Noble-gas cations

Think About It For ions that are isoelectronic with noble gases, cations should have no dots remaining around the element symbol, whereas anions should have eight dots around the element symbol. Note, too, that we put square brackets around the Lewis dot symbol for an ion and place the negative charge outside the brackets. 

SiH3+ ions are easily formed in the reactions of noble gas cations and other small ions with SiFLt via dissociative charge transfer (equation l)11. 

The review of Selig and Holloway [230] on noble-gas cations and anions and the general reviews of Holloway [231] and that of Schrobilgen and Whalen [232] on noble-gas compounds should also be consulted. 

In addition to UV/visible flash photolysis and TRIR spectroscopy, other techniques have been used for the detection of transition metal-noble gas interactions in the gas phase. The interaction of noble gases with transition metal ions has been studied in detail. A series of cationic dimeric species, ML" " (M = V, Cr, Fe, Co, Ni L = Ar, Kr, or Xe), have been detected by mass-spectroscopic methods (55-58). It should be noted that noble gas cations L+ are isoelectronic with halogen atoms, therefore, this series of complexes is not entirely unexpected. The bond dissociation energies of these unstable complexes (Table IV) were determined either from the observed diabatic dissociation thresholds obtained from their visible photodissociation spectra or from the threshold energy for collision-induced dissociation. The bond energies are found to increase linearly with the polarizability of the noble gas. 

Only one set of the MP2 level calculations is shown. Several other gold-noble gas cations were calculated in this study. 

Table 2. Dissociation energies D of diatomic noble gas cations NgNg (in kcal/mol)...

The principle of donor-acceptor interactions in noble gas cations is further exemplified by the theoretical study of Radom and coworkers [95a,c] on the series He C" . Figure 12 shows the optimized geometries for the four cations. The intriguing result of the theoretical studies is the rather short He,C bond length for the triply and quadruply charged species HejC and He4C . The latter molecule, which is isoelectronic with methane CH4, has also been calculated by Schleyer [90d]. and are explosive molecules with... 

Noble gas cationic salts of xenon and krypton have also been isolated from superacid medium. The examples include XeF+, Xe2F, HCNXeF+, XeOE, KrF+, and KT2F3+. 

Schrobilgen published the first structural evidence for this bond type (82), In the intervening time, the majority of new compounds with a xenon-nitrogen bond have resulted from the exploitation of the Lewis acid properties of noble gas cations by Schrobilgen and co-workers (83-88). These workers have further exploited these properties to prepare the first compounds with a krypton-nitrogen bond (87,89). The majority of these new Xe-N and Kr-N compounds are solution species they have been characterized predominately by multinuclear NMR and vibrational spectroscopy and are generally too unstable to be isolated in a pure form. 

To date there is no evidence that sodium forms any chloride other than NaCl indeed the electronic theory of valency predicts that Na" and CU, with their noble gas configurations, are likely to be the most stable ionic species. However, since some noble gas atoms can lose electrons to form cations (p. 354) we cannot rely fully on this theory. We therefore need to examine the evidence provided by energetic data. Let us consider the formation of a number of possible ionic compounds and first, the formation of sodium dichloride , NaCl2. The energy diagram for the formation of this hypothetical compound follows the pattern of that for NaCl but an additional endothermic step is added for the second ionisation energy of sodium. The lattice energy is calculated on the assumption that the compound is ionic and that Na is comparable in size with Mg ". The data are summarised below (standard enthalpies in kJ) ... 

Whether an element is the source of the cation or anion in an ionic bond depends on several factors for which the periodic table can serve as a guide In forming ionic compounds elements at the left of the periodic table typically lose electrons giving a cation that has the same electron configuration as the nearest noble gas Loss of an elec tron from sodium for example yields Na which has the same electron configuration as neon... 

Transfer of an electron from a sodium atom to a chlorine atom yields a sodium cation and a chloride anion both of which have a noble gas electron configuration... 

Several metals that are farther removed from the noble gases in the periodic table form positive ions. These include the transition metals in Groups 3 to 12 and the post-transition metals in Groups 13 to 15. The cations formed by these metals typically have charges of +1, +2, or +3 and ordinarily do not have noble-gas structures. We will postpone to Chapter 4 a general discussion of the specific charges of cations formed by these metals. 

Cations, anions, and atoms with ground state noble-gas electron cnnfigurations. Atoms and ions shown in the same color are isoelectronic tint. is. they have the same electron configiratiorts. 

Noble gas An element in group 18 at the far right of the periodic table, 31 anions, 38 cations, 38 characterization, 190 structures, 150-151... 

Cations with noble gas configurations. The alkali metals, alkaline earths and aluminium belong to this group which exhibit Class A acceptor properties. Electrostatic forces predominate in complex formation, so interactions... 

FIGURE 2.2 When a main-group metal atom forms a cation, it loses its valence s-and p-electrons and acquires the electron configuration of the preceding noble-gas atom. The heavier atoms in Croups 1 S/lll and 14/IV retain their complete subshells of d-electrons. 

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