Structural and thermodynamic aspects of the dissociation of cyclopentadienyl rings from homoleptic cyclopentadienyl early transition metal, cerium, and thorium derivatives

Add time:08/15/2019    Source:sciencedirect.com

Homoleptic cyclopentadienyl derivatives of the early transition metals and lanthanides that have been synthesized include Cp4M (M = Ti, Zr, Ce, Hf, Th), Cp3M (M = Sc, Y, most lanthanides, Ti, Zr, Hf, Th), and Cp2M (M = Ti, V). Density functional theory shows that the Cp4M structures with unusual S4 symmetry are saddle points for the d-block metals Ti, Zr, Hf but genuine minima for the f-block metals Ce and Th. The true equilibrium Cp4M geometries have C1 symmetry with two η5-Cp rings and two η1-Cp rings for M = Ti and Hf but three η5-Cp rings and one η1-Cp ring for M = Zr. The dissociation energies for Cp4M → Cp3M + Cp are substantial and in the order Ti < Ce < Zr ∼ Hf < Th, roughly consistent with the observed relative stabilities of the +4 and +3 oxidation states of these metals. The C3h structures with all η5-Cp rings are genuine minima for most of the Cp3M compounds except for Cp3Ti, which has a Cs symmetry minimum with two η5-Cp rings and one η2-Cp ring, and Cp3V, which has two η5-Cp rings and one η1-Cp ring. The dissociation energies for Cp3M → Cp2M + Cp are in the order V < Ti < Sc < Y < La, consistent with the relative stabilities of the +2 and +3 oxidation states of these metals.

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