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An ambidentate ligand has alternative donor atoms through which it can attach, producing different binding modes. Nitrite can bind through N or O; changing that attachment gives linkage isomerism.

In bidentate coordination, two donor atoms from one ligand bind the same metal. Attachment through both oxygens of nitrite is distinct from choosing between its N and O monodentate modes.

A linkage isomer is one member of a set exhibiting linkage isomerism, such as the N-bound or O-bound form of a coordinated nitrite.

Linkage isomerism occurs when complexes have the same composition but an ambidentate ligand attaches through different atoms. N-bound and O-bound nitrite provide the classical example. This is a difference in attachment connectivity, rather than the different spatial arrangements of unchanged connectivity in stereoisomers.

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 nitroalkane carbon can join the two ends of a five-carbon conjugated ketone framework. carbon–carbon bond formation, thiolate displacement and nitrite elimination can build a substituted phenol. The nitro-bearing carbon, rather than the nitro nitrogen, enters the aromatic ring.

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.

Nitrous acid is $\mathrm{HN}{\mathrm{O}}_2$, the protonated counterpart of nitrite. Its nominal relative molecular mass is 47.

The five-carbon unit is initially an open chain, not a completed carbocycle. Label it $P_1-P_2-P_3-P_4-P_5$, with phenyl-bearing $P_1$, carbonyl $P_3$, amide-substituted $P_4$, and the two ethylthio groups on $P_5$. Its double bonds are initially $P_1=P_2$ and $P_4=P_5$. Let $X$ be the nitro-bearing carbon of nitroethane; its methyl remains a substituent throughout.

DBU removes an alpha proton from nitroethane to give a nitronate. carbon attack at the Michael acceptor $P_1$ forms $X-P_1$, with electron redistribution through $P_2-P_3$ onto the carbonyl oxygen. proton transfer restores the ketone. Removing the remaining X–H proton gives another carbon-nucleophilic nitronate. It attacks the other conjugated end, $P_5$, intramolecularly, closing the six-membered circuit $P_1-P_2-P_3-P_4-P_5-X$. Addition to the ketene dithioacetal and ethylthiolate elimination restore the $P_4=P_5$ bond, leaving one SEt group at $P_5$.

base-assisted loss of the proton at $P_1$, with C–NO2 cleavage at X, creates $P_1=X$ and expels nitrite. Enolization at $P_2-P_3$ supplies the third aromatic double bond and the phenolic OH. proton transfers to the leaving anions complete the acid–base balance. nitroethane supplies the new ring carbon X and its methyl substituent; its nitro group is eliminated. Aromatization, rather than a separate oxidation of an intact six-membered starting ring, provides the final driving force. This is a proposed stepwise nitroalkane benzannulation; related DBU-promoted tandem Michael addition/cyclization/aromatization has direct primary precedent.^[ref]

The nitrite ion is an ambidentate ligand: its Lewis structures leave donor lone pairs on both nitrogen and oxygen. In the N-bound mode, $\mathrm{M}-\mathrm{N}(\mathrm{O}{)}_2$, the nitrogen lone pair makes a sigma bond to the metal, while a suitable unoccupied nitrite molecular orbital can accept metal $d$ electron density by pi backbonding. Thus N-bound nitro is a sigma donor and pi acceptor. The nitrite group remains bent at nitrogen; N coordination does not make it a linear nitrosyl.

In the O-bound mode, $\mathrm{M}-\mathrm{O}-\mathrm{N}=\mathrm{O}$, an oxygen lone pair makes the metal–oxygen sigma bond. A second occupied oxygen orbital can donate sideways into an available metal orbital: O-bound nitrito is a sigma and pi donor, rather than the strong pi acceptor of the N-bound description. oxygen's greater electronegativity lowers its donor orbital energies, but the occupied oxygen orbitals still explain its pi-donor classification. Actual mixing depends on metal oxidation state and geometry.

A further possibility is bidentate coordination, $\mathrm{M}({\kappa }^2\text{-}O,O\text{-}\mathrm{N}{\mathrm{O}}_2)$, with both oxygen atoms attached to one metal. Each attachment has oxygen-to-metal sigma donation; additional pi donation depends on orbital alignment. nitrite can also be a bridging ligand between metals. Neither alternative should be confused with linkage isomerism, which specifically compares attachment through different atoms at the same binding site.