barrier was lowest for the second triplet (S B ¼ 1). This lowering of the barrier
resulted directly from the electron pushing that is taking place in the reaction: for the
singlet state and triplet S A , the number of exchange interactions at the metal
increases only slightly; however, for triplet S B , the reaction along the triplet state
in fact involves a quadruplet at the metal which is coupled anti-ferromagnetically
with the substrate radical. This quartet state at the metal has more exchange
interactions at the metal and hence is much favored for the TS structure. At the
same time, following this spin state also leads to a more favored product, which
according to the Bell-Evans-Polanyi principle will lead to a lowering of the barrier
[179]. And of course, the possibility of reaching the quartet state at the TS needs that
the acceptor orbital is sufficiently low in energy for this to occur; hence, the ligand
field plays an important role as well [180]. These effects of spin state and exchange,
ligand field, and the reaction energy driving force are therefore not mutually
exclusive but probably different interpretations of the same manifestation which
shows that spin-state switching can and does occur.
5 Catalysis
The balance between reactivity and stability is a subtle one, one cannot have both at
the same time. Maximizing reactivity means loss of characterizability, and in
contrast enhanced stability reduces reactivity. In order to improve catalysts, or
even understand how they work, it is therefore necessary to strike the right balance.
This is not always easy, and in particular with highly reactive species, the possibilities to fully characterize them, or even what the actual active species looks like, are
sometimes remote. Often, only a precursor is fully characterized, with indirect
evidence through, e.g., isotope effects on Raman spectra giving insights on what
the active species might look like. Useful evidence is more and more being provided
by computational chemistry, which allows to study the different possibilities of what
the active species might look like, explore their stability and spectroscopy, and
compare these with experimental data. This is not an easy task and often needs a
long-term investment in mutual understanding between experimentalists and theoreticians. Doing a blind test, whereby the experimentalist provides ideas of what the
complexes might look like, but does not provide spectroscopic characterization, has
been shown to provide mutual trust if the theoretician is able to reliably predict what
the spectroscopy would look like. Independent validation of the computational
methods used on known properties helps in understanding catalysis, where only
the unknowns are known.
Dealing with Spin States in Computational Organometallic Catalysis
209
resulted directly from the electron pushing that is taking place in the reaction: for the
singlet state and triplet S A , the number of exchange interactions at the metal
increases only slightly; however, for triplet S B , the reaction along the triplet state
in fact involves a quadruplet at the metal which is coupled anti-ferromagnetically
with the substrate radical. This quartet state at the metal has more exchange
interactions at the metal and hence is much favored for the TS structure. At the
same time, following this spin state also leads to a more favored product, which
according to the Bell-Evans-Polanyi principle will lead to a lowering of the barrier
[179]. And of course, the possibility of reaching the quartet state at the TS needs that
the acceptor orbital is sufficiently low in energy for this to occur; hence, the ligand
field plays an important role as well [180]. These effects of spin state and exchange,
ligand field, and the reaction energy driving force are therefore not mutually
exclusive but probably different interpretations of the same manifestation which
shows that spin-state switching can and does occur.
5 Catalysis
The balance between reactivity and stability is a subtle one, one cannot have both at
the same time. Maximizing reactivity means loss of characterizability, and in
contrast enhanced stability reduces reactivity. In order to improve catalysts, or
even understand how they work, it is therefore necessary to strike the right balance.
This is not always easy, and in particular with highly reactive species, the possibilities to fully characterize them, or even what the actual active species looks like, are
sometimes remote. Often, only a precursor is fully characterized, with indirect
evidence through, e.g., isotope effects on Raman spectra giving insights on what
the active species might look like. Useful evidence is more and more being provided
by computational chemistry, which allows to study the different possibilities of what
the active species might look like, explore their stability and spectroscopy, and
compare these with experimental data. This is not an easy task and often needs a
long-term investment in mutual understanding between experimentalists and theoreticians. Doing a blind test, whereby the experimentalist provides ideas of what the
complexes might look like, but does not provide spectroscopic characterization, has
been shown to provide mutual trust if the theoretician is able to reliably predict what
the spectroscopy would look like. Independent validation of the computational
methods used on known properties helps in understanding catalysis, where only
the unknowns are known.
Dealing with Spin States in Computational Organometallic Catalysis
209