Uranocene, U(C₈H₈)₂, is an organouranium compound composed of a uranium atom sandwiched between two cyclooctatetraenide rings. It was one of the first organoactinide compounds to be synthesized. It is a green air-sensitive solid that dissolves in organic solvents. Uranocene, a member of the " actinocenes," a group of metallocenes incorporating elements from the

actinide The actinide () or actinoid () series encompasses at least the 14 metallic chemical elements in the 5f series, with atomic numbers from 89 to 102, actinium through nobelium. Number 103, lawrencium, is also generally included despite being part ...

series. It is the most studied bis nnulene-metal system, although it has no known practical applications.

Synthesis, structure and bonding

Uranocene was first described in 1968 by the group of Andrew Streitwieser, when it was prepared by the reaction of dipotassium cyclooctatetraenide and uranium tetrachloride in THF at 0°C: : Uranocene synthesis

Uranocene is highly reactive toward oxygen, being

pyrophoric A substance is pyrophoric (from , , 'fire-bearing') if it ignites spontaneously in air at or below (for gases) or within 5 minutes after coming into contact with air (for liquids and solids). Examples are organolithium compounds and triethylb ...

in air but stable to

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

. The x-ray crystal structure of uranocene was first elucidated by the group of Ken Raymond. Considering the molecule to be U⁴⁺(C₈H₈²⁻)₂, the η⁸- cyclooctatetraenide groups are planar, as expected for a ring containing 10 π-electrons, and are mutually parallel, forming a

sandwich A sandwich is a Dish (food), dish typically consisting variously of meat, cheese, sauces, and vegetables used as a filling between slices of bread, or placed atop a slice of bread; or, more generally, any dish in which bread serves as a ''co ...

containing the

uranium Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...

atom. In the solid state, the rings are eclipsed, conferring ''D_8h'' symmetry on the molecule. In solution the rings rotate with a low

activation energy In the Arrhenius model of reaction rates, activation energy is the minimum amount of energy that must be available to reactants for a chemical reaction to occur. The activation energy (''E''a) of a reaction is measured in kilojoules per mole (k ...

. The uranium-cyclooctatetraenyl bonding was shown by photoelectron spectroscopy to be primarily due to mixing of uranium 6d orbitals into ligand pi orbitals and therefore donation of electronic charge to the uranium, with a smaller such interaction involving the uranium (''5f'')² orbitals. Electronic theory calculations agree with this result and point out that the weaker interaction of the open-shell ''5f'' orbitals with the ligand orbitals determines , ''M_J'', , the magnitude of the angular momentum quantum number along the 8-fold symmetry axis of the ground state.

Spectroscopic properties

Uranocene is

paramagnetic Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, ...

. Its

magnetic susceptibility In electromagnetism, the magnetic susceptibility (; denoted , chi) is a measure of how much a material will become magnetized in an applied magnetic field. It is the ratio of magnetization (magnetic moment per unit volume) to the applied magnet ...

is consistent with values of 3 or 4 for , ''M_J'', , with the accompanying magnetic moment being affected by the spin-orbit coupling. Its

NMR Nuclear magnetic resonance (NMR) is a physical phenomenon in which atomic nucleus, nuclei in a strong constant magnetic field are disturbed by a weak oscillating magnetic field (in the near and far field, near field) and respond by producing ...

spectrum is consistent with an , ''M_J'', value of 3. Electronic theory calculations from the simplest to the most accurate also give , ''M_J'', values of 3 for the ground state and 2 for the first excited state, corresponding to double-group symmetry designations of ''E_3g'' and ''E_2g'' for these states. The green color of uranocene is due to three strong transitions in its

visible spectrum The visible spectrum is the spectral band, band of the electromagnetic spectrum that is visual perception, visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' (or simply light). The optica ...

. In addition to finding vibrational frequencies, Raman spectra indicate the presence of a low-lying (''E_2g'') excited electronic state. On the basis of calculations, the visible transitions are assigned to transitions primarily of ''5f''-to-''6d'' nature, giving rise to ''E_2u'' and ''E_3u'' states.

Analogous compounds

Analogous compounds of the form M(C₈H₈)₂ exist for M = ( Nd, Tb, Yb, Th, Pa, Np, and Pu). Extensions include the air-stable derivative U(C₈H₄Ph₄)₂ and the cycloheptatrienyl species (C₇H₇)₂sup>−. In contrast, bis(cyclooctatetraene)iron has a very different structure, with one each of a η⁶- and η⁴-C₈H₈ ligands.

Synthesis, structure and bonding

Spectroscopic properties

Analogous compounds

References

Further reading