Outer-sphere Anion-Anion Complexes

Bands resulting from contact outer-sphere anion-anion interactions, have been found in the electronic absorption spectra of systems containing simultaneously the anions [FeNO(CN)₅]^2– or [Co edta]^– and nucleophilic anions (I−, SCN− and S2O32−); the fraction of complex anions occurring in the contact interaction with nucleophilic anions can be as high as several tens of percent in concentrated solutions.