Spin-crossover in cyanide-based bimetallic coordination polymers-insight from first-principles calculations

Abstract

Employing density functional theory (DFT) based calculations, we perform a computational investigation to unravel the intricate spin-crossover transitions that have been observed (Niel et al., Angew. Chem., Int. Ed., 2003, 42, 3760) in cyanide-based bimetallic coordination polymers, [Fe(pmd)-(H₂O){M(CN)₂}₂]·H₂O (M = Ag or Au and pmd = pyrimidine). Our calculations provide an explanation for the observed temperature-induced low-spin (LS) to high-spin (HS) spin-crossover occurring in both Ag and Au hydrated networks due to Fe-N bond stretching, concomitant with charge re-distribution from Fe to the ligands. For dehydrated compounds, we find that relativistic effects in Au versus Ag lead to differences in the degree of covalent bonding, which in turn, gives rise to a differential behavior, viz. the Ag dehydrated network exhibits a LS-HS spin transition whereas the Au hydrated network remains in HS state. As a critical test of our first-principles study we propose to investigate a Cu-based bimetallic coordination polymer, which we predict to be in LS state.