Helium hydride ion

From the most recent calculations, it appears probable that the helium hydride ion (HeH)+ is a stable species with dissociation energy between 1 75 and 2 05 eV. Use the method of the discussion at the beginning of this chapter to show why this ion might be a stable species whereas the neutral HeH molecule would not. 

Figure 6.12d The converged condition of the polarized calculation for the helium hydride ion, with the polarizing term turned fully on [ci/c2 set to 1.00 in cells G 24 and H 24],...

This is larger than the corresponding value for Li (57 kJ mol" ) but substantially smaller than the value for F (333kJmol" ). The hydride ion H" has the same electron configuration as helium but is much less stable because the single positive charge on the proton must now control the 2 electrons. The hydride ion is thus readily deformable and this constitutes a characteristic feature of its structural chemistry (see p. 66). 

The noble gas hydride ions should have a bond order of one and tbe diatomic noble gas ions should have a bond order of one-half. Neither type cun be isolated in the form of salts of the type HeH+X or He, X- since the electron affinity of positive helium, etc. is greater than that of any appropriate species X, and so such salts would spontaneously decompose ... 

Protonation of a helium atom gives He-H+, the helium hydride cation, the simplest heteronuclear molecule [60]. Conceptually, of course, this can also be formed by the union of a helium dication and a hydride ion, or a helium cation and a hydrogen atom ... 

The hydrogen atom can gain an electron to form the hydride ion II with the helium electronic configuration... 

According to Cantwell (1956), whose results have been discussed in the previous Section, most of the helium hydride daughter ions are formed in the ground state, and only 11 and 4-5% are formed, respectively, in the first and second excited state. Schwartz s calculations... 

Hydride ion iHt) , an anion of hydrogen possessing the helium electronic configuration. 

The helium hydride two-electron ion has been detected in mass spectra and Szabo and Ostlund (47) report it to be of interest in astrophysics, for example, as the decay product in the /6 emission of the heavy dihydrogen species HT and in proton scattering experiments involving helium atoms. There are, too, accurate calculations by Wolniewicz (84), which suggest an equilibrium bond length of 1.4632 a.u. and an electronic binding energy of 0.0749 a.u. 

Hydrogen is sometimes placed above the halogens in the periodic table because the hydrogen atom can pick up one electron to form the hydride ion, H , which has the same electron configuration as helium. The electron affinity of hydrogen (E = -73 kJ/mol), however, is not as large as tiiat of any halogen the electron affinity of fluorine is —328 kJ/mol, and tiiat of iodine is —295 kj/mol. 

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