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

Abstract

With the aim of improving understanding of M‐L bonds in 3d transition metal complexes, quantitative analysis by Energy Decomposition Analysis and Natural Orbital for Chemical Valence model (EDA‐NOCV) is done on octahedral spin crossover (SCO) complexes, as the transition temperature (T1/2) is sensitive to subtle changes in M‐L bonding. EDA‐NOCV analysis of Fe‐N bonds in 5 [FeII( Lazine )2(NCBH3)2], in both low spin (LS) and paramagnetic high spin (HS) states, led to (a) development of a general, widely applicable, corrected M+L6 fragmentation, tested against a family of 5 LS [FeII( Lazine )3(BF4)2], confirming that 3 Lazine are stronger ligands (ΔEorb,σ+π ≈ ‐370 kcal/mol) than 2 Lazine + 2 NCBH3 (≈ ‐335 kcal/mol), as observed; (b) analysis of Fe‐L bonding on LS → HS, reveals more ionic (ΔEelstat) and less covalent (ΔEorb) character (ΔEelstat:ΔEorb 55:45 LS → 64:36 HS), mostly due to a big drop in σ‐ (ΔEorb,σ ↓50%; ‐310 → ‐145 kcal/mol), and a drop in π‐ contributions (ΔEorb,π ↓90%; ‐30 → ‐3 kcal/mol); (c) strong correlation of observed T1/2 and ΔEorb,σ+π, for both LS and HS families (R2 =0.99 LS, R2 = 0.95 HS), but no correlation of T1/2 and ΔΔEorb,σ+π(LS‐HS) (R2 =0.11). Overall, this study has established and validated a generally applicable fragmentation and computational protocol for EDA‐NOCV M‐L bonding analysis of any diamagnetic or paramagnetic, homoleptic or heteroleptic, octahedral transition metal complex.