Experimental verification of the mechanism of metallotropic migrations in cyclooctatetraeneosmiumtricarbonyl (3) by means of 2D EXSY NMR spectroscopy confirmed the mechanism of [1,2]-Os shifts with low activation barrier (E-A = 5.9 +/- 0.2 kcal mol(-1), ln A = 32 +/- 1). Transition-state structure for this rearrangement obtained at the B3LYP/SDD level of theory testifies for the activation energy of 6.5 kcal mol(-1) and supports well the selective [1,2]-Os shifts observed for 3.