Low-temperature lifetimes of metastable high-spin states in spin-crossover and in low-spin iron(II) compounds: The rule and exceptions to the rule

✍ Scribed by Andreas Hauser; Cristian Enachescu; Max Lawson Daku; Alfredo Vargas; Nahid Amstutz

• Publisher: Elsevier Science
• Year: 2006
• Tongue: English
• Weight: 431 KB
• Volume: 250
• Category: Article
• ISSN: 0010-8545
• DOI: 10.1016/j.ccr.2005.12.006

✦ Synopsis

The high-spin → low-spin relaxation in spin-crossover compounds can be described as non-adiabatic multi-phonon process in the strong coupling limit, in which the low-temperature tunnelling rate increases exponentially with the zero-point energy difference between the two states. Based on the hypothesis that the experimental bond length difference between the high-spin and the low-spin state of ∼0.2 Å is also valid for low-spin iron(II) complexes, extrapolation of the single configurational coordinate model allows an estimate of the zero-point energy difference for low-spin complexes from kinetic data. DFT calculations on low-spin [Fe(bpy) 3 ] 2+ support the structural assumption. However, for low-spin [Fe(terpy) 2 ] 2+ the relaxation rate constant shows an anomalous behaviour in so far as it is more in line with spin-crossover systems. This is attributed to very anisotropic bond length changes associated with the spin state change, and the subsequent breakdown of the single mode model.