The chemistry of cyclopentadienyl nitrosyl and related complexes of molybdenum. Part 7. The fluxional behaviour of bis(cyclopentadienyl)dithiocarbamatomolybdenum complexes, and the application of Forsén–Hoffman spin-saturation method to the nuclear magnetic resonance spectra of a five-spin system

Abstract

The synthesis, and ¹H and ¹³C n. m. r. spectral behaviour over a temperature range, of [Mo(η⁵-C₅H₅)(σ-C₅H₅)(NO)(S₂CNR₂)]⁺(R = Me or Buⁿ) is described. The species undergo three independent fluxional processes: (i) 1,2 shifts of the σ-C₅H₅ ring; (ii)σ-η⁵ ring interchange; and (iii) C–N rotation of the S₂C–NR₂ group. Using Forsén-Hoffman spin-saturation techniques, it has been shown conclusively that the rate of 1,2 shifts of the σ ring is significantly greater than that of 1,3 shifts. The mechanisms of ring hapticity interchange, their relation to the properties of [Mo(C₅H₅)₂(NO)X](X = unidentate ligand), and the synthetic implications of the presence of σ- or η³-C₅H₅ rings in such complexes are discussed.