Compounds of Noble Gases

Any attempts to prepare compounds of noble gases were put to a halt by the success of the valency theory relating to a stable octet configuration of noble gases. 

The first real compound of noble gases was made by Bartlett and Lohman at the University of British Columbia in 1962. Bartlett was studying the properties of platinum hexafluoride, an extremely powerful oxidising agent. Oxygen gets oxidised by it and the result is 

The product was isolated. Bartlett noticed that the ionisation energy of (1165 kJmol ) to be very close to that of xenon (1170 kJmol ), hence xenon was expected to react with PtF. He mixed equivolumes of the two and inunediately a yellow solid was obtained. The product was incorrectly thought to be Xe (PtFJ . The structure of the product was later proved to be wrong. However, the fact that xenon has reacted opened up a lot of research activities in that direction. 

Soon after, formation of xenon tetrafluoride was reported. 

Note that diatomic oxygen is being oxidized by the platinum hexafluoride. The latter must indeed be a strong oxidizing agent. 

It occurred to Bartlett that the ionization energies of the oxygen molecule (1180 kj/mol) and the xenon atom (1167 kj/mol) were remarkably similar. He decided to try the same reaction as above with xenon replacing the diatomic oxygen. He prepared known volumes of xenon (in slight excess) and platinum hexafluoride and carefully noted the pressure of each. When he allowed the two gases to mix. 

Further investigation demonstrated that the reaction was more complicated than Bartlett had originally thought. In fact, this reaction is still not well-understood. Equation (19.3) is probably more representative of what happens when xenon and platinum hexafluoride come together  

The exact nature of this reaction, though important, was not as significant as the fact that it was no longer correct to regard xenon as an inert gas. Noble perhaps it 

A flurry of activity followed Bartlett s announcement of the preparation of the first xenon compound. Only a few months later, a group at Argonne National Laboratory was able to prepare xenon tetrafluoride by direct reaction of the elements. They placed a 1 5 Xe/F2 ratio of the gases in a nickel container, and, after an hour at 400°C and 6 atm, the xenon was completely consumed. They sublimed the mixture to produce brilliant colorless crystals of XeF4. The reaction is summarized in Equation (19.4)  

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For related coordination compounds of noble gases, see Bieske and Maier (1992). 

Despite xenon s relatively high reactivity (which isn t really high at all) the Group 18 elements 2ire considered unreactive monatomic gases. The only other noble gas to form any compound is krypton, which can form a very unstable difluoride binary compound. Compounds of noble gases, typically that of xenon, have oxidation states of +2, +4, +6, and +8. 

Bartlett N (1962) Xenon hexafluoroplatinate Xe [PtF6]. Proc Chem Soc London 218 Bartlett N (1963) New compounds of noble gases the fluorides of xenon and radon. Amer Scientist 51 114-118... 

Some atoms take on more than a full octet s worth of electrons. These atoms are said to be hypervalent or hypercoordinated. The phosphorus of phosphorus pentachloride, PCI5, is an example. These kinds of situations require an atom from Period (row) 3 or higher within the periodic table. The exact reasons for this restriction are still debated. Certainly, the larger atomic size of these atoms allows room to accommodate the bulk of all the binding partners that distribute around the central atom s valence shell. In some cases, even noble gases like xenon (Xe) form compounds. Because noble gases already have a filled valence shell, they automatically violate the octet rule. 

Xenon occurs in the atmosphere to the extent of approximately 0.00087%, making it the least abundant of the rare of noble gases in the atmosphere. In terms of abundance, xenon does not appear on lists of elements in the earth s crust because it does not exist in stable compounds under normal conditions. However, xenon because of its limited solubility in HjO. is found in seawater to the extent of approximately 950 pounds per cubic mile (103 kilograms per cubic kilometer). Commercial xenon is derived from air by liquefaction and fractional distillation. There are nine... 

Before leaving the NgMX complexes and the compound HArF introduced in Section 5.8.1, it is noted that a fuller discussion on the chemistry of noble gases may be found in Section 17.5, including a subsection on gold-xenon complexes in Section 17.5.6. 

Gmelin Handbook of Inorganic Chemistry, System No. 68, Platinum. Main Volume, (a) Part A in Sections 1-6, 1938-1951 (Platinum Metals, Occurrence, History, Preparation, Alloys) (b) Part B The Element in Sections 1-4,1939-1942, Physical and Electrochemical properties of Platinum (Chemical reactions of Pt in Section 4) (c) Part C The Compounds of Platinum. Section 1. Compounds with Noble Gases, H, O, N, Halogens, S, Se, Te, B, C,Si, P, As, Sb, Bi, 1939 (d) Section 2. Compounds with the alkali metals and ammonium, mainly the alkali metal platinum double salts, 1940 . (e) Section 3 Other Compounds, 1940 (f) Part D Complexes with Neutral ligands, 1957. Supplement Volume (g) Part A. Section 1. Technology of Platinum Metals. 1986 (h) Section 2. Isotopes, Atoms, Molecules and Clusters. 1989. 

The term noble gas refers to elements in Group 18 (VIIIA) of the periodic table. The periodic table is a chart that shows how chemical elements are related to each other. These gases have been given the name noble because they act as if they are too arrogant to react with other elements. Until the 1960s, no compound of these gases was known. Since they are so inactive, they are also called the inert gases. Inert means inactive. 

It can be seen from Table 3 that the dissipation of noble gases was particularly intense. This is reasonable, since these gases did not react with the solid or liquid materials forming the planet Earth. On the other hand the loss in the case of hydrogen was relatively small, a significant portion of this element remained in a condensing compound (water). 

Other elements but metals may also be enclosed in the cavities of fuUerenes. Atoms of noble gases are particularly apt to this, but compounds with molecular hydrogen or nitrogen atoms included in the cage are known as weU. 

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