Density functional theory calculations are per formed to explore both concerted chairlike and boatlike as well as stepwise mechanisms of the Cope rearrangement of two hypothetical metalladienes. An osma-1,5-hexadiene is designed by substituting CH2 in 1,5-hexadiene by its isolobal analogue, 16-electron Os(PH3)(4) The energy of activation corresponding to the rearrangement of osma-1,5-hexachene involving the chairlike saddle point is computed as 37.4 kcal/mol, 3 9 kcal/mol above the energy barrier of the parent 1,5-hexadiene calculated with the same method and basis set, and is 4.5 kcal/mol below that of the boatlike pathway. In another isolobal replacement, the CH in 1,5-hexadiene is substituted by a 15-electron Re(PH3)(3) fragment. Now the chairlike rearrangement of the rhenia-1,5-hexachene has an E-a value of 23.0 kcal/mol, 10.8 kcal/mol less than the energy barrier of the parent I,5-hexadiene calculated at the same level of theory. The ring inversion of the chair and osma-chair diradical intermediates of the stepwise reaction pathway is also examined and is found in both cases to proceed through a very flat potential energy surface involving twist intermediates