We have shown in this paper that the photochemical reaction
dynamics of Fe(CO)5 and CpCo(CO)2 are due to the formation
of triplet-state intermediates. The results are in contrast to the
singlet intermediate dynamics seen with CpRh(CO)2. Due to a
change in the reaction mechanism, the overall reactivity of the
triplet species is much greater than that of similar singlet species.
A general mechanism explaining the reactivity of triplet
organometallics may now be proposed. Coordinatively unsaturated
singlet organometallics will tend to associate to most
solvents, even alkanes. These interactions will likely hinder
further reactivity with a stronger coupling site as these alkylsolvated
species may exist for milliseconds in solution. The
triplet species are free to react at a faster rate as they do not
coordinate to the same degree with a weak coupling alkyl site.
The time scale of the triplet metal complex reactivity can be
understood in terms of spin-orbit coupling between the singlet
and triplet surfaces as well as the classical barrier to bondactivation.
This mechanistic change explains the differing reactivity of triplet and singlet coordinatively unsaturated
organometallic complexes in triethylsilane and can likely be
extended to other chemical systems.