title: Closed-shell molecules that ionize more readily than cesium.
abstract: We report a class of molecules with extremely low ionization enthalpies, one member of which has been determined to have a gas-phase ionization energy (onset, 3.51 electron volts) lower than that of the cesium atom (which has the lowest gas-phase ionization energy of the elements) or of any other known closed-shell molecule or neutral transient species reported. The molecules are dimetal complexes with the general formula M2(hpp)4 (where M is Cr, Mo, or W, and hpp is the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine), structurally characterized in the solid state, spectroscopically characterized in the gas phase, and modeled with theoretical computations. The low-energy ionization of each molecule corresponds to the removal of an electron from the delta bonding orbital of the quadruple metal-metal bond, and a strong interaction of this orbital with a filled orbital on the hpp ligands largely accounts for the low ionization energies.
authors.affiliation: Department of Chemistry and Laboratory for Molecular Structure and Bonding, Texas A&M University, Post Office Box 30012, College Station, TX 77842-3012, USA. cotton@tamu.edu
authors.forename: F Albert
authors.initials: FA
authors.lastname: Cotton
authors.forename: Nadine E
authors.initials: NE
authors.lastname: Gruhn
authors.forename: Jiande
authors.initials: J
authors.lastname: Gu
authors.forename: Penglin
authors.initials: P
authors.lastname: Huang
authors.forename: Dennis L
authors.initials: DL
authors.lastname: Lichtenberger
authors.forename: Carlos A
authors.initials: CA
authors.lastname: Murillo
authors.forename: Laura O
authors.initials: LO
authors.lastname: Van Dorn
authors.forename: Chad C
authors.initials: CC
authors.lastname: Wilkinson