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)

Experimental evidence for the existence of two new lanthanide-metalloligand adducts (CpSiMe3)3Ce−ECp* (E = Al, Ga) is presented. Paramagnetic 1H 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 Cp3Ln−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 Ln2+ in the f(n−3)d1 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)d2 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.