Synthesis, structure, and molecular orbital calculations of chromium(II) and iron(III) complexes containing η2-pyrazolato ligands

η2 coordination of pyrazolato ligands is currently unknown for d-block metal complexes with d2 or higher electronic configurations, and all known Group 6-11 metal-pyrazolato complexes contain η1, μ:η1,η1, or η5 ligands. Tris(3,5-di-tert-butylpyrazolato)chromium(III) and tris(3,5-di-tert-butylpyrazolato)iron(III) (see picture), which contain η2-pyrazolato ligands, have been synthesized, and molecular-orbital calculations indicate that η2-pyrazolato coordination is less stable than η1 coordination in a model iron(III) complex.