Ab Initio Ligand Field Molecular Mechanics and the Nature of Metal-Ligand π-Bonding in Fe(II) 2,6-di(pyrazol-1-yl)pyridine Spin Crossover Complexes

Robert J. Deeth, Malcolm A. Halcrow, Laurence J. Kershaw Cook, Paul R. Raithby

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A ligand field molecular mechanics (LFMM) force field has been constructed for the spin states of [Fe(bpp)2]2+ (bpp=2,6-di(pyrazol-1-yl)pyridine) and related complexes. A new charge scheme is employed which interpolates between partial charges for neutral bpp and protonated [H3bpp]3+ to achieve a target metal charge. The LFMM angular overlap model (AOM) parameters are fitted to fully ab initio d orbital energies. However, several AOM parameter sets are possible. The ambiguity is resolved by calculating the Jahn–Teller distortion mode for high spin, which indicates that in [Fe(bpp)2]2+pyridine is a π-acceptor and pyrazole a weak π-donor. The alternative fit, assumed previously, where both ligands act as π-donors leads to an inconsistent distortion. LFMM optimisations in the presence of [BF4] or [PF6] anions are in good agreement with experiment and the model also correctly predicts the spin state energetics for 3-pyrazolyl substituents where the interactions are mainly steric. However, for 4-pyridyl or 4-pyrazolyl substituents, LFMM only treats the electrostatic contribution which, for the pyridyl substituents, generates a fair correlation with the spin crossover transition temperatures, T1/2, but in the reverse sense to the dominant electronic effect. Thus, LFMM generates its smallest spin state energy difference for the substituent with the highest T1/2. One parameter set for all substituted bpp ligands is insufficient and further LFMM development will be required.

Original languageEnglish
Pages (from-to)5204-5212
Number of pages9
JournalChemistry - A European Journal
Issue number20
Publication statusPublished - 6 Apr 2018



  • angular overlap model
  • iron complexes
  • ligand field molecular mechanics
  • spin crossover
  • π-bonding

ASJC Scopus subject areas

  • Chemistry(all)

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