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Nitrite is the bent anion $\mathrm{N}{\mathrm{O}}_2^{-}$. It can act as an ambidentate ligand, giving N-bound nitro ligands or O-bound nitrito ligands.

A nitrito ligand is O-bound nitrite, $\mathrm{M}-\mathrm{O}-\mathrm{N}=\mathrm{O}$. Its oxygen donor provides sigma donation and can provide pi donation; an N-bound nitro ligand is its linkage isomer.

A nitro ligand is N-bound nitrite, $\mathrm{M}-\mathrm{N}(\mathrm{O}{)}_2$, normally treated as a sigma-donor ligand and pi-acceptor ligand. The word here concerns coordinated nitrite, not an independently introduced organic nitro substituent.

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.^[ref]

$A=[\mathrm{Co}(\mathrm{N}{\mathrm{H}}_3{)}_5(\mathrm{OH}){]}^{2+}$, $B=[\mathrm{Co}(\mathrm{N}{\mathrm{H}}_3{)}_5(\mathrm{ONO}){]}^{2+}$, $C=[\mathrm{Co}(\mathrm{N}{\mathrm{H}}_3{)}_5(\mathrm{N}{\mathrm{O}}_2){]}^{2+}$, and the by-product is $\mathrm{HN}{\mathrm{O}}_2$. 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 giveThe 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.

$B$ removes more blue-green light and appears orange-red; $C$ removes shorter-wavelength blue light and appears yellow. The observed color is the combined transmitted part of the visible spectrum, approximately complementary to the absorbed band, rather than the color corresponding to the absorption maximum itself. The longer-wavelength band of the O-bound nitrito ligand complex therefore shifts the transmitted balance toward orange/red compared with the N-bound nitro ligand complex. Band width, other absorption bands, concentration and optical path length affect the shade and darkness; a single maximum does not quantitatively determine the perceived color.