HIGH-PRESSURE NMR KINETICS .7. HIGH-PRESSURE NMR EVIDENCE FOR THE DISSOCIATIVE-ASSOCIATIVE CROSSOVER IN THE LIGAND SUBSTITUTION MECHANISM OF OCTAHEDRAL ADDUCTS OF NIOBIUM(V) AND TANTALUM(V) HALIDES

The ligand-exchange reaction MX5-L+L*⇄MX5-L*+L (M=Nb, Ta; X=Cl, Br) has been studied in CH2Cl2 and CHCl3 as a function of pressure by using 1H FT NMR. Positive volumes of activation ranging from+15.2±1.7 to+30.5±0.8 cm3 mol-1 have been determined for dissociative (D) reactions when L=Me2O, MeCN, Me3CCN, (MeO)Cl2PO, and (Me2N)3PS. Negative volumes of activation ranging from -10.7±0.8 to -19.8±0.9 cm3 mol-1 have been determined for associative-interchange (Ia) reactions when L=Me2S, Me2Se, Me2Te. The ΔV*solv can be neglected, and a model for the estimation of ΔV*0 (volume of activation extrapolated to zero pressure) is used for the D mechanism. It satisfactorily accounts for the changes in ligand, halide, and metal sizes. For the Ia reactions the ΔV*0 is difficult to predict quantitatively due to two opposing effects: the negative and positive contributions due to association and steric expansion in the highly crowded seven-coordinate transition stat. © 1979, American Chemical Society. All rights reserved.