Magnetism and UV-Vis

Question 1

What is the spectrochemical series?

Answer 1

A measure of whether a ligand is a π-donor or π-acceptor.

π-donors include halide ions.
σ-donors include water and NCS^-.
π-acceptors include CO and CN^-.

Question 2

What are the optical transition selection rules?

Answer 2

ΔS = 0 - spin must stay the same.

Δl = ±1 - for example s→p.

In a centrosymmetric molecule, there must be a change in parity. Either g→u or u→g.

Question 3

Which of the optical transition selection rules do d-orbital transitions follow?

Answer 3

They often follow the spin selection rule but do not follow the orbital selection rule or the parity selection rule.

Question 4

Are d-orbital transitions allowed?

Answer 4

They are only allowed through second order effects such as spin-orbit and vibronic coupling.

Each rule broken reduces the intensity by a factor of 10.

Question 5

How can the crystal field term be calculated?

Answer 5

Find the electron configuration in terms of the t_2g and e_g set.

Calculate 2S + 1.

If the t_2g set is unevenly unoccupied, it is a T state.

If the e_g set is unevenly unoccupied, it is an E state.

Otherwise it is an A state.

Question 6

What is the hole electron equivalence and how does it apply to octahedral and tetrahedral structures?

Answer 6

An octahedral d⁸ complex will have the same symmetry as a d² tetrahedral complex.

Additionally, an octahedral d² complex will have the same symmetry as a tetrahedral d⁸ complex.

Additionally, adding five electrons does not change the symmetry although the spin can change.

Question 7

What are ligand related absorption bands?

Answer 7

Electrons can be excited that are not just in the metal d-orbitals.

Question 8

What are the three types of ligand-related absorption bands?

Answer 8

Ligand based - isolated transitions in the ligand.

Ligand-to-metal charge transfer - the ligand HOMO is high enough in energy that an electron can be excited from it to the metal d-orbitals.

Metal-to-ligand charge transfer - the ligand LUMO is low enough to accept an electron from the metal d-orbitals.

Question 9

What are some trends in MLCT and LMCT transitions?

Answer 9

In the excited MLCT state, the metal has been oxidised. This is common in electron poor ligands and low-oxidation state metals.

In the excited LMCT state, the metal is reduced and this favoured by electron rich ligands and high oxidation state metals.

Question 10

How does a Tanabe-Sugano diagram differ from an Orgel diagram?

Answer 10

All spin states are included.

It is quantitative.

X-axis is Δ_o/B.

X-axis does not go to infinity.

Ground state term is set to E = 0.

Question 11

What are trends in electron-electron repulsion (B)?

Answer 11

Electron-electron repulsion tends to increase along the period and with charge as the ions are smaller.

More polarisable and softer ligands give lower B parameters. Equally softer metals have a lower B.

Question 12

What is the nephelauxetic effect?

Answer 12

The value of B can change when ligands are attached. This is the because the d-electrons can delocalise onto the ligands as the metal-ligand bond is partially covalent.

Question 13

How can the size of the Nephelauxetic effect be quantified?

Answer 13

By calculating the ratio of B / B_free.

Question 14

What is spin-crossover?

Answer 14

The high and low spin states are close enough in energy that the complex can swap between the two easily.

This happens when middle first row transition metals are bound to ligands with an intermediate field strength.

Question 15

What are the two sources of paramagnetism in molecular complexes?

Answer 15

The intrinsic spin of the unpaired electron, S.

The orbital motion of the electrons, L.

Question 16

What is the most important factor for magnetism in 3d transition metal complexes?

Answer 16

The spin of the unpaired electron, S, is much more important than L.

L is important for free ions and heavier atoms where spin-orbit coupling is large.

Question 17

What are some key magnetic quantities?

Answer 17

Magnetisation (M).

Field (H)

Magnetic susceptibility (χ = dM/dH). This quantifies how responsive a substance is to a magnetic field and can be approximated at low field by χ = M/H.

Question 18

What is the spin-only formula for magnetic moment?

Answer 18

μ = g√S(S+1).

Question 19

How can magnetism be measured?

Answer 19

Through Faraday induction. This involves moving a magnet through a wire coil which produces a current. This is detected by a SQUID (superconducting quantum interference device).

A SQUID is a split ring made from a superconductor which can detect current with a very high sensitivity through the Josephson effect.

Question 20

What is the Curie constant proportional to?

Answer 20

The Curie constant C = 0.500S(S + 1) is proportional to μ² assuming g = 2.