Tetrakis(hexahydropyrimidinopyrimidine)ditungsten(II), known as ditungsten tetra(hpp), is the name of the

coordination compound A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...

with the formula W₂(hpp)₄. This material consists of a pair of

tungsten Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isol ...

centers linked by the conjugate base of four hexahydropyrimidopyrimidine (hpp)

ligand In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's ele ...

s. It adopts a structure sometimes called a

Chinese lantern structure In chemistry, the Chinese lantern structure is a coordination complex where two metal atoms are bridged by four bidentate ligands. This structure type is also known as a paddlewheel complex. Examples include chromium(II) acetate, molybdenum(II) a ...

or paddlewheel compound, the prototype being

copper(II) acetate Copper(II) acetate, also referred to as cupric acetate, is the chemical compound with the formula Cu(OAc)2 where AcO− is acetate (). The hydrated derivative, Cu2(OAc)4(H2O)2, which contains one molecule of water for each copper atom, is availa ...

. The molecule is of research interest because it has the lowest

ionization energy Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive Electric charge, charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged a ...

(3.51 eV) of all stable

chemical element A chemical element is a species of atoms that have a given number of protons in their nuclei, including the pure substance consisting only of that species. Unlike chemical compounds, chemical elements cannot be broken down into simpler sub ...

s or

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one ele ...

s as of the year 2005. This value is even lower than of

caesium Caesium ( IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that ...

with 3.89 eV (or 375 kJ/mol) located at the extreme left lower corner of the

periodic table The periodic table, also known as the periodic table of the (chemical) elements, is a rows and columns arrangement of the chemical elements. It is widely used in chemistry, physics, and other sciences, and is generally seen as an icon of ...

(although

francium Francium is a chemical element with the symbol Fr and atomic number 87. It is extremely radioactive; its most stable isotope, francium-223 (originally called actinium K after the natural decay chain it appears in), has a half-life of only 22 ...

is at a lower position in the periodic table compared to caesium, it has a higher ionization energy and is

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consi ...

) or known

metallocene A metallocene is a compound typically consisting of two cyclopentadienyl anions (, abbreviated Cp) bound to a metal center (M) in the oxidation state II, with the resulting general formula Closely related to the metallocenes are the metallocene d ...

reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are commonly reducing agents include the Earth m ...

s such as decamethyl

cobaltocene Cobaltocene, known also as bis(cyclopentadienyl)cobalt(II) or even "bis Cp cobalt", is an organocobalt compound with the formula Co(C5H5)2. It is a dark purple solid that sublimes readily slightly above room temperature. Cobaltocene was discover ...

with 4.71 eV.

Preparation

This coordination compound is prepared by the reaction of

tungsten hexacarbonyl Tungsten hexacarbonyl (also called tungsten carbonyl) is the chemical compound with the formula W(CO)6. This complex gave rise to the first example of a dihydrogen complex.Kubas, G. J., Metal Dihydrogen and σ-Bond Complexes, Kluwer Academic/Plenu ...

with 1,3,4,6,7,8-hexahydro-2''H''-pyrimido

,2-''a'' The comma is a punctuation mark that appears in several variants in different languages. It has the same shape as an apostrophe or single closing quotation mark () in many typefaces, but it differs from them in being placed on the baseline of ...

yrimidine (Hhpp) in ''o''-dichlorobenzene at 200 °C: : The reaction gives W₂(hpp)₄Cl₂. Dichlorobenzene provides the chlorine atoms and is itself reductively coupled to 2,2′-dichloro biphenyl. The

bond order In chemistry, bond order, as introduced by Linus Pauling, is defined as the difference between the number of bonds and anti-bonds. The bond order itself is the number of electron pairs ( covalent bonds) between two atoms. For example, in diat ...

between the tungsten centers in W₂(hpp)₄Cl₂ is three. This dichloride is further reduced by

potassium Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosp ...

metal to W₂(hpp)₄. This species has a

quadruple bond A quadruple bond is a type of chemical bond between two atoms involving eight electrons. This bond is an extension of the more familiar types double bonds and triple bonds. Stable quadruple bonds are most common among the transition metals in the m ...

between the two tungsten centers. Related quadruply bonded complexes include ₂Cl₈sup>4− and o₂Cl₈sup>4−. Because of its low ionization energy, W₂(hpp)₄ is easily oxidized back to the dichloride by

dichloromethane Dichloromethane (DCM or methylene chloride, methylene bichloride) is an organochlorine compound with the formula . This colorless, volatile liquid with a chloroform-like, sweet odour is widely used as a solvent. Although it is not miscible wit ...

. It is readily oxidized to the corresponding cation with the oxidants

fullerene A fullerene is an allotrope of carbon whose molecule consists of carbon atoms connected by single and double bonds so as to form a closed or partially closed mesh, with fused rings of five to seven atoms. The molecule may be a hollow sphere, ...

and with

tetracyanoquinodimethane Tetracyanoquinodimethane (TCNQ) is the organic compound with the formula . This cyanocarbon, a relative of para-quinone, is an electron acceptor that is used to prepare charge transfer salts, which are of interest in molecular electronics. Pr ...

Preparation

References

Further reading