The chemistry of niobium and tantalum dithiocarbamato-complexes. Part 5. The kinetics and mechanism of the hydrazine-forming reactions between [{M(S2CNEt2)3}2(µ-N2)](M = Nb or Ta) and acid: rate-limiting protonation

The kinetics of the reactions between [{M(S2CNEt2)3}2(µ-N2)] and HX (M = Nb, X = Cl or Br; M = Ta, X = Cl) to yield [M(S2CNEt2)3X2] and hydrazine have been studied in MeCN at 25.0 °C. In all cases the multistep mechanism involves rate-limiting protonation of the bridging dinitrogen atoms, and in one case (M = Ta) the monoprotonated species [{Ta(S2CNEt2)3}2(µ-N2H)]+ was detected spectrophotometrically. The cleavage of the metal–nitrogen bond occurs after the addition of at least two protons. The factors influencing the rates of protonation of bridging and end-on co-ordinated dinitrogen molecules are discussed.