Molecular Graphics and Computation Facility, University of California, Berkeley, California 94720, USA.
Inorg Chem. 2011 Jan 3;50(1):345-57. doi: 10.1021/ic102028d. Epub 2010 Dec 8.
Experimental evidence for the existence of two new lanthanide-metalloligand adducts (CpSiMe(3))(3)Ce-ECp* (E = Al, Ga) is presented. Paramagnetic (1)H NMR titration experiments were employed to derive thermodynamic parameters for Ce-Ga dative bond formation, and competition experiments with the U analogue were performed. Density functional theory calculations were undertaken using model complexes Cp(3)Ln-ECp where Ln = La-Lu and E = Al, Ga. The Ln-E bond distances were predicted to decrease more sharply across the Ln series than those involving hard Lewis bases; however, local increases were observed at Eu and Yb. Electronic analyses were performed in the natural bond orbital-natural localized molecular orbital (NBO/NLMO) formalism, indicating that the E→Ln acceptor orbital is primarily of d character in all cases. The Cp(-) ligands donate significant electron density to the Ln d manifold and thus in its bonding interactions with a dative ligand the Ln center may be considered to be Ln(2+) in the f((n-3))d(1) electronic configuration (n = 3 for La, etc.). Molecular dipole moments, NLMO and natural population analyses, bond order indices, measures of E→Ln charge transfer, and calculated Ln-E heterolytic bond disruption enthalpies were found to follow saw-tooth trends, which correlate to varying degrees with the ionization potentials of the Ln(+) ions (corrected for their ground state-to-f((n-3))d(2) excitations). It is proposed that a steric-strain component which increases with the lanthanide contraction in this case balances the Ln-E bond stabilizing effect of core-orbital contraction. All data indicate that the Ln-E bonding interactions are predominantly of covalent or nonpolar donor-acceptor character. However, the formation of a strong covalent bond is not observed because of resistance to reduction of an effectively divalent Ln center.
本文呈现了两个新的镧系金属-金属配合物加合物（CpSiMe(3))(3)Ce-ECp*（E=Al，Ga）的实验证据。采用顺磁（1）H NMR 滴定实验得出 Ce-Ga 配位键形成的热力学参数，并与 U 类似物进行了竞争实验。使用模型配合物 Cp(3)Ln-ECp（其中 Ln=La-Lu，E=Al，Ga）进行了密度泛函理论计算。预测 Ln-E 键距离在镧系元素系列中比涉及硬路易斯碱的距离下降更急剧；然而，在 Eu 和 Yb 处观察到局部增加。电子分析在自然键轨道-自然局域分子轨道（NBO/NLMO）形式中进行，表明在所有情况下，E→Ln 受体轨道主要具有 d 特征。Cp(-)配体向 Ln d 简并轨道提供显著的电子密度，因此在与 dative 配体的成键相互作用中，Ln 中心在 f((n-3))d(1)电子构型（n=3 对于 La 等）中可以被认为是 Ln(2+)。分子偶极矩、NLMO 和自然布居分析、键序指数、E→Ln 电荷转移测量以及计算的 Ln-E 异裂键断裂焓被发现遵循锯齿形趋势，这些趋势与镧系元素离子的电离势（考虑到它们的基态到 f((n-3))d(2)激发）在不同程度上相关。提出的是，在这种情况下，由于镧系收缩导致的空间应变分量与核心轨道收缩对 Ln-E 键稳定化效应相平衡。所有数据表明，Ln-E 成键相互作用主要具有共价或非极性给体-受体特征。然而，由于有效二价 Ln 中心的还原阻力，没有观察到形成强共价键。
