Uranocene
Encyclopedia
Uranocene U2 is the most notable cyclooctatetraenide
of the f elements
and one of the first organouranium compounds
to be synthesized. Uranocene is a member of the actinocenes, a group of metallocene
s incorporating elements
from the actinide
series. It is the most studied bis[8]annulene
-metal system.
and dipotassium cyclooctatetraenide
, viz.
, and stable to hydrolysis
. 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
containing the uranium
atom. In the solid state, the rings are eclipsed, conferring D8h symmetry on the uranocene molecule. In solution the rings rotate with a low energy barrier.
. Its measured magnetic susceptibility
was found to be consistent with values of 3 or 4 for the magnitude of the (approximate) quantum number
for angular momentum
around the 8-fold axis. The NMR
spectrum of uranocene was analyzed and found to be consistent with an angular momentum quantum number of 3. Electronic theory calculations from the simplest to the most accurate have always given angular momentum quantum numbers of 3 for the ground state and 2 for the first excited state, corresponding to symmetry designations of E3g and E2g for these states.
. Raman spectra
, in addition to finding vibrational frequencies, found the low-lying excited electronic state. It was noted that one of the visible bands originated in this excited state rather than the ground state. Electronic theory calculations showed that the visible transitions are to two excited states, with symmetry labels E2u and E3u, both of which have major components with 5f-to-6d orbital
excitation with smaller components of ligand-pi-to-metal-5f excitation.
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 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 the angular momentum quantum number of the ground state.
, Tb
, Pu
, Pa
, Np
, Th
, and Yb
). Extensions include the air-stable derivative U(C8H4Ph4)2 and the cycloheptatrienyl species [U(C7H7)2]−.
Cyclooctatetraene
1,3,5,7-Cyclooctatetraene is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as [8]annulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature...
of the f elements
F-block
The f-block of the periodic table of the elements consists of those elements whose atoms or ions have valence electrons in f-orbitals. Actual electronic configurations may be slightly different from what is predicted by the Aufbau principle...
and one of the first organouranium compounds
Organouranium chemistry
Organouranium chemistry is the science exploring the properties, structure and reactivity of organouranium compounds, which are organometallic compounds containing a carbon to uranium chemical bond...
to be synthesized. Uranocene is a member of the actinocenes, a group of metallocene
Metallocene
A metallocene is a compound typically consisting of two cyclopentadienyl anions bound to a metal center in the oxidation state II, with the resulting general formula 2M. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride, vanadocene dichloride...
s incorporating elements
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
from the actinide
Actinide
The actinide or actinoid series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.The actinide series derives its name from the group 3 element actinium...
series. It is the most studied bis[8]annulene
Annulene
Annulenes are completely conjugated monocyclic hydrocarbons. They have the general formula CnHn or CnHn+1...
-metal system.
Synthesis
Uranocene was first prepared by the reaction of uranium tetrachlorideUranium tetrachloride
Uranium tetrachloride is compound of uranium in oxidation state +4. It was used in the electromagnetic isotope separation process of uranium enrichment. It is one of the main starting materials for organouranium chemistry.- Synthesis :...
and dipotassium cyclooctatetraenide
Cyclooctatetraene
1,3,5,7-Cyclooctatetraene is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as [8]annulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature...
, viz.
- 2 K + C8H8 → K2(C8H8)
- 2 K2(C8H8) + UCl4 → U(C8H8)2 + 4 KCl.
Structure and chemical properties
Uranocene is pyrophoricPyrophoricity
A pyrophoric substance is a substance that will ignite spontaneously in air. Examples are iron sulfide and many reactive metals including uranium, when powdered or sliced thin. Pyrophoric materials are often water-reactive as well and will ignite when they contact water or humid air...
, and stable to hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
. Considering the molecule to be U4+(C8H82-)2, the η8
Hapticity
The term hapticity is used to describe how a group of contiguous atoms of a ligand are coordinated to a central atom. Hapticity of a ligand is indicated by the Greek character 'eta', η. A superscripted number following the η denotes the number of contiguous atoms of the ligand that are bound to...
-cyclooctatetraenide groups are planar, as expected for a ring containing 10 π-electrons
Conjugated system
In chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in compounds with alternating single and multiple bonds, which in general may lower the overall energy of the molecule and increase stability. Lone pairs, radicals or carbenium ions may be part of the...
, and are mutually parallel, forming a sandwich
Sandwich compound
In organometallic chemistry, a sandwich compound is a chemical compound featuring a metal bound by haptic covalent bonds to two arene ligands. The arenes have the formula CnHn, substituted derivatives and heterocyclic derivatives...
containing the uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
atom. In the solid state, the rings are eclipsed, conferring D8h symmetry on the uranocene molecule. In solution the rings rotate with a low energy barrier.
Magnetic properties and angular momentum
Uranocene is paramagneticParamagnetism
Paramagnetism is a form of magnetism whereby the paramagnetic material is only attracted when in the presence of an externally applied magnetic field. In contrast with this, diamagnetic materials are repulsive when placed in a magnetic field...
. Its measured magnetic susceptibility
Magnetic susceptibility
In electromagnetism, the magnetic susceptibility \chi_m is a dimensionless proportionality constant that indicates the degree of magnetization of a material in response to an applied magnetic field...
was found to be consistent with values of 3 or 4 for the magnitude of the (approximate) quantum number
Quantum number
Quantum numbers describe values of conserved quantities in the dynamics of the quantum system. Perhaps the most peculiar aspect of quantum mechanics is the quantization of observable quantities. This is distinguished from classical mechanics where the values can range continuously...
for angular momentum
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...
around the 8-fold axis. The NMR
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
spectrum of uranocene was analyzed and found to be consistent with an angular momentum quantum number of 3. Electronic theory calculations from the simplest to the most accurate have always given angular momentum quantum numbers of 3 for the ground state and 2 for the first excited state, corresponding to symmetry designations of E3g and E2g for these states.
Visible spectrum
The green color of uranocene is due to three strong transitions in its visible spectrumVisible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
. Raman spectra
Raman spectroscopy
Raman spectroscopy is a spectroscopic technique used to study vibrational, rotational, and other low-frequency modes in a system.It relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range...
, in addition to finding vibrational frequencies, found the low-lying excited electronic state. It was noted that one of the visible bands originated in this excited state rather than the ground state. Electronic theory calculations showed that the visible transitions are to two excited states, with symmetry labels E2u and E3u, both of which have major components with 5f-to-6d orbital
Atomic orbital
An atomic orbital is a mathematical function that describes the wave-like behavior of either one electron or a pair of electrons in an atom. This function can be used to calculate the probability of finding any electron of an atom in any specific region around the atom's nucleus...
excitation with smaller components of ligand-pi-to-metal-5f excitation.
Uranium-COT bond
The uranium-cyclooctatetraenyl bondingChemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
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 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 the angular momentum quantum number of the ground state.
Analogous compounds
Some examples of analogous compounds of the form M(C8H8)2 exist for M = (NdNeodymium
Neodymium is a chemical element with the symbol Nd and atomic number 60. It is a soft silvery metal that tarnishes in air. Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach. It is present in significant quantities in the ore minerals monazite and bastnäsite...
, Tb
Terbium
Terbium is a chemical element with the symbol Tb and atomic number 65. It is a silvery-white rare earth metal that is malleable, ductile and soft enough to be cut with a knife...
, Pu
Plutonium
Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...
, Pa
Protactinium
Protactinium is a chemical element with the symbol Pa and atomic number 91. It is a dense, silvery-gray metal which readily reacts with oxygen, water vapor and inorganic acids. It forms various chemical compounds where protactinium is usually present in the oxidation state +5, but can also assume...
, Np
Neptunium
Neptunium is a chemical element with the symbol Np and atomic number 93. A radioactive metal, neptunium is the first transuranic element and belongs to the actinide series. Its most stable isotope, 237Np, is a by-product of nuclear reactors and plutonium production and it can be used as a...
, Th
Thorium
Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
, and Yb
Ytterbium
Ytterbium is a chemical element with the symbol Yb and atomic number 70. A soft silvery metallic element, ytterbium is a rare earth element of the lanthanide series and is found in the minerals gadolinite, monazite, and xenotime. The element is sometimes associated with yttrium or other related...
). Extensions include the air-stable derivative U(C8H4Ph4)2 and the cycloheptatrienyl species [U(C7H7)2]−.
Further reading
- The f elements, Nikolas Kaltsoyannis and Peter Scott. ISBN 0-19-850467-5
- Chemistry of the Elements, N. N. Greenwood and A. Earnshaw. ISBN 0-08-022057-6
- Lanthanides & Actinides: Organoactinides