Abstract
To improve understanding ofM-Lbonds 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 theM-Linteractions. 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 inM-Lbonding. Specifically, EDA-NOCV analysis of Fe-N bonds in five [Fe-II(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+L(6)fragmentation, tested against a family of five LS [Fe-II(L-azine)(3)](BF4)(2)complexes; this confirmed that threeL(azine)are stronger ligands (Delta E-orb,E-sigma+pi=-370 kcal mol(-1)) than2 L-azine+2 NCBH3(=-335 kcal mol(-1)), as observed. 2) Analysis of Fe-Lbonding on LS -> HS, reveals more ionic (Delta E-elstat) and less covalent (Delta E-orb) character (Delta E-elstat:Delta E(orb)55:45 LS -> 64:36 HS), mostly due to a big drop in sigma (Delta E-orb,E-sigma down arrow 50 %; -310 ->-145 kcal mol(-1)), and a drop in pi contributions (Delta E-orb,E-pi down arrow 90 %; -30 ->-3 kcal mol(-1)). 3) Strong correlation of observedT(1/2)and Delta E-orb,E-sigma+pi, for both LS and HS families (R-2=0.99 LS,R-2=0.95 HS), but no correlation ofT(1/2)and Delta Delta E-orb,E-sigma+pi(LS-HS) (R-2=0.11). Overall, this study has established and validated an EDA-NOCV protocol forM-Lbonding 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.