Past exam of the chemistry B course of the University of Cambridge / 2024 / Paper 2 / B8 / a / i

Treat the two donors of 2-Picolylamine as distinct: $P$ denotes pyridyl N and $A$ denotes amino N. Each ligand must join two adjacent octahedral positions because its five-membered chelate ring cannot span a trans pair. The three $P$ sites can either occupy a triangular face, giving a facial isomer, or a meridian containing a trans pair, giving a meridional isomer. Each geometry has a pair of nonsuperposable mirror images. Therefore there are four configurational stereoisomers: fac-$\Delta$, fac-$\Lambda$, mer-$\Delta$ and mer-$\Lambda$. Rapid ring puckering or amine inversion does not introduce additional persistent configurations in this count.

The mer geometry is the reasonable steric prediction for the more stable form. It spreads the bulky pyridyl rings farther apart, allowing one pyridyl–pyridyl trans relationship, whereas the fac geometry groups all three pyridyl donors on one face. This is a qualitative preference, not a proof independent of counterions and solvent: hydrogen bonding and packing can favor the alternative. Primary structures of different salts contain both geometries, so neither should be claimed impossible.^[ref]