Quantitative Assessment of Ligand Substituent Effects on σ‐ and π‐Contributions to Fe−N Bonds in Spin Crossover Fe II Complexes

The effect of para-substituent X on the electronic structure of sixteen tridentate 4-X-(2,6‐di(pyrazol‐1‐yl))-pyridine (bppX) ligands and the corresponding solution spin crossover [FeII(bppX)2]2+ complexes is analysed further, to supply quantitative insights into the effect of X on the σ-donor and π-acceptor character of the Fe-NA(pyridine) bonds. EDA-NOCV on the sixteen LS complexes revealed that neither ΔEorb,σ+π (R2=0.48) nor ΔEorb,π (R2=0.31) correlated with the experimental T1/2 values, but ΔEorb,σ correlates well (R2=0.82) and implies that as X changes from EDGEWG (Electron Donating to Withdrawing Group), the ligand becomes a better σ-donor. This counter-intuitive result was further probed by Mulliken analysis of the NA atomic orbitals: NA(px) involved in the Fe-N σ-bond vs. the perpendicular NA(pz) employed in the ligand aromatic π-system. As X changes EDGEWG, the electron population on NA(pz) decreases, making it a better π-acceptor, whilst that in NA(px) increases, making it a better σ-bond donor; both increase ligand field, and T1/2 as observed.