A Unified Treatment of the Relationship Between Ligand Substituents and Spin State in a Family of Iron(II) Complexes

The influence of ligands on the spin state of a metal ion is of central importance for bioinorganic chemistry, and the production of base‐metal catalysts for synthesis applications. Complexes derived from [Fe(bpp)2]2+ (bpp=2,6‐di{pyrazol‐1‐yl}pyridine) can be high‐spin, low‐spin, or spin‐crossover (SCO) active depending on the ligand substituents. Plots of the SCO midpoint temperature (T1/2 ) in solution vs. the relevant Hammett parameter show that the low‐spin state of the complex is stabilized by electron‐withdrawing pyridyl (“X”) substituents, but also by electron‐donating pyrazolyl (“Y”) substituents. Moreover, when a subset of complexes with halogeno X or Y substituents is considered, the two sets of compounds instead show identical trends of a small reduction in T1/2 for increasing substituent electronegativity. DFT calculations reproduce these disparate trends, which arise from competing influences of pyridyl and pyrazolyl ligand substituents on Fe‐L σ and π bonding. Höhen und Tiefen: Der Low‐spin‐Zustand des abgebildeten Komplexes wird durch elektronenziehende Substituenten X an den Pyridylringen stabilisiert, durch elektronenschiebende Substituenten Y an den Pyrazolylringen hingegen schon. Dichtefunktionalrechnungen bestätigen diese Befunde, die sich mit konkurrierenden Einflüssen der Substituenten an den Pyridyl‐ and Pyrazolylliganden auf die σ‐ und π‐Fe‐L‐Bindung begründen.