Covalency in f-element complexes

Single-molecule magnets (SMMs) are compounds that, below a blocking temperature, exhibit stable magnetization purely of molecular origin, and not caused by long-range ordering of magnetic moments in the bulk. They thus show promise for applications such as data storage of ultra-high density. The stability of the magnetization increases with increasing ground-state spin and magnetic anisotropy. Transition-metal SMMs typically possess high-spin ground states, but insufficient magnetic anisotropies. Lanthanide SMMs exhibit large magnetic anisotropies, but building high-spin ground states is difficult because they tend to form ionic bonds that limit magnetic exchange coupling. In contrast, the significant covalent bonding and large spin–orbit contributions associated with uranium are particularly attractive for the development of improved SMMs. Here we report a delocalized arene-bridged diuranium SMM. This study demonstrates that arene-bridged polyuranium clusters can exhibit SMM behaviour without relying on the superexchange coupling of spins. This approach may lead to increased blocking temperatures. Single-molecule magnets (SMMs) are multinuclear clusters whose behaviour typically relies on intramolecular spin-coupling interactions between neighbouring metal ions. A diuranium–arene complex has now been prepared that shows behaviour characteristic of an SMM without relying on this type of superexchange mechanism. This may enable the construction of SMMs that maintain their magnetism at higher temperatures.

A delocalized arene-bridged diuranium single-molecule magnet

The actinyl ions exhibit an unusually robust covalent bond which has a profound influence on their chemistry. Their electronic structure has been unravelled by the use of a variety of optical measurements and by photoelectron spectroscopy, which together establish the composition and role of the valence orbitals. The experimental energy level scheme can be compared with the results of molecular orbital calculations of varying degrees of sophistication, the most successful being SCF calculations incorporating the effects of relativity. An important contribution to the bonding comes from the pseudo-core 6p shell, which is important in determining the linearity of the ions. A new concept, the Inverse trans Influence, is introduced in this context. There is some evidence that (d-orbital π-bonding is more important than σ-bonding. The remarkable strength of the actinyl bond can be attributed to the presence in the valence shell of both f and d metal orbitals, giving each actinide-oxygen bond a formal bond order of three.

Electronic structure and bonding in actinyl ions

Covalency in Ln–Cl bonds of Oh-LnCl6x– (x = 3 for Ln = CeIII, NdIII, SmIII, EuIII, GdIII; x = 2 for Ln = CeIV) anions has been investigated, primarily using Cl K-edge X-ray absorption spectroscopy (XAS) and time-dependent density functional theory (TDDFT); however, Ce L3,2-edge and M5,4-edge XAS were also used to characterize CeCl6x– (x = 2, 3). The M5,4-edge XAS spectra were modeled using configuration interaction calculations. The results were evaluated as a function of (1) the lanthanide (Ln) metal identity, which was varied across the series from Ce to Gd, and (2) the Ln oxidation state (when practical, i.e., formally CeIII and CeIV). Pronounced mixing between the Cl 3p- and Ln 5d-orbitals (t2g* and eg*) was observed. Experimental results indicated that Ln 5d-orbital mixing decreased when moving across the lanthanide series. In contrast, oxidizing CeIII to CeIV had little effect on Cl 3p and Ce 5d-orbital mixing. For LnCl63– (formally LnIII), the 4f-orbitals participated only marginally in covalent bo...

Covalency in lanthanides. An X-ray absorption spectroscopy and density functional theory study of LnCl6(x-) (x = 3, 2).

A parametric model for lanthanide and actinide atomic and crystal energy levels is presented that correlates trends in Hartree–Fock calculations with empirically determined atomic parameters in such a way that predictions for unclassified complex cases can be made from the analysis of simpler ones. When appropriate effective operators, including electrostatic operators up to the third rank for fN configurations and magnetic operators up to the second rank for fN and fNd shells, are used in the parametric Hamiltonian, statistical errors of 10–20 cm−1 are typical for the simpler examples presented.

Parametric model for f-shell configurations. I. The effective-opertor Hamiltonian

The spectrum of U3+:LaCl3 has been analyzed; 53 crystal states of electronic origin have been identified, many of them accompanied by extensive vibronic sidebands having a recognizable structure similar to that for lanthanide‐doped LaCl3. Seventeen energy level parameters were varied simultaneously in a least‐squares adjustment yielding a mean error of 26 cm−1. Values for the crystal field parameters are: B20=260±64 cm−1, B40=−532±139 cm−1, B60=−1438±113 cm−1 and B66=1025±88 cm−1.

Spectrum analysis of U3+:LaCl3

The spectrum of U/sup 3 +/:LaCl/sub 3/ has been analyzed; 53 crystal states of electronic origin have been identified, many of them accompanied by extensive vibronic sidebands having a recognizable structure similar to that for lanthanide-doped LaCl/sub 3/. Seventeen energy level parameters were varied simultaneously in a least-squares adjustment yielding a mean error of 26 cm/sup -1/. Values for the crystal field parameters are: B/sup 2//sub 0/=260 +- 64 cm/sup -1/, B/sup 4//sub 0/=-532 +- 139 cm/sup -1/, B/sup 6//sub 0/=-1438 +- 113 cm/sup -1/ and B/sup 6//sub 6/=1025 +- 88 cm/sup -1/.

Spectrum analysis of U/sup 3 +/:LaCl/sub 3/

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Spectrum analysis of U3+:LaCl3

Spectrum analysis of U/sup 3 +/:LaCl/sub 3/