The process of iron- and ruthenium-assisted alkene isomerization is investigated with ultrafast time-resolvedinfrared spectroscopy under ambient conditions. M(CO)
4(
2-1,4-PD) (M = Ru, Fe; PD = pentadiene) wasirradiated with a 100 femtosecond pulse of 266 nm light in a solution of cyclohexane, and a single carbonylwas photodissociated. When M = Ru, experimental results indicate that both Ru(CO)
3(
4-1,4-PD) and HRu(CO)
3(
3-allyl) form within a few picoseconds, and their populations do not change significantly out to 800ps. When M = Fe, the coordinatively unsaturated triplet metal complex,
3Fe(CO)
3(
2-1,4PD), was observedto form rapidly and rearranged to HFe(CO)
3(
3-allyl) at a rate of (4.8 ± 0.2) × 10
8 s
-1. Surprisingly, thetriplet iron complex did not coordinate the free alkene on the pentadiene ligand on the ultrafast time scale.The difference in reactivity between the iron- and the ruthenium-assisted alkene isomerization is due todifferences in spin state of the coordinatively unsaturated metal center.