Review of the structure of tris(β-diketonato)manganese complexes

This review present the reported experimental structures and theoretically calculated structures of tris(β-diketonato)manganese complexes. Experimental high spin tris(β-diketonato)manganese(II) has an octahedral MnO₆ core with six equal Mn-O bond. However, the geometry of experimental high spin tris(β-diketonato)manganese(III) complexes exhibits Jahn-Teller geometrical distortion due to one electron in the eg orbital group of an octahedral geometry, leading to two degenerate electronic states. This geometrical distortion leads to a distorted regular octahedral geometry, such as orthorhombic distortion, tetragonal compression (TC) or elongation (TE); all experimentally reported. The TE form is reported to be the “natural” distorted form for tris(acetylacetonate)manganese(III), on ground of high-frequency and high-field electron paramagnetic resonance (HFEPR) spectroscopy results on a frozen solution, free of crystal packing effects, of tris(acetylacetonate)manganese(III). Theoretical calculations, using density functional theory, indicated that both the TE and TC forms could energetically exist, with the TE more stable by ca 0.04 eV. DFT calculations further indicate that for tris(β-diketonato)manganese(III) molecules containing unsymmetrically substituted β-diketonato ligands, both the TC and TE forms are possible for each of the fac and mer isomers of these molecules. Furthermore, DFT calculations identified three different TE forms for mer isomers, depending on the ligands attached to the elongated Mn-O bonds, with energies within 0.06 eV. The different geometrical Jahn-Teller distorted forms (one fac TE, one fac TC and three mer TE forms), theoretically calculated for tris(β-diketonato)manganese(III) molecules containing unsymmetrically substituted β-diketonato ligands, were referred to as bond stretch isomers. The limits of the current experimental and theoretical studies, and possible perspectives are also discussed.