The spin-forbidden reaction mechanism of Ta ( 4 F, 5d 3 6s 2 ) with CH 3 CN, on two different potential surfaces (PESs) has been investigated at the B3LYP, MP2, and CCSD level of theory. Crossing points between the PESs are located using different methods, and possible spin inversion processes are discussed by means of spin-orbit coupling calculations. As a result, the reaction system will change its spin multiplicities near this crossing seam, leading to a significant decrease in the barrier of 2-4 TS3 from 24.17 to 5.36 kcal/mol, which makes the reaction access to a lower energy pathway and accelerate the reaction rate.