Experimental IR multiphoton dissociation spectra of cationic nobium-acetonitrile complexes with the formal stoichiometry [Nb,nCH(3)CN](+), n = 4, 5 (the notation [Nb,nCH(3)CN](+) was chosen in order to emphasize the formal stoichiometry of a cluster without implying any structural information), as provided by the Free Electron Laser at CLIO, Orsay, France, are compared to ab initio IR absorption spectra throughout the spectral "fingerprinting" range 780-2500 cm(-1). For n = 4 the vibrational spectrum in combination with the performed ab initio calculations provides strong evidence for a square-planar high-spin quintet [Nb-I(NCCH3)(4)](+) complex. For n = 5, additional vibrational bands between 800 and 1550 cm-1 are interpreted in terms of covalent C-C coupling in [Nb,5CH(3)CN](+). Screening on the basis of ab initio calculations leads to the assignment of the recorded spectrum to the metallacyclic species [Nb-III(NCCH3)(3)(N=C(CH3)C(CH3)=N)](+) with an electronic triplet state. The deduced processes upon 4-fold and 5-fold coordination of Nb-I with CH3CN in the gas-phase are complexation only and reductive nitrite coupling, respectively. The minimum energy pathways of the reductive nitrite coupling reaction in [Nb-I(NCCH3)(n)](+), n = 4, 5, investigated for singlet, triplet, and quintet states (S = 0, 1, 2) by density functional theory, account well for the observed (non)reactivity. In ground state (triplet, S = 1) [(NbNCCH3)-N-I)(5)](+) the reaction is found to be exothermic and the activation barrier amounts to approximately 49 kj mol(-1), whereas for ground state (quintet, S = 2) [Nb-I(NCCH3)(4)](+) the corresponding reaction is endothermic and would require an activation of more than 116 U mol(-1).