Covalency in the 4f Shell oftris-Cyclopentadienyl Ytterbium (YbCp3)—A Spectroscopic Evaluation

Abstract

Evidence is presented of significant covalency in the ytterbium 4f shell of tris-cyclopentadienyl ytterbium (YbCp₃) in its electronic ground state, that can be represented by the superposition of an ionic configuration Yb(III):4f¹³(Cp₃) and a charge-transfer configuration Yb(II):4f¹⁴(Cp₃)⁻¹. The relative weights of these configurations were determined from (i) the difference in their 4f photoionization cross sections, (ii) the accumulation of spin-density centered on the ¹³C atoms of the Cp ring, as measured by a pulsed EPR (HYSCORE) experiment, (iii) the reduction in the spin-density in the 4f shell, manifest in the ¹⁷¹Yb hyperfine interaction, and (iv) the principal values of the g-tensor, obtained from the EPR spectrum of a frozen glass solution at 5.4 K. Each of these methods finds that the spin density attributable to the charge transfer configuration is in the range 12–17%. The presence of configuration interaction (CI) also accounts for the highly anomalous energies, intensities, and vibronic structure in the “f–f” region of the optical spectrum, as well as the strict adherence of the magnetic susceptibility to the Curie law in the range 30–300 K.