Di-tungsten tetra(hpp)

Di-tungsten tetra(hpp) or W₂(hpp)₄ is a di-tungsten paddlewheel compound with four hpp or hexahydropyrimidopyrimidine ligands. This compound has the lowest ionization energy (3.51 eV) of all stable chemical elements or chemical compounds measured to date. [cite journal | title = Expeditious Access to the Most Easily Ionized Closed-Shell Molecule, W₂(hpp)₄ | author = F. Albert Cotton, James P. Donahue, Dennis L. Lichtenberger, Carlos A. Murillo, and Dino Villagrán | journal = J. Am. Chem. Soc. | volume = 127 | issue = 31 | pages = 10808 -10809 | year = 2005 | doi = 10.1021/ja0535458] This value is even lower than of caesium with 3.89 eV (or 375 kJ/mol) located at the extreme left lower corner of the periodic table (although francium is at a lower position in the periodic table compared to caesium, it has a higher ionization energy and is radioactive) or known metallocene reducing agents such as the cobaltocene di(tetramethylcyclopentadienyl)cobalt with 4.71 eV.

This organometallic compound is prepared by conversion of tungsten hexacarbonyl with 1,3,4,6,7,8-hexahydro-2H-pyrimido [1,2-a] pyrimidine (Hhpp) in o-dichlorobenzene at 200 °C to W₂(hpp)₄Cl₂. Dichlorobenzene provides the chlorine atoms and is itself reduced to dichlorobiphenyl. The tungsten to tungsten bond in this intermediate has an order of three. The final complex has bond order of 4, a quadruple bond, so that every tungsten atom obeys the 18 electron rule (8 (2 e⁻ x 4 from shared quadruple bond)+ 16 electrons donated (8 nitrogens x 2 e⁻) and 12 electrons (6 e⁻ x 2 W atoms) = 36. 36/2 = 18 per W) and has a closed shell configuration. The dichloride is stable and easy to handle. It can be further reduced by potassium metal to W₂(hpp)₄. Because of its low ionization energy it can easily oxidized back to the dichloride by compounds such as dichloromethane. It also reacts readily with fullerene and with tetracyanoquinodimethane.

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