= Solution
\b[$A=[\mathrm{Co(NH_3)_5(OH)}]^{2+}$, $B=[\mathrm{Co(NH_3)_5(ONO)}]^{2+}$, $C=[\mathrm{Co(NH_3)_5(NO_2)}]^{2+}$, and the by-product is $\mathrm{HNO_2}$.] The first transformation is <deprotonation> of coordinated <water>, not loss of an <ammonia> ligand. The <hydroxo ligand> in $A$ is then nitrosated at <oxygen>, retaining its original Co–O attachment in the initially formed O-bound <nitrito ligand>. Atom and <electric charge> balance give
$$
[\mathrm{Co(NH_3)_5(OH)}]^{2+}+\mathrm{N_2O_3}
\longrightarrow[\mathrm{Co(NH_3)_5(ONO)}]^{2+}+\mathrm{HNO_2}.
$$
The <relative molecular mass> of <nitrous acid> is approximately $1+14+2(16)=47$. Heating converts the O-bound <nitrito ligand> to the N-bound <nitro ligand>; these are <linkage isomers>, with Co remaining in <oxidation state> $+3$ throughout.
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