Q1(b). Naming the conformations of chelate rings
Describe any three systems of nomenclature of conformational isomers of complexes.
Answer in brief
A five-membered M(en) ring is puckered, so complexes have ring conformers. They are named by (1) the IUPAC δ/λ skew-line convention, (2) the lel/ob convention relative to the C₃ axis, and (3) the Corey–Bailar k/k' labels; a full name combines these with Δ/Λ.
Key points
- Draw one line through the two donor atoms (N···N) and a second through the two ring carbons (C–C). The two lines are skew, like two turns of a helix.
- Looking along the common perpendicular, if the C–C line is turned clockwise from the N–N line the ring is δ; anticlockwise gives λ (same rule as Δ/Λ for whole chelates).
- A tris-chelate is named by configuration and each ring: Δ(δδδ), Δ(δδλ), Δ(δλλ), Δ(λλλ) and their Λ partners.
- In a tris-chelate the C–C bond of each ring is either parallel to the C₃ axis (lel) or oblique to it (ob).
The full model answer
Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.
Why conformers exist: in a chelated ethylenediamine ring the N–C–C–N unit is gauche; one carbon lies above and the other below the M,N,N plane. The mirror-image puckers interconvert quickly in solution but give diastereomers once the metal centre itself is chiral.
(1) IUPAC δ and λ (skew lines)
- Draw one line through the two donor atoms (N···N) and a second through the two ring carbons (C–C). The two lines are skew, like two turns of a helix.
- Looking along the common perpendicular, if the C–C line is turned clockwise from the N–N line the ring is δ; anticlockwise gives λ (same rule as Δ/Λ for whole chelates).
- A tris-chelate is named by configuration and each ring: Δ(δδδ), Δ(δδλ), Δ(δλλ), Δ(λλλ) and their Λ partners.
(2) lel and ob
- In a tris-chelate the C–C bond of each ring is either parallel to the C₃ axis (lel) or oblique to it (ob).
- For Δ, a λ ring is lel and a δ ring is ob; for Λ the reverse. The four conformers are lel₃, lel₂ob, lel ob₂ and ob₃.
(3) Corey–Bailar k and k'
- Corey and Bailar (1959), working with models, labelled the two puckers k and k' and compared the steric interactions between the NH₂ groups of neighbouring rings, as in cyclohexane conformations.
- They found Δλλλ (all lel) about 7.5 kJ mol⁻¹ more stable than Δδδδ; for Λ, δδδ is preferred. Experiment confirms this; in solution the small gap gives an equilibrium mixture, the commonest form of the Λ isomer being δδλ.
Related label: when coordination makes a ligand donor atom a stereocentre (the secondary N of dien or trien), it is named R or S as in organic chemistry.
Application. ring conformations change the rates of aqua-ligand exchange in lanthanide chelates used as MRI contrast agents, and direct the stereochemistry of metal-catalysed asymmetric syntheses.
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All parts of Question 1
- 1(a)(i) Covalent character of NaBr and MgBr₂5 marks
- 1(a)(ii) Order of lattice energies: LiCl, KCl, KBr, MgBr₂5 marks
- 1(b) Naming the conformations of chelate rings10 marks
- 1(c) Internal pressure (∂ U/∂ V)_T of a van der Waals gas10 marks
- 1(d) Uncertainty in the position of a honeybee10 marks
- 1(e) Maxwell distribution of molecular speeds and the effect of temperature10 marks