High-yield reactions of the pentafluoride complexes MF5 (M=Nb, Ta) with a variety of diethers, ROCH2CH(R')OR '', afford the ionic derivatives [MF4{kappa(2)-ROCH2CH(R')OR ''}(2)][MF6] (2a-2e) or [MF4{ROCH2CH(R')OR ''}(2)][M2F11] (3a-3f) according to the metal/diether ratio employed, The structures of [MF4(kappa(2)-dme)(2)][MF6] [M=Nb (2a), Ta (2b); dme=MeOCH2CH2OMe] 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 [NbCl4(dme)(2)][NbCl6]. This prediction has been confirmed by NMR identification at -60 degrees C of the complexes [MCl4(kappa(2)-dme)(kappa(1)-dme)][MCl6] [M=Nb (4a), Ta (4b)], obtained by the addition of dme to MCl5 in CDCl3. 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, OR2 (R=Me, Et) and 1,4-dioxane have been obtained selectively upon thermal reaction of MF5 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 NbF5. The activation of dme by NbF5 follows a pathway different from that previously reported for NbCl5, and an explanation based on the thermodynamic parameters calculated at the B3LYP/LANL2DZ level is given: the formation of Me2O and dioxane is the most favored reaction between NbF5 and dme (Delta G(r)degrees = -15.16 kcal . mol(Nb)(-1)), while the formation of MeCl and dioxane is the most favored reaction in the case of NbCl5/dme (Delta G(r)degrees = -53.13 kcal . mol(Nb)(-1)).