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 (T 1/2 ) 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 T 1/2 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.

The abrupt [HS-HS] ↔ localized [HS-LS] spin crossovers of a new triazole-based diiron(II) complex result in a record-equaling thermal hysteresis loop width for a dinuclear complex (ΔT = 22 K by SQUID magnetometer in "settle" mode) and show a remarkable scan rate dependence of only the cooling branch, as revealed by detailed magnetic, DSC, and Mössbauer studies.

Structure:function relationships are surveyed relating the spin-crossover (SCO) midpoint temperature (T½) in the solid state, for 43 members of the iron(II)/dipyrazolylpyridine family of SCO compounds. The difference between T½ in the solid state and in solution [T(latt)] is proposed as a measure of the lattice contribution to the transition temperature. Negative linear correlations between SCO temperature and the magnitude of the rearrangement of the coordination sphere during SCO are evident among isostructural or near-isostructural subsets of compounds; that is, a larger change in molecular structure during SCO stabilizes the high-spin state of a material. Improved correlations are often obtained when T(latt), rather than the raw T½ value, is considered as the measure of SCO temperature. Different lattice types show different tendencies to stabilize the high-spin or low-spin state of the molecules containing them, which correlates with the structural changes that most influence T(latt) in each case. These relationships are mostly unaffected by the SCO cooperativity in the compounds, or by the involvement of any crystallographic phase changes. One or two materials within each subset are outliers in some or all of these correlations however which, in some cases, can be attributed to small differences in their ligand geometry or unusual phase behavior during SCO. A reinvestigation of the structural chemistry of [Fe(3-bpp)2][NCS]2•nH2O (3-bpp = di{1H-pyrazol-3-yl}pyridine; n = 0 or 2), undertaken as part of this study, is also presented. .

. Solid (R)-3·MeNO 2 exhibits an unusualv ery gradual, but discontinuoust hermals pin-crossover with an approximate T1 = 2 of 350 K. The discontinuity around 240 Kl ies well below T1 = 2 , and is unconnected to ac rystallographic phase change occurring at 170 K. Rather,i tc an be correlated with ag radual ordering of the ligand conformation as the temperature is raised. The other solid compounds either exhibit spin-cross-