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

Abstract

Homoleptic cyclopentadienyl derivatives of the early transition metals and lanthanides that have been synthesized include Cp 4M (M = Ti, Zr, Ce, Hf, Th), Cp 3M (M = Sc, Y, most lanthanides, Ti, Zr, Hf, Th), and Cp 2M (M = Ti, V). Density functional theory shows that the Cp 4M structures with unusual S 4 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 Cp 4M geometries have C 1 symmetry with two η 5-Cp rings and two η 1-Cp rings for M = Ti and Hf but three η 5-Cp rings and one η 1-Cp ring for M = Zr. The dissociation energies for Cp 4M → Cp 3M + 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 C 3 h structures with all η 5-Cp rings are genuine minima for most of the Cp 3M compounds except for Cp 3Ti, which has a C s symmetry minimum with two η 5-Cp rings and one η 2-Cp ring, and Cp 3V, which has two η 5-Cp rings and one η 1-Cp ring. The dissociation energies for Cp 3M → Cp 2M + 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.