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  • Title: Covalent lanthanide chemistry near the limit of weak bonding: observation of (CpSiMe3)3Ce-ECp* and a comprehensive density functional theory analysis of Cp3Ln-ECp (E = Al, Ga).
    Author: Krinsky JL, Minasian SG, Arnold J.
    Journal: Inorg Chem; 2011 Jan 03; 50(1):345-57. PubMed ID: 21141834.
    Abstract:
    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.
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