Q1. The spectra of green [Ni(H2O)6]2+ and purple [Ni(en)3]2+ are shown below. Explain why there are three bands for each compound and explain why the bands shift to higher energy for the en complex. (en = ethylenediamine)

Background

Topic: Electronic Spectra of Transition Metal Complexes (Crystal Field Theory)

This question tests your understanding of d-d transitions in octahedral complexes, crystal field splitting, and how ligand field strength affects the absorption spectrum.

Key Terms and Formulas

• Crystal Field Splitting Energy (Δo): The energy difference between t2g and eg orbitals in an octahedral field.

• d-d Transitions: Electronic transitions between split d-orbitals in transition metal complexes.

• Ligand Field Strength: Stronger field ligands (like en) cause a larger Δo than weaker field ligands (like H2O).

Step-by-Step Guidance

1. Recognize that both [Ni(H2O)6]2+ and [Ni(en)3]2+ are octahedral d8 complexes, leading to specific electronic transitions.

2. Recall that in an octahedral field, the d-orbitals split into t2g and eg sets, and transitions between these give rise to absorption bands.

3. Understand that three bands are observed due to three possible spin-allowed transitions for a d8 ion in an octahedral field.

4. Consider that the position (energy) of these bands depends on the crystal field splitting energy (Δo), which is larger for stronger field ligands (en) than for weaker ones (H2O).

5. Set up your explanation for why the bands for [Ni(en)3]2+ are at higher energy (shorter wavelength) compared to [Ni(H2O)6]2+ by relating this to the relative ligand field strengths.

Try solving on your own before revealing the answer!

Q2. What do you understand by the Jahn-Teller effect?

Background

Topic: Coordination Chemistry – Electronic Structure

This question tests your understanding of the Jahn-Teller effect, which describes how certain electronic configurations in transition metal complexes lead to geometric distortions to lower the overall energy.

Key Terms and Concepts

• Jahn-Teller Effect: The distortion of non-linear molecules in degenerate electronic states to remove degeneracy and lower energy.

• Degeneracy: When two or more electronic states have the same energy.

Step-by-Step Guidance

1. Recall that the Jahn-Teller effect applies to non-linear molecules with degenerate electronic ground states.

2. Understand that the effect predicts a geometric distortion (often elongation or compression) to remove degeneracy and stabilize the molecule.

3. Think about which d-electron configurations (e.g., d9, high-spin d4) are most susceptible to this effect in octahedral complexes.

Try explaining in your own words before revealing the answer!

Q3. For the complex [V(H2O)6]3+, Dq/B = 3.0 and B = 600 cm-1.

(a) Calculate the value of Dq and Δo.

Background

Topic: Crystal Field Theory – Spectrochemical Series

This question tests your ability to use the relationship between Dq, B, and Δo (octahedral crystal field splitting energy) for a transition metal complex.

Key Terms and Formulas

• Dq: A measure of crystal field splitting (1 Dq = 1/10 Δo).

• B: Racah parameter, related to electron-electron repulsion.

• Δo = 10 Dq

Step-by-Step Guidance

1. Start by using the given ratio: Dq/B = 3.0 and B = 600 cm-1.

2. Calculate Dq by multiplying the ratio by the value of B.

3. Recall that Δo = 10 Dq, and set up the calculation for Δo using your value for Dq.

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Q4. For the complex [V(H2O)6]3+, what is the ground state free atom term?

Background

Topic: Term Symbols for Transition Metal Ions

This question tests your knowledge of how to determine the ground state term symbol for a free ion using Hund's rules and electron configuration.

Key Terms and Concepts

• Term Symbol: Notation that describes the electronic state of an atom (e.g., , ).

• Hund's Rules: Used to determine the ground state term symbol based on maximum multiplicity and maximum L value.

Step-by-Step Guidance

1. Determine the electron configuration for V3+ (atomic number 23, so remove 3 electrons: 3d2).

2. Apply Hund's rules to find the term symbol for a d2 configuration.

Try working out the term symbol before revealing the answer!

Q5. For the complex [V(H2O)6]3+, give the molecular terms for the first and second spin-allowed transitions.

Background

Topic: Tanabe-Sugano Diagrams and Electronic Transitions

This question tests your ability to identify the ground and excited state terms for spin-allowed transitions in an octahedral d2 complex.

Key Terms and Concepts

• Spin-Allowed Transitions: Transitions that do not involve a change in spin multiplicity.

• Tanabe-Sugano Diagram: Used to predict the energies and terms of electronic transitions in transition metal complexes.

Step-by-Step Guidance

1. Recall the ground state term for a d2 octahedral complex (from previous question).

2. Use the Tanabe-Sugano diagram for d2 to identify the first and second spin-allowed excited states.

3. Write the molecular terms for these transitions (e.g., , ).

Try identifying the transitions before revealing the answer!

Q6. Calculate the value of B and Δo for the Cr3+ ion in [Cr(H2O)6]3+ if ν1 = 17,000 cm-1, ν2 = 24,000 cm-1, and ν3 = 37,000 cm-1.

Background

Topic: Spectrochemical Calculations for Transition Metal Complexes

This question tests your ability to use the observed transition energies (from spectra) to calculate the Racah parameter (B) and crystal field splitting energy (Δo) for a d3 octahedral complex.

Key Terms and Formulas

• Racah Parameter (B): Measures inter-electronic repulsion in transition metal ions.

• Δo: Octahedral crystal field splitting energy.

• For d3 ions, use the Tanabe-Sugano diagram and the following relationships: ν1 = ν2 = ν3 =

Step-by-Step Guidance

1. Write down the given transition energies: ν1, ν2, and ν3.

2. Recall the equations relating these transitions to Δo and B for a d3 octahedral complex (from Tanabe-Sugano diagram or standard tables).

3. Set up the system of equations using the given ν values and solve for B and Δo (you may need to use simultaneous equations or substitution).

4. Prepare to substitute the given values into your equations to find B and Δo.

Try setting up the equations and solving before revealing the answer!