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

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 ) is sensitive to subtle changes in M-L bonding. Specifically, EDA-NOCV analysis of Fe-N bonds in five [Fe (L ) (NCBH ) ], in both low-spin (LS) and paramagnetic high-spin (HS) states led to: 1) development of a general, widely applicable, corrected M+L fragmentation, tested against a family of five LS [Fe (L ) ](BF ) complexes; this confirmed that three L are stronger ligands (ΔE =-370 kcal mol ) than 2 L +2 NCBH (=-335 kcal mol ), as observed. 2) Analysis of Fe-L bonding on LS→HS, reveals more ionic (ΔE ) and less covalent (ΔE ) character (ΔE :ΔE 55:45 LS→64:36 HS), mostly due to a big drop in σ (ΔE ↓50 %; -310→-145 kcal mol ), and a drop in π contributions (ΔE ↓90 %; -30→-3 kcal mol ). 3) Strong correlation of observed T and ΔE , for both LS and HS families (R =0.99 LS, R =0.95 HS), but no correlation of T and ΔΔE (LS-HS) (R =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.