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 [FeII(L azine)2(NCBH3)2], in both low‐spin (LS) and paramagnetic high‐spin (HS) states led to: 1) development of a general, widely applicable, corrected M+L6 fragmentation, tested against a family of five LS [FeII(L azine)3](BF4)2 complexes; this confirmed that three L azine are stronger ligands (ΔE orb,σ+π=−370 kcal mol−1) than 2 L azine +2 NCBH3 (=−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.
A generally applicable fragmentation and computational protocol for EDA‐NOCV M−L bonding analysis of any diamagnetic or paramagnetic, homoleptic or heteroleptic, octahedral complex is established and validated in the first EDA‐NOCV analysis of spin crossover‐active complexes. As expected the combination of the σ and π bonding (ΔE orb,σ+π) in the Fe−L bonds correlates very well with the observed switching temperature (T 1/2).