The novel μ-oxo complexes NbOX₃[κ²-O(Me)CH₂CO₂Me]NbX₅ (X = Cl, 3a; X = Br, 3b), NbOCl₃[κ²-(MeO₂C)CHCH(CO₂Me)]NbCl₅ (7) and NbOCl₃[κ²-CH₂(CO₂Me)₂]NbCl₅ (8) have been prepared in good yields by 1 : 1 molar ratio reactions of the halides NbX₅ (X = Cl, Br) with methoxymethyl acetate [MeOCH₂CO₂Me], dimethyl maleate [(MeO₂C)CHCH(CO₂Me)] and dimethyl malonate [CH₂(CO₂Me)₂] respectively, in different experimental conditions. NMR studies have indicated that the oxo unit of 3, 7 and 8 forms via selective fragmentation (C–O bonds cleavage) of half equivalent of the organic material. The ionic complexes [NbX₄{O–O}₂][NbX₆] [X = Cl, O–O = MeOCH₂CO₂Me, 4; X = Cl, O–O = (MeO₂C)CHCH(CO₂Me), 5b; X = Cl, O–O = CH₂(CO₂Me)₂, 5e] have been identified as intermediates of the reactions affording 3a, 7 and 8. The stable complexes [NbX₄{O–O}₂][NbX₆] [X = F, O–O = (MeO₂C)CHCH₂(CO₂Me), 5a; X = Cl, O–O = (EtO₂C)CHCH₂(CO₂Et), 5c; X = F, O–O = CH₂(CO₂Me)₂, 5d] and [NbCl₄{(EtO₂C)CHCH(CO₂Et)}][NbCl₆] (6) have been prepared by reacting NbX₅ (X = F, Cl) with variable amounts of the appropriate diester. Alternatively, NbX₅ (X = F, Cl) react with diethyl fumarate in 2 : 1 molar ratio affording the dinuclear [NbX₅]₂{μ-κ²-(EtO₂C)CHCH(CO₂Et)} (X = F, 9a; X = Cl, 9b), in high yields. The neutral, monomeric, compounds MX₅(L) [L = HCO₂Me, Et₂NCO₂Me, OMe₂, OEt₂, OMeCH₂Cl, OMeCH₂CH₂Cl, OMeCH₂CH₂Br, O(CH₂CH₂Cl)₂, 1,4-dioxane] have been obtained upon addition of the appropriate O-donor to MX₅. Complexes NbX₅(OMeCH₂CH₂Y) (X = Br, Y = Cl, 10i; X = Y = Br, 10j; X = I, Y = Br, 10k) slowly convert into the corresponding alkoxides [NbX₄(OCH₂CH₂Y)]₂ (X = Br, Y = Cl, 11a; X = Y = Br, 11b; X = I, Y = Br, 11c) in CDCl₃. The further conversion of 11b to NbOBr₃, MeBr and Br(CH₂)₂Br takes place at 60 °C. N₂CHCO₂Et behaves as a ligand with respect to NbF₅, whereas it undergoes fragmentation and halogenation when contacted with NbCl₅. The new compounds have been characterised by NMR and IR spectroscopies, elemental analyses and solution electrical conductivity measurements. The latter have proved to be efficient for discriminating between the neutral or the cationic nature of the products. Moreover, the solid state structures of 3a,b, 7 and 8 have been ascertained by X-ray diffraction studies.