Theory of cooperative high-spin⇄low-spin transitions in iron (III) compounds induced by the molecular distortions

Abstract

The high-spin?low-spin transitions in iron (III) compounds are studied on the basis of the ligand field theory using the model in which the coupling between the iron ion and the intramolecular distortion is the driving force for the spin state transitions. The intermolecular coupling between the molecular distortions is taken into account and the distortions are treated dynamically. Two kinds of the molecular distortions, the uniform dilation and the uniaxial distortion, are considered. The discontinuous high-spin?low-spin transitions may be induced by the coupling with the uniaxial distortion, while the continuous spin state transitions arise from the coupling with the uniform dilation. Temperature dependences of equilibrium cooperative displacement of the ligands, high-spin and low-spin fractions of iron (III) ions, effective magnetic moment, quadrupole splitting in Mössbauer spectra, and internal magnetic field in NMR spectra are calculated based on the model as functions of the coupling strength between iron and the molecular distortion. It is shown that the essential features of the observed temperature dependences may be reproduced consistently in the various quantities by the present model.

Spin crossover iron(III) complexes

“…After standing for 10 days, the complex showed SCO with a hysteresis loop (width of 70 K) centered at 286 K. SCO behavior in [Fe(pap) 2 ]X·solv (X = NO 3 , BPh 4 , PF 6 , ClO 4 , NCS) is also significantly affected by counter anions and solvent molecules [8e]. Solvent free NO 3 − and BPh 4 − salts are HS, whereas the PF 6 − salt is LS. Freshly prepared [Fe(pap) 2 ]ClO 4 exhibits SCO with hysteresis at T 1/2 ↑ = 262 K and T 1/2 ↓ = 242 K (T 1/2 ↑ and T 1/2 ↓ represent transition temperatures in heating and cooling processes, respectively), respectively [20].…”

“…Elemental analyses and powder X-ray diffraction data suggested Scheme 1. 6 ·MeOH exhibit abrupt spin transitions with hysteresis (T 1/2 ↑ = 180 K and T 1/2 ↓ = 165 K) and T 1/2 = 288 K without hysteresis, respectively [21,22].…”

“…The magnetic susceptibility measurements show that the HS fraction of the PF 6 − salt is always higher than that of the BPh 4 − salt at any temperature. The HS state ( 6 A 1g ) for the PF 6 − salt is always placed below the 6 A 1g state of the BPh 4 − salt, assuming both complexes having the ground LS state ( 2 T 2g ) in the same energy. Taking the longer HS bond lengths for the PF 6 − salt than those for the BPh 4 − salts into account, the potential surface of the 6 A 1g state for the PF 6 − salt is placed further from the 2 T 2g state than that of the BPh 4 − salt (Fig.…”

“…doi:10.1016/j.ccr.2007.08.007 carbamato)iron(III) complexes [1], the spin crossover phenomena in iron(II) [2] and iron(III) [3] complexes have been extensively studied [4]. SCO complexes are classified into two groups as stated in the abstract, and their spin transition behavior depends upon intramolecular Jahn-Teller distortions [5] and intermolecular interactions [6]. Heat capacity measurements for SCO systems revealed that the total entropy changes ( S) accompanied with spin transitions are much larger than the values expected for the spin conversions between the LS and HS states: S = 9.13 (=R ln 3) and 13.38 (=R ln 5) J K −1 mol −1 for iron(III) and iron(II) systems, respectively.…”

“…The different spin transition behavior can be explained by theoretical models, such as the domain model, regular solution model and elastic phonon model [7], and herein we address the first model. The domain size (a number of molecules in the domain) for ST types (95 for [Fe(phen) 2 (NCS) 2 ]) is larger than that for SE types (5 for [Fe III (acpa) 2 ]PF 6 ). This suggests that the spin transitions in ST type complexes couple strongly with the lattice phonon system.…”