10
K. P. Kepp
ligands than for weak-field ligands. The largest structural effects are, interestingly,
seen for Mn(II) and Fe(II). A change of 0.2 Å or 0.3 Å is considerable; it occurs on
average for all six bonds in a six-coordinate complex. For Fe(II) SCO systems with
N-donor ligands, one can expect a typical average increase in Fe–N bond length of
~0.2 Å, as shown in black color for Fe(II) in Fig. 3b [52].
As discussed below, metal ions with large structural relaxation upon SCO are
expected to also show more abrupt transitions with hysteresis. Accordingly, DFT as
summarized in Fig. 3b largely explains why Fe(II) SCO systems commonly display
high hysteresis but also predicts that Mn(II) systems should have similar or even larger
ability to do so, of course under a modulating influence of other effects outside the
first coordination sphere.
Since the geometric and environmental effects required to cause SCO can be
subtle, SCO may be induced by adsorption of one molecular system to another, as
the adsorbed state affects the molecular environment of the SCO system. The classic
example is the host–guest systems [82], such as that of Halder and Kepert et al.
[83] Supramolecular cages with SCO properties constitute one class of systems [84],
whereas a recent example of “on-surface” SCO is that of Kumar et al. [85] Hemes,
which are well known to change spin state upon changes in coordination environment,
also seem to undergo on-surface SCO upon relatively weak adsorption to, e.g., gold
surfaces [86], although this finding needs stronger experimental bearing.
2.7 The Nature of the SCO Transition
The actual transition from one spin state to the other can occur either gradually,
or abruptly, and be subject to small or large hysteresis, as shown in Fig. 1. It is
also possible, although not shown in Fig. 1, to have a multistep transition, and twostep processes have recently been studied by DFT [27]. Such multistep processes
typically arise from a heterogeneous SCO sample, i.e., the presence of two or more
the individual and distinct sites undergoing SCO, or possibly from restructuring of
the system (including solvent) near the transition temperature.
The SCO transition curve has a sigmoidal form characteristic of a cooperative
process. The cooperativity can be partly due to magnetic alignment as seen in an Ising
model, and partly to structural phase transitions occurring locally: If the molecules
interact closely, the intermolecular interactions produce free energy minima distinct
to the larger HS and smaller LS volume. Accordingly, the conversion into a given
spin state of one molecule makes it more favorable for neighbor molecules to attain
the same spin state [27]. The extent of cooperativity, and accordingly the abruptness
of the transition, thus depends greatly on the surroundings of the single molecule.
In a solid, each magnetic center has contact with several neighbors, and thus the
geometric spin-state preference induces a friction in the tendency to change spin state,
which is greatly influenced by intermolecular interactions. In a solvent, cooperative
interactions can be modulated by the presence of counter ions and solvent molecules,
and the solvent can separate the SCO solutes so well that the spin transition becomes
K. P. Kepp
ligands than for weak-field ligands. The largest structural effects are, interestingly,
seen for Mn(II) and Fe(II). A change of 0.2 Å or 0.3 Å is considerable; it occurs on
average for all six bonds in a six-coordinate complex. For Fe(II) SCO systems with
N-donor ligands, one can expect a typical average increase in Fe–N bond length of
~0.2 Å, as shown in black color for Fe(II) in Fig. 3b [52].
As discussed below, metal ions with large structural relaxation upon SCO are
expected to also show more abrupt transitions with hysteresis. Accordingly, DFT as
summarized in Fig. 3b largely explains why Fe(II) SCO systems commonly display
high hysteresis but also predicts that Mn(II) systems should have similar or even larger
ability to do so, of course under a modulating influence of other effects outside the
first coordination sphere.
Since the geometric and environmental effects required to cause SCO can be
subtle, SCO may be induced by adsorption of one molecular system to another, as
the adsorbed state affects the molecular environment of the SCO system. The classic
example is the host–guest systems [82], such as that of Halder and Kepert et al.
[83] Supramolecular cages with SCO properties constitute one class of systems [84],
whereas a recent example of “on-surface” SCO is that of Kumar et al. [85] Hemes,
which are well known to change spin state upon changes in coordination environment,
also seem to undergo on-surface SCO upon relatively weak adsorption to, e.g., gold
surfaces [86], although this finding needs stronger experimental bearing.
2.7 The Nature of the SCO Transition
The actual transition from one spin state to the other can occur either gradually,
or abruptly, and be subject to small or large hysteresis, as shown in Fig. 1. It is
also possible, although not shown in Fig. 1, to have a multistep transition, and twostep processes have recently been studied by DFT [27]. Such multistep processes
typically arise from a heterogeneous SCO sample, i.e., the presence of two or more
the individual and distinct sites undergoing SCO, or possibly from restructuring of
the system (including solvent) near the transition temperature.
The SCO transition curve has a sigmoidal form characteristic of a cooperative
process. The cooperativity can be partly due to magnetic alignment as seen in an Ising
model, and partly to structural phase transitions occurring locally: If the molecules
interact closely, the intermolecular interactions produce free energy minima distinct
to the larger HS and smaller LS volume. Accordingly, the conversion into a given
spin state of one molecule makes it more favorable for neighbor molecules to attain
the same spin state [27]. The extent of cooperativity, and accordingly the abruptness
of the transition, thus depends greatly on the surroundings of the single molecule.
In a solid, each magnetic center has contact with several neighbors, and thus the
geometric spin-state preference induces a friction in the tendency to change spin state,
which is greatly influenced by intermolecular interactions. In a solvent, cooperative
interactions can be modulated by the presence of counter ions and solvent molecules,
and the solvent can separate the SCO solutes so well that the spin transition becomes