Theoretical study of spin-orbit coupling and intersystem crossing in the two-state reaction between Nb(NH 2 ) 3 and N 2 O

The two-state mechanism of the reaction of Nb(NH 2 ) 3 with N 2 O on the singlet and triplet potential energy surfaces has been investigated at the B3LYP level. Crossing points between the potential energy surfaces have been located using different methods. Analysis of the strain model shows that the singlet state of the four-coordinate (N 2 O)Nb(NH 2 ) 3 complex with N 2 O bonded via terminal N atom coordination ( 1 2 ) is more stable in the initial stage of reaction, since the bending of the N 2 O fragment [ E def (N 2 O) = 86.1 kcal mol −1 ] results in an energy splitting of the doubly degenerate LUMO; the low-energy LUMO can now strongly couple with the occupied Nb-localized d orbitals, forming a back-bond and transferring charge ( q = 0.82 e) from Nb(NH 2 ) 3 to the N 2 O ligand. Going from 3 2 to 1 2 , the reacting system changes spin multiplicity near the MECP (minimal energy crossing point) region, which takes place with a spin crossing barrier of 9.6–10.0 kcal mol −1 . Analysis of spin-orbit coupling (SOC) indicates that MECP will produce a significant SOC matrix element. The value of SOC is 111.52 cm −1 , due to the electron shift between two perpendicular ϕ orbitals with the same rotation direction, and the magnitude of the spin-multi-plicity mixing increases in the small energy gap between high- and low-spin states, greatly enhancing the probability of intersystem crossing. The probabilities of single ( P 1 ISC ) and double ( P 2 ISC ) passes estimated at MECP (SOC = 111.52 cm −1 ) are approximately 1.17×10 −2 and 2.32×10 −2 , respectively.