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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.

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.

Linkage isomerization changes which atom of an ambidentate ligand is bound to a central atom. Its rate depends on a reaction barrier, whereas the final equilibrium depends on relative free energies.

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.

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.

A is the N-bound linkage isomer, $[(\mathrm{N}{\mathrm{H}}_3{)}_5\mathrm{Co}(\mathrm{NCS}){]}^{2+}$: five ammines plus NCS account for $\mathrm{CoC}{\mathrm{H}}_{15}{\mathrm{N}}_6\mathrm{S}$.

The natural route is conjugate-base substitution. Hydroxide reversibly removes an ammine proton:The amido donor promotes loss of X${}^{-}$, producing a short-lived five-coordinate $[(\mathrm{N}{\mathrm{H}}_3{)}_4(\mathrm{N}{\mathrm{H}}_2)\mathrm{Co}{]}^{2+}$ intermediate. That leaving-group-sensitive loss is the slower stage. Subsequent trapping/proton transfers are fast and no longer depend on which halide left.

Water capture followed by proton transfer gives the hydroxo complex. Competing SCN${}^{-}$ capture can occur at either end of this ambidentate ligand, producing Co-SCN and Co-NCS. Any initially captured S-bound material can undergo the base-assisted linkage rearrangement considered next. Water is overwhelmingly abundant, so the stable N-bound product can remain a minor overall product. The slow X-dependent and fast X-independent stages fit loss of X followed by chemistry of a common intermediate; a single rate-limiting direct attack by thiocyanate would not explain that separation naturally.