The molecular geometric and electronic structures of tris(pentamethylcyclopentadienyl) rare earth metal complexes (C₅Me₅)₃Ln (Ln=Sc, Y, La) were investigated by means of ab initio HF, MP2 and density functional theory (DFT) methods. The bonding energies of Ln-C₅Me₅ were also analyzed on the basis of calculations. To study the effect of the exchange-correlation (XC) functional on the structural parameters and bonding energies, LDA (Xα, SVWN3, SVWN5), GGA (BP86, BLYP), meta-GGA (TPSS) and hybrid (B3LYP, PBE0, TPSSh, M06, M06-HF, M06-2X) functionals have been used. Calculations were carried out with the TZVPP basis sets. The geometry structural parameters from the different methods were compared to the experimental values. The Ln-C distance of (C₅Me₅)₃Ln shows a dramatic reduction of about 15 pm from HF to MP2 level. These differences demonstrates the importance of electron correlation on the Ln-C₅Me₅ bonds. In (C₅Me₅)₃Ln complexes, the geometric parameters differ much for different DFT exchange-correlation functional. For the Ln-C bond length, the theoretical values closest to the experiment were the M06-2X and M06 methods, with the deviation of only 1.2 and 2.5 pm, respectively. Thus, the M06-2X methods appear to be the most reliable for predicting the molecular structures of (C₅Me₅)₃Ln. The studies of the electronic structures showed that the (C₅Me₅)₃Ln satisfied the 18-electron rule, so these complexes had high stability. The (n-1)d and ns orbitals of Ln atom had a better overlap with the π orbitals of C₅Me₅. Our calculations revealed that the geometries of (C₅R₅)₃Ln (R=H, Me) were influenced significantly by the “Ln-C₅R₅ bonding effect” and “C₅R₅-C₅R₅ steric effect”, while the thermal stabilities were influenced by whether atomic radius of Ln was suitable for the ring size of (C₅R₅)₃. Therefore, the introduction of specific substituents into C₅R₅ could affect the π orbital energies, Ln-C₅R₅ distance and C₅R₅-C₅R₅ distance, and then control the Ln-C₅R₅ bonding strength and stability of (C₅R₅)₃Ln.

Key words: tris(pentamethylcyclopentadienyl) rare earth metal complexes, ab initio, density functional theory, electronic structure, metal-ligand bonding energy