In order to explore the details of the reaction mechanisms of Nb atoms with acetonitrile molecules, the sextet, quartet, and doublet spin state potential energy surfaces have been investigated. Density functional theory (DFT) with the relativistic zero-order regular approximation at the PW91/TZ2P level has been applied. The complicated minimum energy reaction path involves four transition states (TS), stationary states (1) to (5) and two intersystem crossings from spin sextets to quartets to doublets (indicated by ⇒): ⁶Nb + NCCH₃ → ⁶Nb η¹-NCCH₃ (⁶1) → ⁶TS_1/2 ⇒ ⁴Nb η²-(NC)CH₃ (⁴2) → ⁴TS_2/3 → ⁴NbH η³-(NCCH₂) (⁴3) → ⁴TS_3/4 → CNNbCH₃ (⁴4) ⇒ ²TS_4/5 → CN(NbH)CH₂ (²5). The minimum energy crossing points were determined with the help of the DFT fractional-occupation-number approach. The first spin inversion leads from the sextet to an energetically low intermediate quartet (⁴2) with final insertion of Nb into the C–C bond. The second one from the quartet to the doublet state facilitates the activation of a C–H bond, lowering the rearrangement-barrier by 44 kJ mol⁻¹. The overall reaction is calculated to be exothermic by about 170–180 kJ mol⁻¹. All intermediate and product species were frequency and NBO analyzed. The species can be rationalized with the help of Lewis type formulas.