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

## TL;DR

This paper explores how tetracyanometalates react in superacids, forming new complexes and revealing unique bonding interactions in the solid state.

## Contribution

The study reports the 4-fold protonation of tetracyanometalates and identifies novel π-hole and regium bonding interactions in the solid state.

## Key findings

- Homoleptic hydrogen isocyanide complexes are formed from tetracyanometalates in superacids.
- Strong H···F and M···F interactions are observed, confirmed by QTAIM and NBO analyses.
- Density functional theory calculations reveal electrophilic π-hole regions on metal centers.

## Abstract

Reaction of the group
10 tetracyanometalates K2[MII(CN)4] (M = Ni, Pd, Pt) and tetracyanoaurate K­[AuIII(CN)4] with an excess of the superacid HF/SbF5 results
in the formation and structural characterization
of homoleptic hydrogen isocyanide complexes [MII(CNH)4]­[SbF6]2 (M = Ni, Pd, Pt) and [AuIII(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 NiII,
PdII, and PtII. 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.