Noble gases oxidation numbers

For many species the effective atomic number (FAN) or 18- electron rule is helpful. Low spin transition-metal complexes having the FAN of the next noble gas (Table 5), which have 18 valence electrons, are usually inert, and normally react by dissociation. Fach normal donor is considered to contribute two electrons the remainder are metal valence electrons. Sixteen-electron complexes are often inert, if these are low spin and square-planar, but can undergo associative substitution and oxidative-addition reactions. 

Since the discovery of the first noble gas compound, Xe PtF (Bartlett, 1962), a number of compounds of krypton, xenon, and radon have been prepared. Xenon has been shown to have a very rich chemistry, encompassing simple fluorides, XeF2> XeF, and XeF oxides, XeO and XeO oxyf luorides, XeOF2> XeOF, and Xe02 2 perxenates perchlorates fluorosulfates and many adducts with Lewis acids and bases (Bartlett and Sladky, 1973). Krypton compounds are less stable than xenon compounds, hence only about a dozen have been prepared KrF and derivatives of KrF2> such as KrF+SbF, KrF+VF, and KrF+Ta2F11. The chemistry of radon has been studied by radioactive tracer methods, since there are no stable isotopes of this element, and it has been deduced that radon also forms a difluoride and several complex salts. In this paper, some of the methods of preparation and properties of radon compounds are described. For further information concerning the chemistry, the reader is referred to a recent review (Stein, 1983). 

The effective atomic number rule (the 18-electron rule) was described briefly in Chapter 16, but we will consider it again here because it is so useful when discussing carbonyl and olefin complexes. The composition of stable binary metal carbonyls is largely predictable by the effective atomic number (EAN) rule, or the "18-electron rule" as it is also known. Stated in the simplest terms, the EAN rule predicts that a metal in the zero or other low oxidation state will gain electrons from a sufficient number of ligands so that the metal will achieve the electron configuration of the next noble gas. For the first-row transition metals, this means the krypton configuration with a total of 36 electrons. 

Stable binary ionic compounds are formed from ions that have noble gas configurations. None of the compounds meet this requirement. First of all, C04 is not an ionic compound at all because it is a covalent compound, made from 2 nonmetals. Even so, C04 is not stable because with O2, C would have an oxidation number of +8, which is very unlikely. Consider the following ionic compounds composed of a metal and... 

Iron oxide yellows, 19 399—401 Iron pellets, 14 498—499 Iron pentacarbonyl, 7 591 14 550 16 71 effective atomic number of noble gas, 7 590t... 

At first glance, the standard potentials listed in Table 1 are largely nondescript. All are quite similar, with the possible exception of the sodium couple, which might appear to be anomalously positive. These values are qualitatively consistent with the simple picture that develops upon consideration of the electronic structures of the metals and their oxidized monovalent cations. Each of the metals exhibits an electronic structure that can be symbolized by (noble gas) s, where the principal quantum number (n) ranges from 2 < < 7. For example, the electronic structure for potassium is [Ar]4s, that is, ls 2s 2p 3s 3p 4sk Each of the alkali metals can easily lose one electron to give a stable monovalent metal cation that is isoelectronic with the noble gas... 

For many elements, the atomization efficiency (the ratio of the number of atoms to the total number of analyte species, atoms, ions and molecules in the flame) is 1, but for others it is less than 1, even for the nitrous oxide-acetylene flame (for example, it is very low for the lanthanides). Even when atoms have been formed they may be lost by compound formation and ionization. The latter is a particular problem for elements on the left of the Periodic Table (e.g. Na Na + e the ion has a noble gas configuration, is difficult to excite and so is lost analytically). Ionization increases exponentially with increase in temperature, such that it must be considered a problem for the alkali, alkaline earth, and rare earth elements and also some others (e g. Al, Ga, In, Sc, Ti, Tl) in the nitrous oxide-acetylene flame. Thus, we observe some self-suppression of ionization at higher concentrations. For trace analysis, an ionization suppressor or buffer consisting of a large excess of an easily ionizable element (e g. caesium or potassium) is added. The excess caesium ionizes in the flame, suppressing ionization (e g. of sodium) by a simple, mass action effect ... 

For the group 6 and 7 elements, also the oxide/hydroxide molecules have been synthesized. For elements of group 8, the tetroxide is the species of choice, since this molecule is very volatile. For future studies with the p-elements around atomic number 114 the elements are expected to be volatile in their atomic state and should behave like noble metals or even like a noble gas. 

Xenon is much more reactive, forming a number of different fluorides, fluorocations, fluoroanions, oxides, and oxofluorides. The first noble gas compound to be discovered contained xenon— the orange/yeUow solid Xe+LPtFel" discovered by Bartlett 12) in 1962. The oxidation... 

Elements 104 to 112 are transition elements (6d s to 6d s ). For the first half of these elements high oxidation states are predicted. Elements 112 and 114 are of special interest, because of the relativistic effects of the filled 7s level of 112 and the filled 7pyj sublevel of 114, which give these elements a noble character. The formation of the 7pi/2 sublevel is also expected to influence the oxidation states of elements 115 to 117. With increasing atomic number, the energy differenee between the pi/2 and P3/2 sublevels increases with the result that only the P3/2 eleetrons will be available as valenee electrons. Element 118 should be a noble gas but, due to its low ionization energy, compounds should easily be formed in which this element has the oxidation state IV or VI. Some chemical properties predieted for elements 104 to 121 are summarized in Table 14.7. 

Apparently, epitaxial thin-film model catalysts provide a well-defined initial state for a systematic study of microstructural changes and structure-activity correlations. Model catalysts were prepared for various noble metal-oxide combinations, including Pt, Rh, Ir, Pd, Re supported by Al Oj, SiO, TiO, CeO, VO, Ga Oj, etc. The number density of the metal particles (island density particles per cm ) and their size can be controlled via the NaCl(OOl) substrate temperature during evaporation and the amount of metal deposited (as measured by a quartz microbalance), respectively (Pig. 15.4). 

Xenon is the noble gas element with the richest chemistry. It forms several compounds with fluorine and oxygen, and even compounds with Xe-N and Xe-C bonds. The compound, Xep2, is thermodynamically stable. Xenon is found in compounds with oxidation numbers of+2, +4, +6, and -1-8. 

Semiempirical calculations of free energies and enthalpies of hydration derived from an electrostatic model of ions with a noble gas structure have been applied to the ter-valent actinide ions. A primary hydration number for the actinides was determined by correlating the experimental enthalpy data for plutonium(iii) with the model. The thermodynamic data for actinide metals and their oxides from thorium to curium has been assessed. The thermodynamic data for the substoicheiometric dioxides at high temperatures has been used to consider the relative stabilities of valence states lower than four and subsequently examine the stability requirements for the sesquioxides and monoxides. Sequential thermodynamic trends in the gaseous metals, monoxides, and dioxides were examined and compared with those of the lanthanides. A study of the rates of actinide oxidation-reduction reactions showed that, contrary to previous reports, the Marcus equation ... 

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