Exchange Coupling and Its Chemical Trend Studied by High-Frequency EPR on Heterometallic [Ln₂Ni] Complexes

We applied high-frequency electron paramagnetic resonance to trinuclear 4f–3d heterometallic complexes, [{Ln­(hfac)3}2{Ni­(dpk)2(py)2}] (Ln = Y, Gd, Tb, and Ho; hfac = hexafluoroacetylacetonate, dpk = di-2-pyridyl ketoximate, and py = pyridine), and determined the exchange parameter JLn–Ni as well as nickel­(II) zero-field splitting parameters. In contrast to the antiferromagnetic Dy analogue, ferromagnetic couplings were precisely characterized as JGd–Ni/kB = +0.301(4) K, JTb–Ni/kB = +0.216(12) K, and JHo–Ni/kB = +0.110(3) K (defined as −JLn–Ni∑JLnzSNi).