Stacking the Deck to Make Materials With Atypically Short Heterometallic d⁸ M(II)···Au(III) Bonds

The first Pt^II···Au^III metallophilic interaction is reported in K₄[Pt(CN)₄][Au(CN)₄]Cl·6H₂O, which consists of a 1D chain of stacking [Pt(CN)₄]^2- and [Au(CN)₄]^- units bridged by potassium cations, with extremely short Pt^II···Au^III distances of 3.0024(4) and 3.0057(4) Å at 80 K. The chloride ions and water molecules are bound to the potassium cation. Addition of KCl to the reaction mixture is necessary for the metallophilic Pt^II···Au^III chain to assemble. The isostructural [Pd(CN)₄]^2- and [Ni(CN)₄]^2- analogs can be rationally synthesized in a similar manner, which feature the first Pd^II···Au^III and Ni^II···Au^III interactions, with the Ni^II analog having the shortest metal-metal distances of 2.968(3) and 2.978(3) Å. The bromide versions of all compounds were also synthesized by substituting KBr for KCl in the reaction mixture. ¹⁹⁵Pt solid-state NMR of the title compound revealed highly anisotropic, axially symmetric magnetic shielding which resembles that found in systems with Pt^II···Pt^II interactions, such as K₂[Pt(CN)₄]·H₂O. The Pd^II and Pt^II analogs are emissive, with λ_max of 555 and 570 nm. Theoretical calculations indicate that the positive charge of the K⁺ ions effectively neutralizes the electrostatic repulsion between the cyanometallates, resulting in significant metal-based donor-acceptor interactions. These novel d⁸···d⁸ interactions provide a conceptual framework for the rational design of future metallophilic compounds.