MChem Master of Chemistry Master of Chemistry: Inorganic Chemistry 4 — Questions and Answers
Question 1: The Cotton effect observed in circular dichroism (CD) spectroscopy of a chiral metal complex is most directly associated with:
- Electronic transitions that are magnetically allowed but electrically forbidden
- Electronic transitions that have both electric and magnetic transition dipole moments of non-zero magnitude (Correct answer)
- Vibrational transitions of chiral ligands only
- Spin-forbidden d-d transitions in octahedral complexes
Correct answer: Electronic transitions that have both electric and magnetic transition dipole moments of non-zero magnitude
The Cotton effect arises from electronic transitions that are simultaneously electric-dipole and magnetic-dipole allowed, which occurs in inherently chiral chromophores.
Question 2: In the Pauson–Khand reaction, what is the role of the cobalt carbonyl complex [Co₂(CO)₈]?
- It acts as a stoichiometric oxidant regenerated after the cycle
- It coordinates the alkyne and mediates [2+2+1] cycloaddition with an alkene and CO (Correct answer)
- It provides a radical CO source via homolytic Co–C bond cleavage
- It catalyzes a Diels–Alder [4+2] cycloaddition
Correct answer: It coordinates the alkyne and mediates [2+2+1] cycloaddition with an alkene and CO
Co₂(CO)₈ coordinates to the alkyne forming a stable alkyne–dicobalt complex that then undergoes an intramolecular [2+2+1] cycloaddition with an alkene and one CO to give a cyclopentenone.
Question 3: Which of the following best describes the Jahn–Teller effect in a high-spin d⁴ octahedral complex?
- Elongation along the z-axis due to unequal occupation of the eg orbitals (Correct answer)
- Compression along the z-axis due to unequal occupation of the t₂g orbitals
- No distortion because the d⁴ configuration is orbitally non-degenerate
- Tetrahedral distortion to relieve spin frustration
Correct answer: Elongation along the z-axis due to unequal occupation of the eg orbitals
High-spin d⁴ gives an eg¹ configuration (one electron in dz² or dx²-y²), causing a first-order Jahn–Teller distortion that typically elongates the z-axis bonds.
Question 4: Perovskite oxides (ABO₃) are structurally tolerant of A- and B-site substitution. The Goldschmidt tolerance factor t is used to predict:
- The magnetic moment of the B-site transition metal cation
- The degree of structural distortion from the ideal cubic perovskite (Correct answer)
- The ionic conductivity through the oxygen sublattice
- The bandgap of the semiconductor perovskite
Correct answer: The degree of structural distortion from the ideal cubic perovskite
The tolerance factor t = (rA + rO)/[√2(rB + rO)] quantifies how well the ionic radii fit the ideal cubic structure; t < 1 leads to tilting of BO₆ octahedra.
Question 5: In organometallic chemistry, an agostic interaction refers to:
- A 3c-2e C–H⋯M interaction where the C–H bond donates electron density to an electrophilic metal center (Correct answer)
- A hydrogen bond between a metal hydride and a N–H donor ligand
- A π interaction between an aromatic ring and a metal in η⁶ coordination
- A dihydrogen complex formed by oxidative addition of H₂ to a metal
Correct answer: A 3c-2e C–H⋯M interaction where the C–H bond donates electron density to an electrophilic metal center
An agostic interaction involves intramolecular donation of a C–H bonding electron pair to a coordinatively unsaturated metal, forming a 3-center 2-electron bond.
Question 6: The Ellingham diagram is used in inorganic chemistry to predict:
- The thermodynamic feasibility of reducing one metal oxide with another metal or carbon (Correct answer)
- The kinetics of corrosion in aqueous environments at various pH values
- The electrode potentials of metals in standard electrochemical cells
- The crystal field stabilization energies of transition metal oxides
Correct answer: The thermodynamic feasibility of reducing one metal oxide with another metal or carbon
The Ellingham diagram plots ΔG° of oxide formation vs. temperature; a metal or carbon whose line lies below another oxide's line can thermodynamically reduce that oxide.
Question 7: Which bonding model best explains the unusually short P–N bond in phosphazenes such as (NPCl₂)₃?
- Pure σ-bond framework with no π character
- Delocalized π bonding involving N lone pair donation into P d-orbitals or P–N π* orbitals (Correct answer)
- Ionic bonding between P³⁺ and N³⁻ ions in a ring
- Hyperconjugation from N–H bonds into P–Cl σ* orbitals
Correct answer: Delocalized π bonding involving N lone pair donation into P d-orbitals or P–N π* orbitals
Phosphazenes exhibit delocalized π bonding via N lone pair→P d-orbital (or σ*) interactions, which shortens the P–N bond and gives the ring aromatic-like stability.
MChem Master of Chemistry Master of Chemistry: Inorganic Chemistry 4
The Cotton effect observed in circular dichroism (CD) spectroscopy of a chiral metal complex is most directly associated with: