Transition-metal nitrosyl compounds. Part V. Nuclear magnetic resonance temperature-dependence studies on mono- and di-nitrosyl-(dithiocarbamato)-complexes of molybdenum and tungsten

Abstract

At 25°C the ¹H n.m.r. spectrum of [M(NO)₂(S₂CNMe₂)₂](M = Mo or W) is consistent with a rigid cis-octahedral geometry. At higher temperatures (80–140°C) a rapid interconversion of the two types of N-methyl group takes place. The 25°C ¹H n.m.r. spectrum of [Mo(NO)(S₂CNMe₂)₃] may be interpreted in terms of a pentagonal bipyramidal structure. At elevated temperatures (60–128°C) interconversion of both the N-methyl groups and the two types of dithiocarbamato-ligands occurs. Possible mechanisms for these interconversions or rearrangements are discussed. The most favourable mechanism for both compounds is probably that involving partial dissociation of a dithiocarbamato-ligand, together with a simultaneous rotation of the N-methyl groups about the C–N bond. ΔG* is about 21 kcal mole^–1 for the dinitrosyl complexes, and about 18 kcal mole^–1 for the mononitrosyl complex.