Emergence of the structure-directing role of f-orbital overlap-driven covalency

Erli Lu,Saira Sajjad,Ashley J Wooles,Nikolas Kaltsoyannis,Stephen T Liddle,Victoria E J Berryman

doi:10.1038/s41467-019-08553-y

Erli Lu, Saira Sajjad + Show 4 more

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https://doi.org/10.1038/s41467-019-08553-y

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Abstract

FEUDAL (f’s essentially unaffected, d’s accommodate ligands) is a longstanding bonding model in actinide chemistry, in which metal-ligand binding uses 6d-orbitals, with the 5f remaining non-bonding. The inverse-trans-influence (ITI) is a case where the model may break down, and it has been suggested that ionic and covalent effects work synergistically in the ITI. Here, we report an experimentally grounded computational study that quantitatively explores the ITI, and in particular the structure-directing role of f-orbital covalency. Strong donor ligands generate a cis-ligand-directing electrostatic potential (ESP) at the metal centre. When f-orbital participation, via overlap-driven covalency, becomes dominant via short actinide-element distances, this ionic ESP effect is overcome, favouring a trans-ligand-directed geometry. This study contradicts the accepted ITI paradigm in that here ionic and covalent effects work against each other, and suggests a clearly non-FEUDAL, structure-directing role for the f-orbitals.

Highlights

FEUDAL (f’s essentially unaffected, d’s accommodate ligands) is a longstanding bonding model in actinide chemistry, in which metal-ligand binding uses 6d-orbitals, with the 5f remaining non-bonding
It is clear from the computed data that the thorium complexes have much the strongest preference for a cis geometry
If considering orbital arguments, this is counterintuitive because the bonding of thorium is more ionic than uranium and so orbital factors, and the cis effect, should be diminished

Summary

Introduction

FEUDAL (f’s essentially unaffected, d’s accommodate ligands) is a longstanding bonding model in actinide chemistry, in which metal-ligand binding uses 6d-orbitals, with the 5f remaining non-bonding. This model advances the notion that actinides bind primarily using their d-orbitals and the f-orbitals remain mainly non-bonding[34,35] This view seems to hold for ions like uranium when bonded to expansive ligands that have low angular requirements, e.g. C5–8-arenes[36,37,38,39,40,41], but in recent years this has been increasingly challenged when small ligands with more acute angular requirements, e.g. nitrides, are considered[42,43]. The former is called overlap-driven covalency, and it is on this aspect that discussions in this paper will focus

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