= Solution
Treat each <semiquinonate ligand> as a symmetric O,O <chelate>, with the <free radical> delocalized over the dioxolene framework. The ideal octahedral donor-connectivity structures are \b[one trans form and two mirror-related cis forms], giving \b[three <stereoisomers>]. Trans refers to the two <pyridine> N donors, not to the two O atoms of a <chelate>, which must remain adjacent.
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2022/paper-2-isomers.png]
{title=The achiral trans arrangement and the two mirror-related cis donor arrangements}
In the diagram, coloured curves join the two O donors belonging to one ligand. For an unambiguous coordinate specification, the trans form has Py at $+z,-z$, with <chelate> pairs $(+x,+y)$ and $(-x,-y)$. A cis pair has Py at $+z,+x$, with <chelates> $(-z,+y)$ and $(-x,-y)$; reflecting $y\mapsto-y$ gives the other cis form. Proper rotations cannot superpose those two cis arrangements. Ring <conformations> and <pyridine> rotations are not extra donor-connectivity <isomers>.
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