Composés d'addition des halogénures de niobium (V) et de tantale(V): III. Détermination de constantes de stabilité par résonance magnétique nucléaire

In view of a systematic study of the Lewis acidity of niobium (V) and tantalum (V) halides, NMR. methods for the determination of stability constants are discussed. In the case of rapid exchange chemical shifts are used to determine the relative amounts of adduct(s) and free base(s) in equilibrium. In the case of slow exchange the relative concentrations of the species are given by the peak areas.     

Composés d'addition des halogénures de niobium(V) et de tantale(V): VI. Réactions d'échange des composés du chlorure de niobium(V) avec quelques nitriles

The ligand exchange reactions NbCl₅·?N + RCN* ? NbCl₅ · RCN* + RCN are studied by NMR. spectroscopy for R = Me₃C, Me, FCH₂, CICH₂, BrCH₂, ICH₂. The reaction is of zero order in RCN and of first order in NbCl₅ · RCN and thus a dissociative mechanism is suggested for all the ligands studied. The enthalpies and entropies of activation are determined over 50° to 90° temperature ranges. There is a linear correlation between ΔG≠ and the free enthalpy of formation of NbCl₅ RCN. However this correlation is shown to hold only for series of adducts having the same donor group.     

Composés d'addition des halogénures de niobium(V) et de tantale(V) IV. Stabilité relative des composés de chlorures avec quelques nitriles

The relative stability constants of the adducts of MCl₅ (M ? Nb, Ta) with acetonitrile, halogenoacetonitriles, pivalonitrile, acrylonitrile and benzonitriles are determined in dilute solutions by NMR. methods. The stability of the adducts is controlled by inductive factors. Chemical shifts and analysis of the new compounds are reported.