to be magnetically dilute?

When compounds with paramagnetic centres are separated from each other by diamagnetic atoms (e.g = ligands)

When is a compound considered to be magnetically non-dilute?

If ligands are removed, paramagnetic centres will interact with eachother

What is ferromagnetism?

All paramagnetic centres alligned in same direction

What is antiferromagnetism?

All paramagnetic centres alternating directions

What is ferrimagnetism?

Paramagnetic centres alternating directions but having an overall resultant moment

What is antiferro-magnetism?

When two transition metals both have unpaired electrons and are bonded together (usually by bridging ligand)

How do you show that two metal centres have opposite spins and therefore are antiferromagnetically linked by ligand using e_g, p_z, e_g orbitals?

How do you show that two metal centres have opposite spins and therefore are antiferromagnetically linked by ligand using e_g, p_x, e_g orbitals?

How do you show that two metal centres have same spins and therefore are ferromagnetically linked by ligand using e_g, p_z, t₂g orbitals?

• Unpaired electrons in orthogonal orbitals (zero overlap integeral) tend to align themselves with parallel spins - arrangements are exhange-stabilised.

How do you show that two metal centres have same spins and therefore are ferromagnetically linked by ligand using e_g, p_z, py, eg orbitals?

• Creates right angle

• Two ligand orbitals are orthogonal to eachother

If both antiferromagnetic and ferromagnetic pathways are avaliable, which one will dominate more?

Antiferromagnetic

What is super-exchange?

General mechanism involving ligand orbitals to mediate coupling between metal electrons

→ Doesn’t matter whether antiferromagnetic or ferromagnetic

How do you quantify J (energy difference between singlet and triplet (exchange energy)) in bimetallic complexes?

x = J / kT

• J = Energy difference between singlet and triplet (exchange energy)

What happens if J (energy difference between singlet and triplet (exchange energy)) is positive?

Exchange energy favours electrons with parallel spins

• Think antiferromagnetism and ferromagnetism

Show an example of antiferromagnetism (use magnetic unit cell)

• Increasing covalency leads to increasing T_N

• T_N is higher in oxides than fluorides

Give an example of ferrimagnetism (think of unit cells)

• Inverse spinel and magnetite structures leads to magnetic moment

• Acute angle = Sets up semimagnetic coupling

What’s the formula for an inverse spinel structure?

B(ABO₄)

What’s the formula for a magnetite structure?

FeIII (FeIIFeIIIO4)