The novel mu-oxo complexes NbOX(3)[kappa(2)-O(Me)CH(2)CO(2)Me]NbX(5) (X = Cl, ; X = Br, ), NbOCl(3)[kappa(2)-(MeO(2)C)CH[double bond, length as m-dash]CH(CO(2)Me)]NbCl(5) () and NbOCl(3)[kappa(2)-CH(2)(CO(2)Me)(2)]NbCl(5) () have been prepared in good yields by 1 : 1 molar ratio reactions of the halides NbX(5) (X = Cl, Br) with methoxymethyl acetate [MeOCH(2)CO(2)Me], dimethyl maleate [(MeO(2)C)CH[double bond, length as m-dash]CH(CO(2)Me)] and dimethyl malonate [CH(2)(CO(2)Me)(2)] respectively, in different experimental conditions. NMR studies have indicated that the oxo unit of , and forms via selective fragmentation (C-O bonds cleavage) of half equivalent of the organic material. The ionic complexes [NbX(4){O-O}(2)][NbX(6)] [X = Cl, O-O = MeOCH(2)CO(2)Me, ; X = Cl, O-O = (MeO(2)C)CH[double bond, length as m-dash]CH(CO(2)Me), ; X = Cl, O-O = CH(2)(CO(2)Me)(2), ] have been identified as intermediates of the reactions affording , and . The stable complexes [NbX(4){O-O}(2)][NbX(6)] [X = F, O-O = (MeO(2)C)CH[double bond, length as m-dash]CH(2)(CO(2)Me), ; X = Cl, O-O = (EtO(2)C)CH[double bond, length as m-dash]CH(2)(CO(2)Et), ; X = F, O-O = CH(2)(CO(2)Me)(2), ] and [NbCl(4){(EtO(2)C)CH[double bond, length as m-dash]CH(CO(2)Et)}][NbCl(6)] () have been prepared by reacting NbX(5) (X = F, Cl) with variable amounts of the appropriate diester. Alternatively, NbX(5) (X = F, Cl) react with diethyl fumarate in 2 : 1 molar ratio affording the dinuclear [NbX(5)](2){mu-kappa(2)-(EtO(2)C)CH[double bond, length as m-dash]CH(CO(2)Et)} (X = F, ; X = Cl, ), in high yields. The neutral, monomeric, compounds MX(5)(L) [L = HCO(2)Me, Et(2)NCO(2)Me, OMe(2), OEt(2), OMeCH(2)Cl, OMeCH(2)CH(2)Cl, OMeCH(2)CH(2)Br, O(CH(2)CH(2)Cl)(2), 1,4-dioxane] have been obtained upon addition of the appropriate O-donor to MX(5). Complexes NbX(5)(OMeCH(2)CH(2)Y) (X = Br, Y = Cl, ; X = Y = Br, ; X = I, Y = Br, ) slowly convert into the corresponding alkoxides [NbX(4)(OCH(2)CH(2)Y)](2) (X = Br, Y = Cl, ; X = Y = Br, ; X = I, Y = Br, ) in CDCl(3). The further conversion of to NbOBr(3), MeBr and Br(CH(2))(2)Br takes place at 60 degrees C. N(2)CHCO(2)Et behaves as a ligand with respect to NbF(5), whereas it undergoes fragmentation and halogenation when contacted with NbCl(5). 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 ,, and have been ascertained by X-ray diffraction studies.