The Electronic Determinants of Spin Crossover Described …
5
T ½ H SCO /S SCO , which can, as discussed below, be obtained from quantumchemical computations at variable accuracy.
2.2 The Spectrochemical Series
In text books, the spectrochemical series [38, 46, 47] is traditionally used to estimate
the preference for either HS or LS in a given coordination complex. This series is
based on spectroscopic measurements of the absorption peak for the d–d transitions
of cobalt(III) complexes and ranks common coordinating ligands according to o
in an octahedral field. A rough rule-of-thumb order is:
I
−
< Br
−
< Cl
−
< F
−
< S σ-donor < O σ-donor < N σ-donor < π-acceptors
Although sometimes forgotten, this series is mainly based on Co(III), and the
series, more specifically, the ligand field stabilization energy calculated from o ,
estimates the relative preference for HS versus LS if this preference was only due to
electronic energy as measured by the absorption spectroscopy. The series, moreover,
reflects a non-thermal electronic excitation, whereas the SCO systems of interest
involve the thermal excitation of typically two electrons. Although widely used and
displayed in textbooks, the estimates based on the spectrochemical series thus miss
vibrational relaxation, spin pairing, and entropic effects, and they do not necessarily
accurately convey the spin-state preference in a real chemical system at thermal equilibrium. Still, because the energy described by absorption maximum is a large part
of the typical thermodynamic preference between the spin states, it is often accurate
when applied to trend predictions, which largely explains its success [48–51].
2.3 The Thermochemical Spin Series
The real thermochemical spin-state preference and thus the adequate tool for rationalizing and predicting SCO can be argued to be a “thermochemical series” of spin-state
propensity [52]. This series takes into account ground-state geometry relaxation of
the HS state and entropy terms that also favor HS [31, 52, 53]. This series is straightforward achievable from DFT computations of the fully relaxed ground-state geometry of the HS state, which corrects the spectrochemical series based on electronic
transitions in which the HS state features as an excited state. Furthermore, DFT can
compute the vibrational entropy term with decent accuracy [36, 52, 54] so that the
real preference as given by the free energy in (2) is honored. The series is importantly independent on the functional used [52], because the trend of interest involves
cancellation of the major systematic errors in DFT that are discussed below.
5
T ½ H SCO /S SCO , which can, as discussed below, be obtained from quantumchemical computations at variable accuracy.
2.2 The Spectrochemical Series
In text books, the spectrochemical series [38, 46, 47] is traditionally used to estimate
the preference for either HS or LS in a given coordination complex. This series is
based on spectroscopic measurements of the absorption peak for the d–d transitions
of cobalt(III) complexes and ranks common coordinating ligands according to o
in an octahedral field. A rough rule-of-thumb order is:
I
−
< Br
−
< Cl
−
< F
−
< S σ-donor < O σ-donor < N σ-donor < π-acceptors
Although sometimes forgotten, this series is mainly based on Co(III), and the
series, more specifically, the ligand field stabilization energy calculated from o ,
estimates the relative preference for HS versus LS if this preference was only due to
electronic energy as measured by the absorption spectroscopy. The series, moreover,
reflects a non-thermal electronic excitation, whereas the SCO systems of interest
involve the thermal excitation of typically two electrons. Although widely used and
displayed in textbooks, the estimates based on the spectrochemical series thus miss
vibrational relaxation, spin pairing, and entropic effects, and they do not necessarily
accurately convey the spin-state preference in a real chemical system at thermal equilibrium. Still, because the energy described by absorption maximum is a large part
of the typical thermodynamic preference between the spin states, it is often accurate
when applied to trend predictions, which largely explains its success [48–51].
2.3 The Thermochemical Spin Series
The real thermochemical spin-state preference and thus the adequate tool for rationalizing and predicting SCO can be argued to be a “thermochemical series” of spin-state
propensity [52]. This series takes into account ground-state geometry relaxation of
the HS state and entropy terms that also favor HS [31, 52, 53]. This series is straightforward achievable from DFT computations of the fully relaxed ground-state geometry of the HS state, which corrects the spectrochemical series based on electronic
transitions in which the HS state features as an excited state. Furthermore, DFT can
compute the vibrational entropy term with decent accuracy [36, 52, 54] so that the
real preference as given by the free energy in (2) is honored. The series is importantly independent on the functional used [52], because the trend of interest involves
cancellation of the major systematic errors in DFT that are discussed below.