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An Unprecedented Square Planar Fe(III) Complex Exhibiting Spin Crossover Between the Spin-Admixed States.
Kamal Uddin Ansari1, Dipanti Borah1, Lena Spillecke2
1Department of Chemistry, Indian Institute of Technology Bombay, Mumbai, Maharashtra, India.
Chemistry (Weinheim an Der Bergstrasse, Germany)
|April 18, 2026
Summary
This study introduces a novel ferric ion complex exhibiting spin crossover (SCO) between intermediate-spin and low-spin states, a rare phenomenon. The complex displays unique spin-admixed behavior, offering new insights into molecular magnetism.
Area of Science:
- Coordination Chemistry
- Molecular Magnetism
- Quantum Spin Dynamics
Background:
- Strong spin-orbit coupling can lead to spin-admixed states in ferric complexes.
- Spin crossover (SCO) typically involves transitions between distinct spin states.
- Previous research has not documented spin admixing between intermediate-spin (IS) and low-spin (LS) states.
Purpose of the Study:
- To report a unique square planar ferric ion complex.
- To investigate the unprecedented observation of spin crossover between spin-admixed states.
- To elucidate the electronic structure and underlying mechanisms of quantum spin admixing and SCO.
Main Methods:
- Synthesis and characterization of a novel square planar ferric ion complex, [Na(DME)3][Fe(L12–)2] (1).
- Variable temperature magnetic susceptibility measurements.
- Field- and temperature-dependent Mössbauer spectroscopy.
- Variable temperature X-band Electron Paramagnetic Resonance (EPR) and multifrequency EPR (MF-EPR) spectroscopy.
- Theoretical calculations for electronic structure analysis.
Main Results:
- The complex [Na(DME)3][Fe(L12–)2] (1) exhibits spin crossover (SCO).
- SCO occurs between spin-admixed states, specifically involving intermediate-spin (IS; S = 3/2) and low-spin (LS; S = 1/2) contributions.
- This spin-admixed state is distinct from previously reported spin-admixed states involving high-spin (HS) and IS states.
- Experimental data and theoretical calculations support the observed quantum spin admixing and SCO phenomena.
Conclusions:
- A rare square planar ferric ion complex demonstrates unprecedented spin crossover between spin-admixed IS and LS states.
- The findings challenge existing understanding of spin crossover mechanisms and spin-state mixing in ferric complexes.
- This work provides a foundation for designing new molecular materials with tunable magnetic properties.
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