New iron(II) complexes were synthesized with two tridentate hybrid ligands having phosphorous and nitrogen donor sites, in order to quantitatively estimate the difference of the ligand-field strengths of phosphorous and nitrogen donor sites in cationic metal complexes. Iron(II) complexes with bis(dimethylphosphinoethyl)amine (PNP) and 2,6-bis(diphenylphosphinomethyl)pyridine (PpyP) ligands crystallized as un-symmetric facial-[Fe(PNP)<inf>2</inf>](PF<inf>6</inf>)<inf>2</inf>·CH<inf>3</inf>NO<inf>2</inf> and mer-[Fe(PpyP)<inf>2</inf>](CF<inf>3</inf>SO<inf>3</inf>)<inf>2</inf>, respectively, as expected from the steric congestion and from the tendency to avoid the mutual trans influence between two phosphorous donor sites. Both complexes are in the low-spin electronic state up to 400 K. The pseudo-D <inf>4h</inf> coordination geometry of the PpyP complex made it possible to separate axial (2 × N) and equatorial (4 × P) contributions to the overall ligand-field by means of a spectrometric method: the difference in the ligand-field strengths by the equatorial Ph<inf>2</inf>P-donor sites and by the axial 2,6-disubstituted pyridine donor sites is ca. 13,200 cm<sup>-1</sup>. A significantly reduced inter-electronic repulsion parameter (425 cm<sup>-1</sup> for both PNP and PpyP complexes) from the value of the free ion (1,060 cm<sup>-1</sup>) indicates covalent interaction between the Fe(II) and P atoms even in these cationic metal complexes. It is shown that the degree of covalency as well as the coordination bond strengths between various metal ions and phosphorous/nitrogen donor atoms is successfully explained by the relative energy levels of interacting atomic orbitals calculated on the basis of the Thomas-Fermi-Dirac potential.
- 2, 6-Bis(diphenylphosphinomethyl)pyridine (PpyP)
- Bis(dimethylphosphinoethyl)amine (PNP)
- Iron(II) complexes
- Ligand field
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Molecular Biology