The distinct affinity of cyclopentadienylligands towards trivalent uranium over lanthanide ions. Evidence for cooperative ligation and back-bonding in the actinide complexes

Abstract

The mono and bis(cyclopentadienyl) compounds [M(C₅H₄Bu^t)I₂] and [M(C₅H₄Bu^t)₂I] (M = U, La, Ce, Nd) were formed in thf by comproportionation reactions of [M(C₅H₄Bu^t)₃] and LnI₃ or [UI₃(L)₄] (L = thf or py) in the molar ratio of 1 : 2 and 2 : 1, respectively, while treatment of [UI₃(py)₄] or LnI₃ (Ln = La, Ce, Nd) with 1 or 2 mol equivalents of LiC₅H₄Bu^t in thf afforded the [M(C₅H₄Bu^t)I₂] and [M(C₅H₄Bu^t)₂I₂]⁻ compounds, respectively. The X-ray crystal structures of [M(C₅H₄Bu^t)I₂(py)₃] (M = U, La, Ce, Nd), [{Ce(C₅H₄Bu^t)₂(μ-I)}₂] and [M(C₅H₄Bu^t)₂I(py)₂] (M = U, Nd) have been determined; the differences between the average M–C distances in the mono(cyclopentadienyl) complexes correspond to the variation in the ionic radii of the trivalent uranium and lanthanide ions while the U–N and U–I bond lengths seem to be smaller than those predicted from a purely ionic bonding model. The distinct affinity of the cyclopentadienyl ligands towards Ln(III) and U(III) was revealed by two series of competing reactions: the ligand exchange reactions between [Ln(C₅H₄Bu^t)_n′I_3−n′] (Ln = La, Ce, Nd) and [U(C₅H₄Bu^t)_n″I_3−n″] species (1 ≤ n′ + n″ = n ≤ 5), and the addition of n mol equivalents of LiC₅H₄Bu^t (1 ≤ n ≤ 5) to a 1 : 1 mixture of LnI₃ and [UI₃(thf)₄] or [UI₃(py)₄]. The stability of the [M(C₅H₄Bu^t)I₂] species was found to vary in the order Nd > Ce > U > La, a trend which is in accord with an electrostatic bonding model. However, the bis and tris(cyclopentadienyl) complexes of uranium are more stable than their lanthanide analogues. This difference can be accounted for by a higher degree of covalency in the U–C₅H₄Bu^t bond, resulting from the late appearance of back-bonding which would emerge only after the first cyclopentadienyl ligand is bound.