SYNTHESES, STRUCTURES AND FLUXIONAL BEHAVIOR OF ETA(4)-1-DI-TERT-BUTYLBUTA-1,3-DIENE COMPLEXES OF IRON AND RUTHENIUM

The complexes [M(CO)(3)(eta(4)-diene)] (M = Fe or Ru; diene = 1,4-di-t ert-butylbuta-1,3-diene) have been synthesised and their crystal struc tures and solution fluxional behaviour determined. While they adopt sq uare-pyramidal geometries in the solid state, the carbonyl ligands sho w fluxional behaviour in solution (E(4) = 44.6, Delta H-double dagger = 42.2 kJ mol(-1), Delta S-double dagger = -29.8 J K-1 mol(-1) where M = Fe; 52.6, 50.0 kJ mol(-1) and -30.5 J K-1 mol(-1) where M = Ru). Th ese activation parameters are the largest reported for simple [M(CO)(3 )(diene)] complexes. An inverse correlation between E(a) and the diene 'bite' angle theta(1) for a range of [M(CO)(3)(diene)] complexes is d escribed and used to account for variations in activation barriers.