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.