Issue 7, 2005

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(C5H4But)I2] and [M(C5H4But)2I] (M = U, La, Ce, Nd) were formed in thf by comproportionation reactions of [M(C5H4But)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 LiC5H4But in thf afforded the [M(C5H4But)I2] and [M(C5H4But)2I2] compounds, respectively. The X-ray crystal structures of [M(C5H4But)I2(py)3] (M = U, La, Ce, Nd), [{Ce(C5H4But)2(μ-I)}2] and [M(C5H4But)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(C5H4But)nI3−n] (Ln = La, Ce, Nd) and [U(C5H4But)nI3−n] species (1 ≤ n′ + n″ = n ≤ 5), and the addition of n mol equivalents of LiC5H4But (1 ≤ n ≤ 5) to a 1 : 1 mixture of LnI3 and [UI3(thf)4] or [UI3(py)4]. The stability of the [M(C5H4But)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–C5H4But bond, resulting from the late appearance of back-bonding which would emerge only after the first cyclopentadienyl ligand is bound.

Graphical abstract: The distinct affinity of cyclopentadienyl ligands towards trivalent uranium over lanthanide ions. Evidence for cooperative ligation and back-bonding in the actinide complexes

Supplementary files

Article information

Article type
Paper
Submitted
20 Dec 2004
Accepted
22 Feb 2005
First published
04 Mar 2005

Dalton Trans., 2005, 1263-1272

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

T. Mehdoui, J. Berthet, P. Thuéry and M. Ephritikhine, Dalton Trans., 2005, 1263 DOI: 10.1039/B419088B

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