Some structural and electronic properties of MX3 (M = Ln, Sc, Y, Ti+, Zr+, Hf+; X = H, Me, Hal, NH2) from DFT calculations.

Abstract

The geometry of trivalent homoleptic d0 MX3 (X = H, Me, Hal, NH2) complexes for the entire lanthanide family, neutral group 3 and cationic group 4 metal center complexes have been studied with DFT(B3PW91) calculations. The geometrical parameters are in good agreement with the available experimental data. The degree of pyramidalization of the metal is discussed. The hydride and alkyl complexes are strongly pyramidal. In the case of the halide, a pyramidal structure is preferred for fluoride and the systems become increasingly planar with heavier halides. The geometrical trends with X are similar for group 3, group 4 and lanthanide complexes. However group 3 complexes are almost planar, group 4 strongly pyramidal and lanthanide intermediate. The lanthanide contraction is reproduced. A natural bond orbital (NBO) charge analysis is used to rationalize the results. This highlights the similarities and differences in the M-X bonding in the three families of complexes. In all cases, the pyramidalization is related to the participation of the valence d orbitals in the M-X bonds but the M-X bond is mostly ionic in lanthanide and considerably more covalent for the d transition metals. The hydride and alkyl complexes, which have more covalent character than the halide complexes are more pyramidal. In the case of the halide complexes, the fluoride complexes, in which there is the least population of the M d orbitals, are found to be more pyramidal because the increasing covalency with heavier halide stabilizes the planar structure through dpi-ppi interaction. The electronic metal d-p transition of the free ion is shown to be a good indicator of the pyramidalization at M although these values should only be used qualitatively. The strong ionic character of the Ln-X bond gives a rationale for the more important elongation of the beta Si-C bond in La(CH(SiMe3)2)3 than in La(N(SiMe3)2)3. The elongation is shown to be in part due to the negative hyperconjugation of the lone pair used for the Ln-ligand bonds in the beta bonds.