Spiro[2,4]hepta-4,6-diene and spiro[4,4]nona-1,3-diene react with [Mo(CO)(2)Cp'(NCCH3)(2)][BF4] (Cp' = Cp, Ind; Ind = eta(5)-C9H7) to afford the corresponding diene complex [Mo(diene)(CO)(2)Cp'](+). When Cp' = Ind, the reaction proceeded forward leading to ring opening in the case of the small spiro ring. Although this and another product resulting from migration of the side arm to the carbonyl were detected when Cp' = Cp, they did not form from the diene complex. A DFT/PBE1PBE study was carried out and showed a kinetically controlled reaction pathway leading from the [Mo(diene)(CO)(2)Ind](+) to the reaction product, with an activation barrier of 21.3 kcal mol (1). The thermodynamic preferred species was the non-observed complex (insertion), and its formation required higher barriers. In the presence of Cp, all the barriers increased significantly, explaining the inertness of the initial diene complex. The interpretation of this behaviour is associated with the ease of the eta(5) -> eta(3) haptotropic rearrangement of the indenyl, which helps to lower some relevant barriers. This route is not available for the Cp analogue.