Abstract The cationic complexes 2 [Nb(η-C5H5)2(Bu)(η2-C(S)SR]]X are formed from [Nb(η-C5H5)2(Bu)[η2-CS2]] (1) and RX (R = Me, Et, i-Pr, CH2COOEt; X = I. R = CH2C6H5, CH2CH2CH2Cl, CH2CHCH2; X = Br). They react with NaBH4 in THF giving complexes 4. These decompose in solution and give, after column chromatography, neutral complexes 3 of the type [Nb(η-C5H5)2(Bu)[η-CH(S)SR]]. The conversion of 2 into 3, which corresponds to nucleophilic attack by the hydride ion on the CS2 carbon atom, seems to proceed via formation of intermediate derivatives 4 containing a niobium-coordinated BH4 group. Reaction of 1 with diiodomethane as alkylating reagent results in the formation of a dimeric niobium compound containing the NbC(S )SCH2S( S)CN b unit, which reacts further with NaBH4.