Keywords: Imides / Molybdenum / Tungsten / Cyclopentadienyl ligands / RearrangementsThe new imido compounds [(η 5 -C 5 H 5 )M(NR) 2 (η 1 -C 5 H 5 )] (R = tBu: M = Mo 2a, W 2b; R = Mes: M = W 2c) have been prepared and the crystal structure of 2c has been determined. Three fluxional processes can be detected by variable temperature proton NMR spectroscopic studies of these cyclopentadienyl complexes, namely: (i) rotation of the σ-C 5 H 5 li-