ULTRAFAST STUDIES OF METAL-METAL BOND-CLEAVAGE IN FE-3(CO)(12) IN SOLUTION

The ultrafast dynamics following metal-metal bond breaking in Fe-3(CO) (12) have been investigated. Following excitation at 590 nm, the groun d state Fe-3(CO)(12) is converted to a coordinatively unsaturated isom er with bridging COs, Fe-3(CO)(12)(unsat.), within the instrument resp onse of 1.5 ps. The unsaturated isomer then returns to the ground stat e with an exponential lime constant of 150 ps. This rate of isomerizat ion back to the ground state implies a barrier of 4.3 kcal mol(-1). Th is is in agreement with the results of Cotton and Hunter who put an up per limit of 6 kcal mol(-1) on the isomerization of Fe-3(CO)(12) from an all terminal structure to a bridging structure. A portion (20%) of the excited molecules do not return to the ground state on the time sc ale of this experiment (< 1 ns). This is attributed to fragmentation o f Fe-3(CO)(12) to mono- and dinuclear species. Faster disappearance of the Fe-3(CO)(12)(unsat.) at longer wavelengths indicates that fragmen tation is promoted by excess vibrational energy.