= Solution
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 \b[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.
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2024/paper-2-isomers.png]
{title=Distinct fac and mer arrangements of an unsymmetrical bidentate ligand; each has a mirror partner}
\b[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.<Spin Crossover in [Fe(2-Picolylamine)3]2+ Adjusted by Organosulfonate Anions by Xin-Hua Zhao and others>[2015, pp. 7857–7867, mer structures in compounds 1 and 2 and fac structure in compound 3]{ref}
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