Abstract
Crystal packing energy calculations are applied to the [Fe(PM-L)2(NCS)2] family of spin crossover (SCO) complexes (PM-L = 4-substituted derivatives of the N-(2-pyridylmethylene)-4-aminobiphenyl ligand) with the aim of relating quantitatively the cooperativity of observed SCO transitions to intermolecular interactions in the crystal structures. This approach reveals a linear variation of the transition abruptness with the sum of the magnitudes of the interaction energy changes within the first molecular coordination sphere in the crystal structure. Abrupt transitions are associated with the presence of significant stabilising and destabilising changes in intermolecular interaction energies. While the numerical trend established for the PM-L family does not directly extend to other classes of SCO complex in which the intermolecular interactions may be very different, a plot of transition abruptness against the range of interaction energy changes normalised by the largest change shows a clustering of complexes with similar transition abruptness. The changes in intermolecular interactions are conveniently visualised using energy difference frameworks, which illustrate the cooperativity pathways of an SCO transition.
Highlights
In octahedral complexes in which the metal has a d4–d7 electronic con guration, the occupation of eg and t2g orbitals is directed by the nature of the ligand eld
The crystal structures in the [Fe(PM-L)2(NCS)2] family can be described in terms of the formation of layers in which the molecules are arranged so that a molecular axis drawn between the metal atom and the molecular centroid is parallel to the layer (ESI, Section S3†).[19,38,49,59]
The study has concentrated on the PM-L family of Fe(2+) complexes
Summary
In octahedral complexes in which the metal has a d4–d7 electronic con guration, the occupation of eg and t2g orbitals is directed by the nature of the ligand eld. Weak eld ligands result in a small energy difference between t2g and eg orbitals (DO), while strong eld ligands lead to a larger DO This leads to two possible electron distributions dependent on the size of DO relative to the spin pairing energy (EP). In spin crossover (SCO) complexes, the values of DO and Ep energies are similar, allowing a complex to exist as either HS or LS dependent on the amount of energy applied to a system in the form of temperature, pressure, or light. Such complexes can be reversibly switched between spin states, resulting in different
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