6
K. P. Kepp
The resulting thermochemical series resembles the spectrochemical series, but
notably differs in several aspects. One of the most interesting differences is that Cl
−
and Br
− have similar spin-state propensity once the thermochemical, spin-pairing,
and vibrational-structural corrections are accounted for [52]. In the spectrochemical
series, they separate clearly in the halide trend. Another difference is the preference
for negatively charged versus neutral π-acceptors such as CN
− and CO, which change
place in the thermochemical series depending on the oxidation state of the metal ion.
A third difference relates to coordination isomers such as SCN
− and NCS
− that also
change relative position depending on metal oxidation state [52], of relevance to the
much studied SCO system [Fe(SCN) 2 (Phen) 2 ] and its derivatives.
The thermochemical series is quantitative and includes vibrational relaxation and
entropy terms directly [52]. Thus, one can predict that Mn(III) systems will have
relatively similar ligand preferences as Fe(II) in order to induce SCO, whereas Fe(III)
should have a slightly weaker total ligand field; Co(II) SCO systems are predicted
to be realized with stronger ligand fields than for Mn(III), Fe(III), and Fe(II), and
even stronger ligand fields are required for Mn(II) such that even CO and CN
−
become relevant. In contrast, Co(III) requries very weak ligands due to its strong LS
preference, with SCO most likely occuring between F
− and weak O-donor ligands
such as water, ethers, and alcohols [52]. Examples of SCO systems that illustrate these
preferences are shown in Fig. 2, with 6N coordination for Fe(II) (Fig. 2a), weaker
4N2O coordination for Fe(III) (Fig. 2b), tunable ligand fields for porphyrins with
Fe(II)/Fe(III) (Fig. 2c), 6N coordination for Co(II) (Fig. 2d), and correspondingly
weaker 6O ligand field for Co(III) (Fig. 2e).
2.4 The Oxidation State on the Central Metal Ion
The metal ion’s oxidation state plays a major role in defining the spin-state propensity,
with higher oxidation state favoring LS. This effect (“the spectrochemical series of the
central ions”) was originally formulated by Jørgensen on the basis of spectroscopic
data [39], but its fundamental truth is easily recovered and even quantified using
modern DFT [52]. Specifically, a difference in LS preference of Fe(II) and Fe(III)
of ~50 kJ/mol has been estimated [52]; for Co(III) versus Co(II), the difference is
even higher and can reach 100 kJ/mol [52]. This effect clearly needs to be considered
if SCO systems of variable metal oxidation states are to be developed by rational
screening.
The fact that Fe(III) favors LS more than Fe(II) is easily understood from the
stronger and shorter metal–ligand bonds formed in the higher oxidation states, which
increases the σ-donation and thus the ligand field splitting and tendency toward low
spin. Because of this, the most common coordination structure for Fe(II) SCO systems
is 6N [55], whereas for Fe(III) SCO systems, the 4N2O coordination structure is
common [6], where two nitrogen donors have been changed into weaker oxygen
donor atoms. There are many exceptions to this preference: For example, some 4N2O
iron(II) systems have been reported [56], and an Fe(II) 5N1S system has been made
K. P. Kepp
The resulting thermochemical series resembles the spectrochemical series, but
notably differs in several aspects. One of the most interesting differences is that Cl
−
and Br
− have similar spin-state propensity once the thermochemical, spin-pairing,
and vibrational-structural corrections are accounted for [52]. In the spectrochemical
series, they separate clearly in the halide trend. Another difference is the preference
for negatively charged versus neutral π-acceptors such as CN
− and CO, which change
place in the thermochemical series depending on the oxidation state of the metal ion.
A third difference relates to coordination isomers such as SCN
− and NCS
− that also
change relative position depending on metal oxidation state [52], of relevance to the
much studied SCO system [Fe(SCN) 2 (Phen) 2 ] and its derivatives.
The thermochemical series is quantitative and includes vibrational relaxation and
entropy terms directly [52]. Thus, one can predict that Mn(III) systems will have
relatively similar ligand preferences as Fe(II) in order to induce SCO, whereas Fe(III)
should have a slightly weaker total ligand field; Co(II) SCO systems are predicted
to be realized with stronger ligand fields than for Mn(III), Fe(III), and Fe(II), and
even stronger ligand fields are required for Mn(II) such that even CO and CN
−
become relevant. In contrast, Co(III) requries very weak ligands due to its strong LS
preference, with SCO most likely occuring between F
− and weak O-donor ligands
such as water, ethers, and alcohols [52]. Examples of SCO systems that illustrate these
preferences are shown in Fig. 2, with 6N coordination for Fe(II) (Fig. 2a), weaker
4N2O coordination for Fe(III) (Fig. 2b), tunable ligand fields for porphyrins with
Fe(II)/Fe(III) (Fig. 2c), 6N coordination for Co(II) (Fig. 2d), and correspondingly
weaker 6O ligand field for Co(III) (Fig. 2e).
2.4 The Oxidation State on the Central Metal Ion
The metal ion’s oxidation state plays a major role in defining the spin-state propensity,
with higher oxidation state favoring LS. This effect (“the spectrochemical series of the
central ions”) was originally formulated by Jørgensen on the basis of spectroscopic
data [39], but its fundamental truth is easily recovered and even quantified using
modern DFT [52]. Specifically, a difference in LS preference of Fe(II) and Fe(III)
of ~50 kJ/mol has been estimated [52]; for Co(III) versus Co(II), the difference is
even higher and can reach 100 kJ/mol [52]. This effect clearly needs to be considered
if SCO systems of variable metal oxidation states are to be developed by rational
screening.
The fact that Fe(III) favors LS more than Fe(II) is easily understood from the
stronger and shorter metal–ligand bonds formed in the higher oxidation states, which
increases the σ-donation and thus the ligand field splitting and tendency toward low
spin. Because of this, the most common coordination structure for Fe(II) SCO systems
is 6N [55], whereas for Fe(III) SCO systems, the 4N2O coordination structure is
common [6], where two nitrogen donors have been changed into weaker oxygen
donor atoms. There are many exceptions to this preference: For example, some 4N2O
iron(II) systems have been reported [56], and an Fe(II) 5N1S system has been made