Synthesis, magnetic properties, and electronic spectra of bis(β-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 beta-diketonates, [Cr(beta-diketonato)(2)(IM(2)py)]PF6, and [Ni(beta-diketonato)(2)(IM(2)py)] (IM(2)py = 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 IM(2)py 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 IM(2)py, respectively, giving a variety of the magnetic coupling constant J values with varying the beta-diketonato ligands. The UV-vis shoulders around (19-20) x 10(3) and (17-18) x 10(3) cm(-1) for the Cr(III) and Ni(II) complexes, respectively, characteristic of the IM(2)py complexes were assigned to the metal-ligand charge-transfer transitions, Cr(t(2g))-SOMO(pi*) and Ni(e(g))-SOMO( pi*) MLCT in terms of the resonance Raman spectra and the variable-temperature absorption spectra. The absorption components centered around (13-14) x 10(3) cm(-1) for the Cr(III) and Ni(II) complexes were due to the formally spin-forbidden d-d transition within the t(2g) and e(g) subshells, associated with the intensity enhancement, The spectroscopic behavior with varying the beta-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.