Covalency in the 4f Shell of tris-Cyclopentadienyl Ytterbium (YbCp3)A Spectroscopic Evaluation

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.本研究报道了三(环戊二烯基)镱（tris-cyclopentadienyl ytterbium，YbCp₃）电子基态下，镱原子4f轨道壳层存在显著共价性的实验证据。该共价性可表述为离子组态Yb(III):4f¹³(Cp₃)与电荷转移组态Yb(II):4f¹⁴(Cp₃)⁻¹的叠加。上述两种组态的相对占比通过四种方法确定：(i) 二者4f轨道的光电离截面差值；(ii) 经脉冲电子顺磁共振（HYSCORE）实验测得的、集中于环戊二烯（Cp）环¹³C原子上的自旋密度累积量；(iii) 4f轨道壳层自旋密度的降低，该效应体现于¹⁷¹Yb的超精细相互作用中；(iv) 于5.4 K下冷冻玻璃态溶液的电子顺磁共振（EPR）光谱中获得的g张量主值。所有实验方法均得到，归因于电荷转移组态的自旋密度占比处于12%~17%的区间内。组态相互作用（configuration interaction，CI）的存在，不仅解释了光谱中“f-f”跃迁区域出现的高度反常的能级、谱线强度与振动-电子结构，同时也阐明了30 K~300 K范围内磁化率严格遵循居里定律的实验现象。