Quantitative and Chemically Intuitive Evaluation of the Nature of M−L Bonds in Paramagnetic Compounds: Application of EDA‐NOCV Theory to Spin Crossover Complexes

To improve understanding of M−L bonds in 3d transition metal complexes, analysis by energy decomposition analysis and natural orbital for chemical valence model (EDA‐NOCV) is desirable as it provides a full, quantitative and chemically intuitive ab initio description of the M−L interactions. In this study, a generally applicable fragmentation and computational protocol was established and validated by using octahedral spin crossover (SCO) complexes, as the transition temperature (T (1/2)) is sensitive to subtle changes in M−L bonding. Specifically, EDA‐NOCV analysis of Fe−N bonds in five [Fe(II)(L (azine))(2)(NCBH(3))(2)], in both low‐spin (LS) and paramagnetic high‐spin (HS) states led to: 1) development of a general, widely applicable, corrected M+L(6) fragmentation, tested against a family of five LS [Fe(II)(L (azine))(3)](BF(4))(2) complexes; this confirmed that three L (azine) are stronger ligands (ΔE (orb,σ+π)=−370 kcal mol(−1)) than 2 L (azine) +2 NCBH(3) (=−335 kcal mol(−1)), as observed. 2) Analysis of Fe−L bonding on LS→HS, reveals more ionic (ΔE (elstat)) and less covalent (ΔE (orb)) character (ΔE (elstat):ΔE (orb) 55:45 LS→64:36 HS), mostly due to a big drop in σ (ΔE (orb,σ) ↓50 %; −310→−145 kcal mol(−1)), and a drop in π contributions (ΔE (orb,π) ↓90 %; −30→−3 kcal mol(−1)). 3) Strong correlation of observed T (1/2) and ΔE (orb,σ+π), for both LS and HS families (R (2)=0.99 LS, R (2)=0.95 HS), but no correlation of T (1/2) and ΔΔE (orb,σ+π)(LS‐HS) (R (2)=0.11). Overall, this study has established and validated an EDA‐NOCV protocol for M−L bonding analysis of any diamagnetic or paramagnetic, homoleptic or heteroleptic, octahedral transition metal complex. This new and widely applicable EDA‐NOCV protocol holds great promise as a predictive tool.