Quantitative and chemically intuitive evaluation of the nature of M‐L bonds in paramagnetic compounds: application of EDA‐NOCV theory to spin crossover complexes

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.