Quantitative Assessment of Ligand Substituent Effects on σ- and π-Contributions to Fe−N Bonds in Spin Crossover FeII 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 solution T1/2 values (which are expected to reflect the ligand field imposed on the iron centre), but that 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. In 2016, Halcrow, Deeth and co-workers proposed an intuitively reasonable explanation of the effect of the para-X substituents on the T1/2 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 T1/2, 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. σp+(X) correlation reported in their work.
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