To explore the details of the reaction mechanisms of Zr atoms with acetonitrile molecules, the triplet and singlet spin-state potential energy surfaces have been investigated. Density functional theory (DFT) with the relativistic zero-order regular approximation at the PW9I/TZ2P level has been applied. The complicated minimum energy reaction path involves four transition states (TS), stationary states 1-5 and one spin inversion (indicated by double right arrow): Zr-3 + NCCH3 -> Zr-3-eta(1)-NCCH3 ((3)1) -> (TS1/2)-T-3 -> Zr-3-eta(2)-(NC)CH3 ((3)2) -> (TS2/3)-T-3 -> (ZrH)-Zr-3-eta(3)-(NCCH2) ((3)3) -> (TS3/4)-T-3 -> CNZrCH3 ((3)4) double right arrow (TS4/5)-T-1 -> CN(ZrH)CH2 ((1)5). The minimum energy crossing point was determined with the help of the DFT fractional-occupation-number approach. The spin inversion leading from the triplet to the singlet state facilitates the activation of a C-H bond, lowering the rearrangement-barrier by 78 kJ/mol. The overall reaction is calculated to be exothermic by about 296 kJ/mol. All intermediate and product species were frequency and NBO analyzed. The species can be rationalized with the help of Lewis type formulas.