= Solution
Neutral <pyridine> contributes no <formal charge>. If $n$ of the two <dioxolene ligands> are dianions, the combined ligand charge is $-2-n$, so the <cobalt> <oxidation state> is $2+n$. The distinct charge assignments are therefore
$$
\boxed{\begin{array}{c|c}
\text{metal}&\text{two dioxolene ligands}\\ \hline
\mathrm{Co^{II}}&L^-+L^-\\
\mathrm{Co^{III}}&L^-+L^{2-}\\
\mathrm{Co^{IV}}&L^{2-}+L^{2-}
\end{array}}
$$
Interchanging the two ligand labels in the middle row does not make another charge combination. Co(IV) belongs in this unrestricted charge-balance list even though the subsequent magnetic analysis excludes it.
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