STUDIES OF THE KINETICS OF THE REACTIONS OF NITRILES WITH DINITROGEN COMPLEXES OF MOLYBDENUM(0) AND TUNGSTEN(0) - FACTORS AFFECTING THE RATE OF DISSOCIATIVE LOSS OF DINITROGEN

The kinetics of the substitution of nitriles for dinitrogen in trans-[M(N2)2-(dppe)2] (M = Mo, W; dppe = 1,2-bis(diphenylphosphino)ethane) and cis-[M(N2)2(PMe2Ph)4] (M = Mo, W) complexes have been studied in toluene solution at various temperatures. In the case of trans-[Mo(N2)2(dppe)2], one dinitrogen is replaced by a nitrile in toluene and the kinetic data support an Sn1 dissociative mechanism (k(PhCN) 3.6 X 10-4 s-1 at 303 K; Ea 28 kcal/mol; ΔH≠ 27 kcal/mol; ΔS≠ 15 cal/mol-deg). The analogous reaction with trans-[W(N2)2(dppe)2] was found to proceed only photochemically, with loss of one dinitrogen. The thermal activation energy for N2 dissociation from trans-[W(N2)2(dppe)2] in toluene is estimated to be 38 kcal/mol. In the cis-[M)N2)2(PMe2Ph)4] (M = Mo, W) reactions in toluene, both dinitrogen ligands are replaced by nitriles in two consecutive dissociative steps (k(Mo)/k(W) ≈ 102 for each step). © 1979.