= Solution
\b[The N-bound isomer $C$ is the thermodynamically favored form; heating accelerates access to it.] Formation of $B$ retains the original metal–<oxygen> attachment and is a plausible <kinetic control> outcome. Rearranging that attachment requires overcoming an <activation free energy>, especially at a substitutionally inert low-spin <cobalt>(III) center. Raising <temperature> speeds the <linkage isomerization>, allowing the more stable N-bound <nitro ligand> to accumulate. Its stronger <ligand field> and different covalent bonding help rationalize the stability, but a larger field splitting alone is not a complete calculation of the isomerization <Gibbs free energy>. The classical crystallographic comparison establishes the different attachments and thermodynamic direction.<Nitrito-Nitro Linkage Isomerization in the Solid State. 2. A Comparative Study of the Structures of Nitrito- and Nitropentaamminecobalt(III) Dichloride by Ingmar Grenthe and Ellika Nordin>[1979, pp. 1869–1874, structural comparison and solid-state isomerization]{ref}
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