Substituents of a given type often are placeable in a sequential fashion at the sites of a fixed molecular skeleton. The differently substituted species form a collection of permutational isomers falling into a (substitution) reaction network, which may be viewed as a partially ordered set (or poset). When the substituent species are paired together (as so‐called "bidentate ligands") to attach to neighbor sites, the permutational isomerism problem and associated reaction‐network poset is modified, as here illustrated. Extension of the ideas beyond the chemical context are noted.