Abstract
Abstract The effect of para ‐substituent X on the electronic structure of sixteen tridentate 4‐ X ‐(2,6‐di(pyrazol‐1‐yl))‐pyridine ( bpp X ) ligands and the corresponding solution spin crossover [Fe II ( bpp X ) 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‐ N A (pyridine) bonds. EDA‐NOCV on the sixteen LS complexes revealed that neither ΔE orb,σ+π (R 2 =0.48) nor ΔE orb , π (R 2 =0.31) correlated with the experimental solution T 1/2 values (which are expected to reflect the ligand field imposed on the iron centre), but that ΔE orb , σ correlates well (R 2 =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 N A atomic orbitals: N A ( p x ) involved in the Fe−N σ‐bond vs. the perpendicular N A ( p z ) employed in the ligand aromatic π‐system. As X changes EDG → EWG , the electron population on N A ( p z ) decreases, making it a better π‐acceptor, whilst that in N A ( p x ) increases, making it a better σ‐bond donor; both increase ligand field, and T 1/2 as observed. In 2016, Halcrow, Deeth and co‐workers proposed an intuitively reasonable explanation of the effect of the para ‐ X substituents on the T 1/2 values in this family of complexes, consistent with the calculated MO energy levels, that M→L π‐backdonation dominates in these M−L bonds. Here the quantitative EDA‐NOCV analysis of the M−L bond contributions provides a more complete, coherent and detailed picture of the relative impact of M−L σ‐versus π‐bonding in determining the observed T 1/2 , refining the earlier interpretation and revealing the importance of the σ‐bonding. Furthermore, our results are in perfect agreement with the ΔE(HS‐LS) vs. σ p + (X) correlation reported in their work.