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 (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.