This Feature Article seeks to present the current state of knowledge, both experimental and theoretical, of the electronic structure and bonding in actinyl ions and related species, such as the isoelectronic imido compounds as well as in linear triatomic actinide molecules of the type X-An-Y.

Electronic Structure and Bonding in Actinyl Ions and their Analogs

Luminescence of inorganic solids: From isolated centres to concentrated systems

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Lecture notes in Chemistry

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

N-Alkyl-N-methylpyrrolidini- um cations have been used for the design of ionic liquid crystals, including a new type of uranium-containing me- A Pyrrolidinium salts with bromide, bis(trifluoromethylsulfonyl)- A tetrafluoroborate, hexafluoro- phosphate, thiocyanate, tetrakis(2- thenoyltrifluoroacetonato)europate(III) and tetrabromouranyl counteranions were prepared. For the bromide salts and tetrabromouranyl compounds, the chain length of the alkyl group CnH2n + 1 was varied from eight to twenty carbon atoms (n = 8, 10-20). The compounds show rich mesomorphic behaviour: highly ordered smectic phases (the crystal smectic E phase and the uncom- mon crystal smectic T phase), smectic A phases, and hexagonal columnar phases were observed, depending on chain length and anion. This work gives better insight into the nature and formation of the crystal smectic T phase, and the molecular requirements for the appearance of this highly or- dered phase. This uncommon tetrago- nal mesophase is thoroughly discussed on the basis of detailed powder X-ray diffraction experiments and in relation to the existing literature. Structural models are proposed for self-assembly of the molecules within the smectic layers. In addition, the photophysical properties of the compounds contain- ing a metal complex anion were inves- tigated. For the uranium-containing mesogens, luminescence can be in- duced by dissolving them in an ionic liquid matrix. The europium-containing compound shows intense red photolu- minescence with high colour purity.

Pyrrolidinium ionic liquid crystals.

The low-temperature single-crystal optical absorption spectrum of Cs2UO2Cl4 is reported in six different polarizations Measurements have also been made on the 18O compound Zeeman-effect measureme

The electronic structure of the uranyl ion: Part I. The electronic spectrum of Cs2UO2Cl4

The experimental spectra of the uranyl ion, in tetragonal and trigonal equatorial fields, are compared with an empirical model of the excited states arising from four possible configurations of the...

The electronic structure of the uranyl ion: III. Theory

Polarized single crystal absorption spectra at 4·2 K are reported for CsUO2(NO3)3 and NaUO2(CH3COO)3 Natural and magnetic circular dichroism and Zeeman effect measurements have been made Measurements have also been made on crystals containing U18O2 2+ In the portion of the spectrum between 21 000 and 30 000 cm-1 seven electronic states have been identified in both CsUO2(NO3)3 and NaUO2(CH3COO)3 In the majority of cases the symmetry of the excited states has been assigned

The electronic structure of the uranyl ion

Polarized high resolution absorption spectra at 4·2 K are reported for single crystals of Cs2UO2Cl4. An analysis of the polarization of the electronic origin bands and the vibronic structure provides evidence for a transition which is allowed by a pure electric-quadrupole mechanism.

D.R. Woodwark

Papers

The electronic structure of the uranyl ion: Part I. The electronic spectrum of Cs2UO2Cl4

The electronic structure of the uranyl ion: III. Theory

The electronic structure of the uranyl ion

Observation of a pure electric quadrupole mechanism for a solid-state electronic transition