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  • 1 CeSNEF, Politecnico di Milano Istituto Ingegneria Nucleare Via Ponzio 34/3 20133 Milan Italy
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In order to ascertain possible relationships between the rate constants, KSE, of positronium, Ps, spin exchange /SE/ reactions promoted by 3d aqua ions and the electron configuration of the ion, the KSE's of Mn aq 2+ and Co aq 2+ aqua ions were measured together with that of [Cr(NH3)6]3+. The KSE's values obtained are discussed together with those of the Cr aq 3+ , Fe aq 2+ , Ni aq 2+ and [Ni(NH3)6]2+ ions previously measured. It was found that Cr aq 3+ and Mn aq 2+ KSE's are 1 M–1ns–1, while those of Fe aq 2+ , Co aq 2+ and Ni aq 2+ ions are 2.5 M–1ns–1. Thus the KSE values of 3d aqua ions do not depend solely on the number of their unpaired electrons as it was found for the 4f aqua ions. The trend observed parallels that of the electron delocalization from the metal atom; the delocalization occurs only with Fe aq 2+ , Co aq 2+ and Ni aq 2+ ions, but not with the Cr aq 3+ and Mn aq 2+ aqua ions. The trend of the KSE's of aqua, ammine and perhaps of chloro complexes parallels that of ligand capabilities to cause d-electron cloud expansion, and thus d-electron delocalization from metal atoms.