Zwischenatome

Zwischenatome is a term used in chemistry, predominantly in German-language discussions, to refer to atoms that appear transiently during a chemical transformation and are not part of the stable reactants or products. The idea captures atoms that are in the process of changing their bonding arrangement as bonds are formed and broken along a reaction pathway, occupying a position between discrete, well-defined species.

In mechanistic discussions, such atoms may be involved in transition states or very short-lived intermediates and may migrate or rearrange as the system moves toward products. The concept is especially common in organometallic chemistry and catalysis, where atoms or ligands can move within a coordination sphere or between fragments during a catalytic cycle. In these contexts, a migrating atom can be described informally as a Zwischenatom, highlighting its role during bond-formation events rather than as a stable fragment.

Because the term is informal and not standardized, many chemists prefer to describe the underlying process with established terms such as transition state, reaction intermediate, or migrating atom, and to specify the exact bonding changes involved. The use of Zwischenatome can aid intuition when teaching mechanisms or outlining plausible pathways, as long as it is accompanied by precise structural and energetic information.

See also: reaction mechanism, transition state, reaction intermediate, migratory insertion, atom transfer.