Synthesis, Magnetic Properties, and Electronic Spectra of Bis(<i>β</i>-diketonato)chromium(III) and Nickel(II) Complexes with a Chelated Imino Nitroxide Radical: X-ray Structures of [Cr(acaMe)₂(IM2py)]PF₆ and [Ni(acac)₂(IM2py)]

Two new series of each of four Cr(III) and Ni(II) imino nitroxide complexes with various kinds of β-diketonates, [Cr(β-diketonato)2(IM2py)]PF6, and [Ni(β-diketonato)2(IM2py)] (IM2py = 2-(2‘-(pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxy)) have been synthesized, and their structures and magnetic and optical properties have been examined. The X-ray analysis demonstrated that a IM2py ligand coordinated to Cr(III) and Ni(II) acts as a five-membered bidentate chelate. The variable-temperature magnetic susceptibility measurements indicated the antiferromagnetic and ferromagnetic interaction of Cr(III) and Ni(II) with IM2py, respectively, giving a variety of the magnetic coupling constant J values with varying the β-diketonato ligands. The UV−vis shoulders around (19−20) × 103 and (17−18) × 103 cm-1 for the Cr(III) and Ni(II) complexes, respectively, characteristic of the IM2py complexes were assigned to the metal−ligand charge-transfer transitions, Cr(t2g)-SOMO(π*) and Ni(eg)-SOMO(π*) MLCT in terms of the resonance Raman spectra and the variable-temperature absorption spectra. The absorption components centered around (13−14) × 103 cm-1 for the Cr(III) and Ni(II) complexes were due to the formally spin-forbidden d−d transition within the t2g and eg subshells, associated with the intensity enhancement. The spectroscopic behavior with varying the β-diketonato ligands is discussed in connection with the antiferromagnetic or ferromagnetic coupling constant J values on the basis of the exchange mechanism along with the coligand effect.