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Herein, we introduce the concept of pseudoisotope to describe particles which have similar chemistry but different masses. Examples include ligands with different substituents or metal ions with identical charges and similar coordination properties. Mixtures of pseudoisotopes may be used to establish rapidly the nuclearity of polynuclear species by electrospray ionization-mass spectrometry. Pseudoisotope exchange allows the study of the dynamics of polynuclear complexes, and shows these reactions may be surprisingly slow. The evolution of the mass spectra indicates the degree of fragmentation occurring during the exchange.

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The complex [Cu2(1)2]2+ (1 = 1,3‐bis(1‐methyl‐1H‐benzimidazol‐2‐yl)benzene) undergoes slow oxidation by dioxygen in DMF solution to give the hydroxylated product [Cu2(2‐H)2]2+ (2 = 2,6‐bis(1‐methyl‐1H‐benzimidazol‐2‐yl)phenol) characterized by an X‐ray crystal‐structure analysis. The oxidation occurs much faster when CuII is mixed with 1 in the presence of H2O2, with 80% hydroxylation observed within a few minutes. The mononuclear complex formed with 1‐methyl‐2‐phenyl‐1H‐benzimidazole (3) shows no hydroxylation under these conditions. It is concluded that the hydroxylation requires the presence of a ligand capable of stabilizing a binuclear species, but no special coordinative activation of the copper is required.