Structural Aspects of Spin Crossover. Example of the [FeIILn(NCS)2] Complexes

Structural Determination of a Short-Lived Excited Iron(II) Complex by Picosecond X-Ray Absorption Spectroscopy

“…The R Fe-N values of 0:2 A extracted from both the XANES and the EXAFS are in very good agreement with the LS-to-HS bond elongation from DFT calculations on Fe II bpy 3 2 [35], and with the values derived for other Fe(II)-based SCO complexes using time resolved [22] and static structural methods [9][10][11][12][13][14][15][16][17][18]20,21]. That the bond elongation is nearly the same for all complexes, despite their largely different HS lifetimes, confirms the fact that the driving force for the HS-LS relaxation is determined by the energetics of the HS state, rather than its structure [36].…”

“…Schematic potential energy curves of Fe(II)-SCO complexes as a function of the Fe-N bond distance, and pathways of nonradiative relaxation leading to the short-lived quintet state (0.6 ns for Fe II bpy 3 2 ), upon excitation of the singlet MLCT state in the UV. [9][10][11][12][13][14][15][16][17][18][19][20][21]. Recently, Fe K-edge steady-state XAS of the LS Fe tren py 3 PF 6 2 and its HS analogue Fe tren 6-Me-py 3 PF 6 2 were combined to interpret the picosecond transient spectra of the photoactivated Fe tren py 3 PF 6 2 complex, showing good agreement with the anticipated result for the HS complex [22].…”

Bulk spin-crossover in the complex [FeL(NCS)2] of a tris(pyridyl)ethane-derived N4-ligand—a temperature-dependent crystallographic study

“…Comparing the data with those of other bis(thiocyanato-κN)iron(II) complexes and taking into account the spin-transition curve (see Fig. 6), it is reasonable to assume a virtually complete SCO for 1a at 273 K. 23,27 The notable changes in molecular geometry are illustrated by an overlay of the complexes at both temperatures (see Fig. 8).…”

“…ΔT 80 is defined as the difference of temperatures at which 80% of the molecules are in the HS and the LS state, respectively, and is considered a measure of the molecular interaction during SCO. 23 The coefficients of determination can be used to judge the adequacy of the model. The values obtained for powder and solution are similar.…”

“…These parameters thereby affect the abruptness of an SCO as well as the broadness of a hysteresis, if present. 23 As the pseudopolymorphs of [FeL(NCS) 2 ] (1) also differ with respect to such interactions, the transitions are expected to show different characteristics.…”