The dynamic carbon-13 nuclear magnetic resonance spectroscopy of some organometallic compounds

The fluxional behavior of 1, 3, 5, 7-tetramethylcyclooctatetraene-tricarbonylchromium, (TMCOT)Cr(CO)₃, previously shown by ¹H NMR to be based on a single, reversible type of 1, 2-shift has been reexamined with ¹³C NMR. The previous conclusions are confirmed. For the cyclooctatetraenetricarbonyl metal compounds, (COT)M(CO)₃, M=Cr, Mo, or W, for which no pathway had previously been established, ¹³C NMR show conclusively that 1, 2-shifts are not the pathway and that only 1, 3-shifts or a process resulting in random shifts are admissible. While 1, 3-shifts cannot be ruled out rigorously, it is argued that a process in which the metal atom moves to a position over the midpoint of a virtually flat octagonal ring whence it can pass with equal probability to any of the eight equilibrium positions is more likely. On this basis the qualitatively different behavior of the (TMCOT)M(CO)₃ and (COT)M(COT)₃ compounds can be understood as arising from the different energies required to flatten the rings. The dynamic ¹³C NMR of several molecules containing Fe-Fe(CO)₃ moieties has been investigated. In (acenaphthylene)Fe₂(CO)₅, which has Fe(CO)₂ and Fe(CO)₃ groups connected by an Fe-Fe bond, the CO groups of FE(CO)₃ scramble among themselves, but do not exchange with those on the FE(CO)₂ group, thus providing that the localized process is important in compounds with Fe-Fe(CO)₃ type groups.