[en] The reactions of ammonium trifluoroacetate and lithium hydride, of ammonium trifluoroacetate and lithium deuteride, and of tetradeuteroammonium trifluoroacetate and lithium hydride were studied in an attempt to prove experimentally nucleophilic displacement through the formation of pentacoordinated NH₅. Significant isotopic scramblings were observed in the reaction products. An analysis of possible side reactions indicated that the most reasonable explanation for the reaction products obtained is the attack of D^- on the quarternary hydrogen of NH₄⁺ in an SN2-like fashion causing exchange to occur via pentacoordinated NH₄D. The possibility of a real pentacoordinated intermediate was also considered. CNDO/2 calculations show that a nucleophilic attack on hydrogen is favored, but the alternative attack on nitrogen can not be dismissed because of the experimental data. The reaction of NF₄⁺SbF₆^- and LiF could indicate the possibility of nucleophilic attack on nitrogen. (U.S.)