High-yield reactions of the pentafluoride complexes MF(5) (M = Nb, Ta) with a variety of diethers, ROCH(2)CH(R')OR'', afford the ionic derivatives [MF(4){kappa(2)-ROCH(2)CH(R')OR''}(2)][MF(6)] (2a-2e) or [MF(4){ROCH(2)CH(R')OR''}(2)][M(2)F(11)] (3a-3f) according to the metal/diether ratio employed. The structures of [MF(4)(kappa(2)-dme)(2)][MF(6)] [M = Nb (2a), Ta (2b); dme = MeOCH(2)CH(2)OMe] have been determined by X-ray diffraction. Moreover, the structure of the cationic part of 2a in the gas phase has been optimized by density functional theory calculations (the B3LYP/LANL2DZ method). An analogous computational study has allowed one to predict the existence of a heptacoordinated niobium cation within the species [NbCl(4)(dme)(2)][NbCl(6)]. This prediction has been confirmed by NMR identification at -60 degrees C of the complexes [MCl(4)(kappa(2)-dme)(kappa(1)-dme)][MCl(6)] [M = Nb (4a), Ta (4b)], obtained by the addition of dme to MCl(5) in CDCl(3). Activation of the coordinated diether in 2 and 3 takes place in solution at high temperature and generally proceeds via cleavage of C-O bonds and coupling of the fragments produced. Thus, OR(2) (R = Me, Et) and 1,4-dioxane have been obtained selectively upon thermal reaction of MF(5) with dme, diglyme, or 1,2-diethoxyethane, followed by treatment with water. In analogous conditions, 1,2-dimethoxypropane converts mainly into acetone and dimethyl ether. The formation of dioxane from dme occurs in high yield also by using a catalytic amount of NbF(5). The activation of dme by NbF(5) follows a pathway different from that previously reported for NbCl(5), and an explanation based on the thermodynamic parameters calculated at the B3LYP/LANL2DZ level is given: the formation of Me(2)O and dioxane is the most favored reaction between NbF(5) and dme (DeltaG(r) degrees = -15.16 kcal x mol(Nb)(-1)), while the formation of MeCl and dioxane is the most favored reaction in the case of NbCl(5)/dme (DeltaG(r) degrees = -53.13 kcal x mol(Nb)(-1)).