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

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.