4-Fold Protonation of Tetracyanometalates in Superacids: Hydrogen and π-Hole Bonding in the Solid State.

The MEP analysis confirmed the electronic transformation of the typically nucleophilic d 8 square-planar cores into strong electrophilic sites, demonstrating a “supramolecular Umpolung” where the metal center is switched from an electron donor to a π-hole electron acceptor. The MEP values showed a clear trend of decreasing electrophilicity among the divalent cations from Ni to Pt, which perfectly correlates with the increase in the experimental M···F distances and the corresponding weakening of the ion-pair binding energies.

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