route) to trigonal prismatic (in the dihydroxo path). It should be added that the
overall rate-determining step is however the O–O bond breaking in the first phase.
6 Conclusions and Perspectives
Spin states are shown to play a vital role in many organometallic and bioinorganic
chemistry reactions and need a combined effort of spectroscopy, theory, synthesis,
and catalysis in order to understand in great detail how the transition-metal complexes are able to perform their catalytic activity. This is in particular true for highly
reactive complexes (e.g., putative Fe
V
(O) species), for which unambiguous proof of
their existence and character may be difficult to obtain. Computational chemistry
should (and does) provide useful insights since it allows for direct comparison with
experiment through, e.g., spectroscopy, but has not yet matured to the level where a
foolproof methodology has been provided that withstands scrutiny by experimentalists and theorists alike. Especially in cases where experimental proof is scarce, and
computational chemistry provides puzzling answers (such as an O–O distance of
2.1 Å, which may hold one electron in it [186, 187] or otherwise be understood as a
transition-state structure), there is no straightforward path leading toward clear
understanding, only toward continued validation, confirmation, prediction, crossvalidation, and agony.
Although clear advances have been made in theory, with spin-state consistent
DFAs (OPBE, SSB-D, S12g), DMRG-PT2, CASPT2/CC, stochastic CASSCF, local
and DLPNO coupled cluster, benchmarks on a variety of systems of interest to the
organometallic and bioinorganic communities have shown that there is not yet a
practical tool that is able to provide results consistent with full CI and/or experiment
for the transition-metal complexes. Moreover, because of the system sizes and the
unfavorable scaling of wavefunction theory, such advances are not foreseen in the
immediate future (coming years); however, with the advent of quantum computing
and deep learning, a decade from now, the picture may have changed completely.
This chapter focused on the chemistry taking place in the catalysis, with a
molecular point of view where a 2D drawing should be able to explain the chemistry.
However, naturally, molecules and transition-metal complexes are not static at all,
and (explicit) solvent effects and molecular dynamics simulations should be taken
into account as well (as described in other chapters). Nevertheless, adding these
effects would make the complex view on transition-metal catalysis even more
complicated and therefore was ignored here.
The analysis of the chemical aspect of calculations, and validation of these with
experiments, is playing an ever larger role in transition-metal chemistry research
nowadays. Within Europe this was no doubt stimulated by the very successful COST
Actions on transition-metal chemistry in recent years (CM1003, CM1205,
CM1305).
Dealing with Spin States in Computational Organometallic Catalysis
215
overall rate-determining step is however the O–O bond breaking in the first phase.
6 Conclusions and Perspectives
Spin states are shown to play a vital role in many organometallic and bioinorganic
chemistry reactions and need a combined effort of spectroscopy, theory, synthesis,
and catalysis in order to understand in great detail how the transition-metal complexes are able to perform their catalytic activity. This is in particular true for highly
reactive complexes (e.g., putative Fe
V
(O) species), for which unambiguous proof of
their existence and character may be difficult to obtain. Computational chemistry
should (and does) provide useful insights since it allows for direct comparison with
experiment through, e.g., spectroscopy, but has not yet matured to the level where a
foolproof methodology has been provided that withstands scrutiny by experimentalists and theorists alike. Especially in cases where experimental proof is scarce, and
computational chemistry provides puzzling answers (such as an O–O distance of
2.1 Å, which may hold one electron in it [186, 187] or otherwise be understood as a
transition-state structure), there is no straightforward path leading toward clear
understanding, only toward continued validation, confirmation, prediction, crossvalidation, and agony.
Although clear advances have been made in theory, with spin-state consistent
DFAs (OPBE, SSB-D, S12g), DMRG-PT2, CASPT2/CC, stochastic CASSCF, local
and DLPNO coupled cluster, benchmarks on a variety of systems of interest to the
organometallic and bioinorganic communities have shown that there is not yet a
practical tool that is able to provide results consistent with full CI and/or experiment
for the transition-metal complexes. Moreover, because of the system sizes and the
unfavorable scaling of wavefunction theory, such advances are not foreseen in the
immediate future (coming years); however, with the advent of quantum computing
and deep learning, a decade from now, the picture may have changed completely.
This chapter focused on the chemistry taking place in the catalysis, with a
molecular point of view where a 2D drawing should be able to explain the chemistry.
However, naturally, molecules and transition-metal complexes are not static at all,
and (explicit) solvent effects and molecular dynamics simulations should be taken
into account as well (as described in other chapters). Nevertheless, adding these
effects would make the complex view on transition-metal catalysis even more
complicated and therefore was ignored here.
The analysis of the chemical aspect of calculations, and validation of these with
experiments, is playing an ever larger role in transition-metal chemistry research
nowadays. Within Europe this was no doubt stimulated by the very successful COST
Actions on transition-metal chemistry in recent years (CM1003, CM1205,
CM1305).
Dealing with Spin States in Computational Organometallic Catalysis
215