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)<sub>2</sub>]<sub>3</sub> (M = Ag, Au), KCd[M(CN)<sub>2</sub>]<sub>3</sub>, and KNi[Au(CN)<sub>2</sub>]<sub>3</sub>) 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 <i>ab</i> plane, where Kagome sheets of M atoms reside, and negative thermal expansion (NTE) along the trigonal <i>c</i> axis, perpendicular to these sheets. The magnitude of thermal expansion is unusually large in all cases (40 × 10<sup>−6</sup> K<sup>−1</sup> < |α| < 110 × 10<sup>−6</sup> K<sup>−1</sup>). The system with the weakest metallophilic interactions, In[Ag(CN)<sub>2</sub>]<sub>3</sub>, shows the most “colossal” thermal expansion of the series (α<sub>a</sub> = 105(2) × 10<sup>−6</sup> K<sup>−1</sup>, α<sub>c</sub> = −84(2) × 10<sup>−6</sup> K<sup>−1</sup> 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<sup>+</sup> counterions also reduces the magnitude of thermal expansion.