8
K. P. Kepp
Homoleptic coordination complexes for a range of different simple monodentate ligands with these d-electron configurations for Mn(II), Mn(III), Fe(II), Fe(III),
Co(II) and Co(III) have been studied by DFT [52] and can help to suggest how
such an “ideal” symmetric homoleptic system may be realized. Using the functionals known to be more accurate for the purpose (e.g., B3LYP* [60, 61] or TPSSh
[62], e.g., as shown previously [63, 64]), one sees that some combinations of metal
ions and ligands can bring a homoleptic complex very close to SCO. Co(III) has the
strongest LS preference of all first-row d-block metal ions up to the III oxidation state
as it has maximal ligand field stabilization energy in LS due to its t
6
2g configuration
and has higher charge than the iso-electronic Fe(II) LS. Thus, Co(III) SCO systems
are rare and require very weak total ligand field strengths to reach SCO; the 6O
coordination structures by Kläui and associates are notable in this regard [65–67].
Examples of Co(III) SCO may also exist in mixed-metal oxides [68], and a new
example of a bistrimetaphosphate Co(III) complex with probable SCO properties
was reported recently (Fig. 2e) [69]. In contrast, Co(II) SCO systems are relatively
common and often feature 6N-coordination structures, e.g., bis-terpyridines (Fig. 2d)
[70], consistent with the discussion above [71–73].
This raises an interesting and important question, namely how far above the LS
state is the HS state of Co(III)(aq)? All other M(aq) systems of the first row of the
d-block are HS due to water’s weak ligand field, and their spin states were recently
studied by CASPT2 [74]. Standing out alone, Co(III) is known to be LS under typical conditions studied (which are very acidic, because Co(III)(aq) readily undergoes
reductive hydrolysis to Co(II) at neutral pH). Moreover, [CoF 6 ]
3− is known to be HS,
and this makes the range to HS very small since H 2 O is close-by in the spectrochemical series [38]. The Co(III)(aq) is assumed (and has been shown in older data) to
feature LS, but this produces several anomalies such as a much faster self-exchange
electron transfer rate and faster ligand substitution than expected. Recent DFT computations [75] of the relative self-exchange rates of hydrated transition metal ions
accurately recover the experimental 10
5 anomaly of Co(II)/Co(III) when plotting
the trend in reorganization energies versus experimental rate constants and using LS
Co(III) as commonly assumed. When doing the same correlation for HS Co(III)(aq),
the anomaly disappears almost completely. DFT can also be used to correct previous
spectroscopic estimates of the HS–LS gap with entropy and vibrational geometry
relaxation showing that Co(III)(aq) is very close to SCO. In conclusion, this analysis converges on the view that the HS state is probably active during much of the
chemistry of the hydrated Co(III), in stark contrast to text book consensus based on
early NMR and absorption spectroscopy measurements in strong acid [76–78], but
explaining the anomalous high ligand substitution and electron transfer capabilities
of Co(III)(aq) [79, 80]. Indeed, several SCO systems of Co(III) with O-donor ligands
are known [66, 68]. Future exploration of the SCO properties of Co(III) in water-like
coordination environments should therefore be of interest.
K. P. Kepp
Homoleptic coordination complexes for a range of different simple monodentate ligands with these d-electron configurations for Mn(II), Mn(III), Fe(II), Fe(III),
Co(II) and Co(III) have been studied by DFT [52] and can help to suggest how
such an “ideal” symmetric homoleptic system may be realized. Using the functionals known to be more accurate for the purpose (e.g., B3LYP* [60, 61] or TPSSh
[62], e.g., as shown previously [63, 64]), one sees that some combinations of metal
ions and ligands can bring a homoleptic complex very close to SCO. Co(III) has the
strongest LS preference of all first-row d-block metal ions up to the III oxidation state
as it has maximal ligand field stabilization energy in LS due to its t
6
2g configuration
and has higher charge than the iso-electronic Fe(II) LS. Thus, Co(III) SCO systems
are rare and require very weak total ligand field strengths to reach SCO; the 6O
coordination structures by Kläui and associates are notable in this regard [65–67].
Examples of Co(III) SCO may also exist in mixed-metal oxides [68], and a new
example of a bistrimetaphosphate Co(III) complex with probable SCO properties
was reported recently (Fig. 2e) [69]. In contrast, Co(II) SCO systems are relatively
common and often feature 6N-coordination structures, e.g., bis-terpyridines (Fig. 2d)
[70], consistent with the discussion above [71–73].
This raises an interesting and important question, namely how far above the LS
state is the HS state of Co(III)(aq)? All other M(aq) systems of the first row of the
d-block are HS due to water’s weak ligand field, and their spin states were recently
studied by CASPT2 [74]. Standing out alone, Co(III) is known to be LS under typical conditions studied (which are very acidic, because Co(III)(aq) readily undergoes
reductive hydrolysis to Co(II) at neutral pH). Moreover, [CoF 6 ]
3− is known to be HS,
and this makes the range to HS very small since H 2 O is close-by in the spectrochemical series [38]. The Co(III)(aq) is assumed (and has been shown in older data) to
feature LS, but this produces several anomalies such as a much faster self-exchange
electron transfer rate and faster ligand substitution than expected. Recent DFT computations [75] of the relative self-exchange rates of hydrated transition metal ions
accurately recover the experimental 10
5 anomaly of Co(II)/Co(III) when plotting
the trend in reorganization energies versus experimental rate constants and using LS
Co(III) as commonly assumed. When doing the same correlation for HS Co(III)(aq),
the anomaly disappears almost completely. DFT can also be used to correct previous
spectroscopic estimates of the HS–LS gap with entropy and vibrational geometry
relaxation showing that Co(III)(aq) is very close to SCO. In conclusion, this analysis converges on the view that the HS state is probably active during much of the
chemistry of the hydrated Co(III), in stark contrast to text book consensus based on
early NMR and absorption spectroscopy measurements in strong acid [76–78], but
explaining the anomalous high ligand substitution and electron transfer capabilities
of Co(III)(aq) [79, 80]. Indeed, several SCO systems of Co(III) with O-donor ligands
are known [66, 68]. Future exploration of the SCO properties of Co(III) in water-like
coordination environments should therefore be of interest.