MChem Master of Chemistry Master of Chemistry: Inorganic Chemistry 5 — Questions and Answers
Question 1: The trans effect series in square planar Pt(II) complexes places H⁻ and CO at opposite ends to Cl⁻. What is the kinetic basis for the strong trans effect of CO?
- CO is a strong σ-donor that destabilizes the trans M–L bond by electron donation
- CO is a strong π-acceptor that stabilizes the transition state for substitution trans to itself by delocalization of the metal's electron density (Correct answer)
- CO causes steric crowding that forces the trans ligand to dissociate
- CO raises the activation energy of trans ligand substitution by back-bonding
Correct answer: CO is a strong π-acceptor that stabilizes the transition state for substitution trans to itself by delocalization of the metal's electron density
CO's strong π-acceptor character stabilizes the trigonal bipyramidal transition state for associative substitution by delocalizing excess electron density, lowering the activation barrier for the trans ligand.
Question 2: In bioinorganic chemistry, the enzyme nitrogenase reduces N₂ to NH₃. Which metal clusters are present in the active site of Mo-nitrogenase?
- A mononuclear Mo site and a dinuclear Fe₂S₂ cluster
- The P-cluster (Fe₈S₇) and the FeMo-cofactor (MoFe₇S₉C homocitrate) (Correct answer)
- A Mn₄CaO₅ cluster similar to photosystem II
- A [NiFe] center analogous to [NiFe]-hydrogenase
Correct answer: The P-cluster (Fe₈S₇) and the FeMo-cofactor (MoFe₇S₉C homocitrate)
Mo-nitrogenase contains the P-cluster (Fe₈S₇) for electron transfer and the FeMo-cofactor (MoFe₇S₉C with homocitrate) as the site of N₂ binding and reduction.
Question 3: Which property of f-block elements makes their coordination chemistry distinctly different from d-block elements?
- f electrons are strongly directional and dominate covalent bonding with ligands
- f orbitals are largely core-like, shielded by outer 5d and 6s electrons, leading to predominantly ionic, electrostatic bonding with ligands (Correct answer)
- Lanthanides and actinides show large crystal field splittings comparable to 4d metals
- f-block ions adopt fixed oxidation states of +2 exclusively
Correct answer: f orbitals are largely core-like, shielded by outer 5d and 6s electrons, leading to predominantly ionic, electrostatic bonding with ligands
The 4f orbitals are radially contracted and shielded by outer shells, so f electrons participate little in bonding; lanthanide coordination chemistry is dominated by ionic, electrostatic interactions.
Question 4: A d⁶ octahedral complex is described as low-spin with a large Δo. Which statement about its magnetic and substitution behavior is correct?
- It is paramagnetic with four unpaired electrons and highly labile
- It is diamagnetic with zero unpaired electrons and typically substitution-inert (Correct answer)
- It is paramagnetic with two unpaired electrons and moderately labile
- It is diamagnetic but highly labile due to strong π-backbonding
Correct answer: It is diamagnetic with zero unpaired electrons and typically substitution-inert
Low-spin d⁶ fills all three t₂g orbitals (t₂g⁶ eg⁰), giving zero unpaired electrons (diamagnetic) and high CFSE that makes the complex kinetically inert toward substitution.
Question 5: In main-group chemistry, the concept of 'lone pair stereochemical activity' explains why SnCl₂ is bent while SnCl₄ is tetrahedral. Which model accounts for this?
- Crystal field theory predicts distortion of Sn²⁺ from d electron repulsion
- VSEPR theory treats the lone pair as an electron domain that influences molecular geometry (Correct answer)
- Molecular orbital theory shows lone pairs occupy non-bonding MOs with no geometric consequence
- The ionic model predicts Sn²⁺ forms a regular coordination environment from lattice energy
Correct answer: VSEPR theory treats the lone pair as an electron domain that influences molecular geometry
VSEPR treats the lone pair on Sn in SnCl₂ as a sterically active electron domain, giving an AX₂E arrangement that results in a bent (V-shaped) molecule.
Question 6: The Zintl concept describes intermetallic compounds formed between electropositive and electronegative metals. In the Zintl phase Na₄Si₄, the Si₄⁴⁻ anion adopts which structure?
- Linear Si₄ chain with cumulated double bonds
- Tetrahedral Si₄⁴⁻ cluster isoelectronic with P₄ (Correct answer)
- Square planar Si₄⁴⁻ ring with delocalized π bonds
- Ladder-type Si₄ oligomer with Si–Si–Si angles of 120°
Correct answer: Tetrahedral Si₄⁴⁻ cluster isoelectronic with P₄
Si₄⁴⁻ is a tetrahedral cluster isoelectronic with white phosphorus (P₄), consistent with the Zintl–Klemm concept where the anion adopts the structure of the isoelectronic main-group element.
Question 7: Which mechanism operates in the water oxidation half-reaction at the oxygen-evolving complex (OEC) of photosystem II?
- Homolytic O–O bond formation between two terminal oxo radicals on adjacent Mn centers (radical coupling)
- Nucleophilic attack of a substrate water molecule on a high-valent electrophilic Mn=O oxo group (Correct answer)
- Reductive elimination of O₂ from a Mn(IV) peroxo intermediate
- Concerted [4+2] cycloaddition between two Mn=O groups
Correct answer: Nucleophilic attack of a substrate water molecule on a high-valent electrophilic Mn=O oxo group
The most supported mechanism involves nucleophilic water attack on a high-valent Mn(IV)=O or Mn(V)=O electrophilic oxo intermediate to form the O–O bond in the S-state cycle.
MChem Master of Chemistry Master of Chemistry: Inorganic Chemistry 5
The trans effect series in square planar Pt(II) complexes places H⁻ and CO at opposite ends to Cl⁻.
What is the kinetic basis for the strong trans effect of CO?