The trihapto bonded cycloheptadienyl complexes [MoX(CO) 2 L 2 (η 3 -C 7 H 9 )] (X = Cl, L 2 = Ph 2 PCH 2 CH 2 PPh 2 (dppe) 1, L 2 = Ph 2 PCH 2 PPh 2 (dppm) 2; X = NCO, L 2 = dppe 3) have been synthesised by reaction of [Mo(CO) 2 L 2 (η 5 -C 7 H 9 )][BF 4 ] with LiCl or KNCO in acetone. Spectroscopic investigations indicate that 1-3 possess an asymmetric structure with one end of the bidentate phosphine ligand located trans to the C 7 H 9 ring. In solution in acetonitrile [Mo(CO) 2 L 2 (η 5 -C 7 H 9 )] + may exist in equilibrium with the adducts [Mo(CO) 2 (NCMe)L 2 (η 3 -C 7 H 9 )] + with the composition of any mixture temperature and L 2 dependent. Variable temperature 3 1 P-NMR studies on [Mo(CO) 2 (dppe)(η 5 -C 7 H 9 )][BF 4 ] in CD 3 CN reveal that the proportion of [Mo(CO) 2 (NCMe)(dppe)(η 3 -C 7 H 9 )] + present is increased as the temperature is lowered. The new complexes [Mo(CO) 2 L 2 (η 5 -C 7 H 9 )][BF 4 ] (L 2 = Me 2 PCH 2 CH 2 PMe 2 (dmpe) 4 or 1,1,1-tris(diphenylphosphinomethyl)ethane (triphos) 5) have been prepared, the latter as an isomer mixture. Complex 4 remains unchanged in acetonitrile but infrared and variable temperature 3 1 P-NMR studies on 5 in acetonitrile establish the partial formation of [Mo(CO) 2 (NCMe)(triphos)(η 3 -C 7 H 9 )] + .