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

Reaction of the group 10 tetracyanometalates K₂[M^II(CN)₄] (M = Ni, Pd, Pt) and tetracyanoaurate K[Au^III(CN)₄] with an excess of the superacid HF/SbF₅ results in the formation and structural characterization of homoleptic hydrogen isocyanide complexes [M^II(CNH)₄][SbF₆]₂ (M = Ni, Pd, Pt) and [Au^III(CNH)₄][SbF₆]₃·2HF, respectively. The intermolecular interactions in the solid state are dominated by strong H···F bonded networks as well as weak contacts between the fluorine atoms and C≡N groups, which are more pronounced for the more electrophilic trication. Additionally, M···F contacts below the sum of van der Waals radii for all compounds are observed, which can be regarded as regium bonding. Furthermore, density functional theory (DFT) calculations were performed to provide an in-depth energetic and electronic characterization of the observed M···F interactions. Molecular electrostatic potential (MEP) surfaces confirm the existence of a π-hole (electrophilic region) over the metal centers, a notable transformation for these typically nucleophilic square-planar complexes of Ni^II, Pd^II, and Pt^II. Quantum theory of atoms in molecules (QTAIM) analysis confirms the noncovalent, closed-shell nature of the M···F contacts. Additionally, natural bond orbital (NBO) analysis quantifies the donor-acceptor character of these regium/π-hole interactions.