Impact of Metallophilicity on “Colossal” Positive and Negative Thermal Expansion in a Series of Isostructural Dicyanometallate Coordination Polymers

Five isostructural dicyanometallate coordination polymers containing metallophilic interactions (In[M(CN)₂]₃ (M = Ag, Au), KCd[M(CN)₂]₃, and KNi[Au(CN)₂]₃) were synthesized and investigated by variable-temperature powder X-ray diffraction to probe their thermal expansion properties. The compounds have a trigonal unit cell and show positive thermal expansion (PTE) in the ab plane, where Kagome sheets of M atoms reside, and negative thermal expansion (NTE) along the trigonal c axis, perpendicular to these sheets. The magnitude of thermal expansion is unusually large in all cases (40 × 10⁻⁶ K⁻¹ < |α| < 110 × 10⁻⁶ K⁻¹). The system with the weakest metallophilic interactions, In[Ag(CN)₂]₃, shows the most “colossal” thermal expansion of the series (α_a = 105(2) × 10⁻⁶ K⁻¹, α_c = −84(2) × 10⁻⁶ K⁻¹ at 295 K), while systems containing stronger Au−Au interactions show relatively reduced thermal expansion. Thus, it appears that strong metallophilic interactions hinder colossal thermal expansion behavior. Additionally, the presence of K⁺ counterions also reduces the magnitude of thermal expansion.