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 (T1/2) is sensitive to subtle changes in M−L bonding. Specifically, EDA‐NOCV analysis of Fe−N bonds in five [FeII(Lazine)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(Lazine)3](BF4)2 complexes; this confirmed that three Lazine are stronger ligands (ΔEorb,σ+π=−370 kcal mol−1) than 2 Lazine+2 NCBH3 (=−335 kcal mol−1), as observed. 2) 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−1), and a drop in π contributions (ΔEorb,π ↓90 %; −30→−3 kcal mol−1). 3) 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 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.

Quantitative Assessment of Ligand Substituent Effects on σ‐ and π‐Contributions to Fe−N Bonds in Spin Crossover FeIIComplexes

“…Note that the development and validation of this general fragmentation method (M + 6 L), for dia-and para-magnetic complexes and the application of it to SCO complexes for the first time in Ref. [20], and again herein, opens the door quantifying the nature of MÀ L bonding in more families of SCO complexes (in which the ligand field strength is very delicately poised) and we expect the resulting findings will continue to be revelatory.…”

“…This is very different from the [Fe(L pyridine ) 2 (NCBH 3 ) 2 ] complex where the ionic bonding is already dominating in the LS state (ΔE elstat :ΔE orb , 55 : 45), and this further increases in the HS state (65 : 35) (Table 2). [20] In conclusion, EDA analysis of these families of complexes, which feature very different types of coordination environments, is shown to correctly incorporate ) 2 ] complexes: [20] all energies are reported in kcal/mol.…”

“…In contrast, the ΔE orb interaction, which accounts for the covalent bonding between the fragments, remains almost constant across the whole range of σ p + values: from X = NMe 2 to X = NO 2 , ΔE orb increases by just + 20 kcal/mol (+ 3.5 %) in the LS [Fe II (bpp X ) 2 ] 2 + and decreased by just À 5 kcal/mol (À 1.5 %) in the HS [Fe II (bpp X ) 2 ] 2 + complexes. Comparing these EDA results with those for the [Fe II (L azine ) 2 (NCBH 3 ) 2 ] family (L azine = 3-(2azinyl)-4-tolyl-5-phenyl-1,2,4-triazole; (Table 2), [20] few differences can be grouped up. The ΔE int energies for the [Fe II (L azine ) 2 (NCBH 3 ) 2 ] family are twice the size of those for the [Fe II (bpp X ) 2 ] 2 + family, [20] but yet, the ΔE orb values are almost the same (Table 2).…”

“…Comparing these EDA results with those for the [Fe II (L azine ) 2 (NCBH 3 ) 2 ] family (L azine = 3-(2azinyl)-4-tolyl-5-phenyl-1,2,4-triazole; (Table 2), [20] few differences can be grouped up. The ΔE int energies for the [Fe II (L azine ) 2 (NCBH 3 ) 2 ] family are twice the size of those for the [Fe II (bpp X ) 2 ] 2 + family, [20] but yet, the ΔE orb values are almost the same (Table 2). The cause of the big difference in ΔE int values is the drop in magnitude observed for the ΔE elstat term in the [Fe II (bpp X ) 2 ] 2 + family vs. the [Fe II (L azine ) 2 (NCBH 3 ) 2 ] family.…”

“…The geometry optimisation computational protocol employed for the sixteen LS and sixteen HS [Fe II (bpp X ) 2 ] 2 + complexes was chosen based on the functional screening we performed previously. [20] The same computational protocol was applied to all of the candidates, in the same CPCM solvent, acetone, albeit the LS forms of the X = NMe 2 or NH 2 complexes were not observed experimentally. Calculating the Root-Mean-Square-Deviation (RMSD) of each atomic position (Eq.…”