Magnetic Anisotropy of Dichlorobis(?5-cyclopentadienyl) Complexes of Vanadium, Niobium, and Tantalum

The magnetic behavior of the mononuclear nd1 systems MCp2Cl2 (M = V4+[3d1], Nb4+[4d1], Ta4+[5d1], space group P21/c, pseudosymmetry of the molecules C2v) deviates from pure single ion spin magnetism on account of ligand field effect (Hlf), spin-orbit coupling (Hso), and intermolecular spin-spin exchange interactions (Hex). For both VCp2Cl2 and NbCp2Cl2 excellent adaptations to the measured susceptibility data were obtained (2 K = T = 300 K) on the basis of spectroscopic data (lf, so) and cooperative metalmetal interactions (ex) of antiferromagnetic nature [molecular field model (mf)]. For TaCp2Cl2 experimental term structure data are not available. Therefore, J circle divide rgensen's spectroscopical series (g-factor of the central ion) was applied to extrapolate the data set for TaCp2Cl2. Hlf, Hso, and Hex (antiferromagnetic) increase in the order 3d1 ? 4d1 ? 5d1 leading, with rising atomic number of the metals, to a distinct enhancement of the magnetic anisotropy. At 4 K the mu eff components mu eff,y (oriented perpendicular to the cgMcg plane; cg = center of gravity of the Cp ring), mu eff,z (oriented along the twofold pseudoaxis), and mu eff,x are 1.73, 1.69, 1.68 (V), 1.73, 1.62, 1.59 (Nb), and 1.71, 1.59, 1.49 (Ta). While mu eff,y is independent of T, both mu eff,z and mu eff,x decrease with decreasing T.