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

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 [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 (Δ E 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 (Δ E elstat ) and less covalent (Δ E orb ) character (Δ E elstat :Δ E orb 55:45 LS → 64:36 HS), mostly due to a big drop in σ (Δ E orb,σ ↓50 %; −310→−145 kcal mol −1 ), and a drop in π contributions (Δ E orb,π ↓90 %; −30→−3 kcal mol −1 ). 3) Strong correlation of observed T 1/2 and Δ E 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 ΔΔ E 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.

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