Brindu

Brenda Vinter 11/01/2007














Noble gases
Compounds

ReviewDiscovered at the end of 1800s noble gases were believed to be unable to form compounds . However , in 1962 when Bartlett isolated the first xenon compound , a new era of noble gases chemistry began .


Introduction

The development of quantum mechanics in the 1920s explained , in theory , the inactivity of noble gases . But even from those early days scientist speculated that inert gases might be able to form compounds with other chemical elements .The first compound reported by Bartlett was XePtF6 [1] , now known to be the ionic salt [XeF]+[PtF6]- .

Compounds with Fluorine and Oxygen

Compounds of xenon chemically bonded to fluoride and oxygen were the first to be synthesised and characterised .
Xenon fluoride (XeF2) was the first to be synthesised shorly after XePtF6 and it is surprising that it remained elusive for a long time since it is fairly simple to be prepared from a gaseous mixture of xenon and fluorine using different types of energy[2] .

F2(g) + Xe(g) XeF2

Other Xe fluorides that have been prepared are XeF4 and XeF6 .

Fig.1 XeF2 and XeF4
http://web1.caryacademy.org/chemistry/rushin/StudentProjects/ElementWebSites/xenon/compounds.htm

Fig.2 XeF6
http://www.3dchem.com/inorganics/XeF6.jpg


XeF2 is one of the most stable and therefore easy to handle noble gas compound and as a result its chemistry is the most extensive . It is a strong Lewis base , a fluorinating and oxidising agent and its cations (XeF+ and Xe2F3+ ) , obtained in anhydrous HF and Lewis acids , are even stronger such agents [3] .

XeF2 XeF+ + F- [4]
2XeF2 Xe2F3+ + F- [5]


Fig.3 V-shaped structure of Xe2F3+ cation in Xe2F3AsF6 [3]



XeF2 readily reacts with metals pentafluorides forming complexes which can be mainly ionic in the case of strong Lewis acids such as SbF5 and AsF5 , or mainly covalent for milder Lewis acids , when XeF2 is coordinated to MF5 via a fluorine bridge .



Fig.4 Structure of XeFAsF6 [6]



XeF2 reacts as well with the weaker Lewis acids tetrafluorides , includind the xenon tetrafluoride ,
XeF4 [7] .

Fig.5 XeF2.CrF4 [8]

Due to its semi ionic character and its small size XeF2 is a very good ligand and a variety of metal complexes have been synthesised , were XeF2 acts as a ligand , either coordinated to one metal centre or as a bridging ligand between two metal centres [3] .

Fig.6 [Mg(XeF2)2](AsF6)2 , an example of metal coordinated by non-bridging XeF2 [9]

Fig.7 [Ba(XeF2)5](AsF6)2 , example of a metal coordinated by bridging XeF2 [3]

Fig. 8 [Ca2(XeF2)9](AsF6)4 , example of a metal coordinated only by XeF2 [10]






Krypton fluorides compounds that have been reported so far are KrF2 and salts of the cations KrF+ and Kr2F3+ ,all thermodynamically unstable , with KrF2 the only krypton compound that has been structurally characterised [11] .
These compounds have similar properties with the xenon fluorides , forming salts with strong fluoride ion acceptors of the type MF6 , where M= As , Sb , Au , Pt , Bi and Ta .





Fig.9 α-KrF2 packaging diagram[11] Fig.10 [Kr2F3][SbF6].KrF2 [11]





There are two known xenon oxides , XeO3 , a solid at room temperature , and XeO4 , a gas at room temperature , both thermodynamically unstable and decompose explosively [12] .

XeO3 XeO4
Fig.11 http://www.3dchem.com/inorganicmolecule.asp?id=817


More versatile are the xenon oxide fluorides XeOF4 and XeO2F2 , which form complexes with strong Lewis acids AsF6 and SbF6 [13] .









Fig.12 XeOF4 Fig.13 XeO2F2
www.apsu.edu/jonesr/Chapter%2010%20Review.pps

Four such complexes have been characterised , α-[XeO2F][SbF6] , β-[XeO2F][SbF6] which exists as a tetramer , [XeO2F][AsF6] and [FO2XeFXeO2F][AsF6] which has a bent F-Xe-F bond [13] .

Fig.14 α-[XeO2F][SbF6] [13] Fig.15 [FO2XeFXeO2F][AsF6] [13]




Fig.16 Tetrameric structure of β-[XeO2F][SbF6] [13]HNgY compouds


Another big group of noble gases compounds are those of the type HNgY , where Y is a fragment with strong electron affinity . This group brought the first xenon-sulfur bond compound , HXeSH ,
a thermodynamically unstable but thermally stable molecule[14] and , most importantly , the first chemically bonded argon compound , HArF [15] .

Fig. 17 Synthetic route of HXeSH [14]

Predicted computationally in 1995 and isolated in 2000 , HArF is a highly polarised , metastable compound . The molecule was experimentally observed in a solid state argon matrix [15] and it was shortly followed by the discovery of HKrF in a solid state krypton matrix[16] .


Fig.18 HKrF in solid Kr a) double-substitutional site and b) single-substitutional site [16]


Other non-metals – noble gas compounds

The first compound of xenon bonded to nitrogen reported in early 70s was FXeN-(SO2F)2 [17] and since a handful of imidodisulfurylfluoride compounds have been characterised such as Xe[N(SO2F)2]2 , F[XeN(SO2F)2]2+ etc . all containing xenon bonded to a sp2 hybridised nitrogen .
Recently a compound with xenon bonded to a sp3 nitrogen has been synthesised as a [AsF6]- salt [18] .

Fig.19 [F5TeN(H)Xe][AsF6] structure [18]

Compounds with krypton-nitrogen bonds such as HC≡ N- KrF+, F3C≡N-KrF+ have been also characterised as salts of [AsF6]- with an octahedral symmetry around the anion [18] .

HArF only opened the first page of argon chemistry . Twelve kinetically stable compounds of noble gases and cyanoacetylene have been reported , four of them with argon . Two types of these compounds were characterised , HNgCCCN and the less stable HNgCCNC [19] .

Fig. 20 Ng-C and Ng-N compounds [19]

Computational research has been carried out on HCCArNC , possibly the first stable neutral compound with an Ar-N bond [19] .

Further theoretical studies have been done on argon-carbon and argon-silicon bonds and two compounds were predicted : FArCCH and FArSiF3 [20] . Computational work showed that the bonding in these compounds is stronger than in HArF , and therefore they are more stable than the only experimentally characterised argon compound [20] . This suggests that a whole family of this type of compounds of argon should be possible to synthesise and that argon chemistry is not science-fiction .









Fig.21 Geometry and electronic density of FArCCH (bond length in Å) [20]



Fig.22 Geometry and electronic density of FarSiF3 (bond length in Å) [20]




Metal – Noble gas bonds

After the discovery of a Au-Xe bond in the bulk compound AuXe42+[SbF11-] [21] a new chapter in the noble gases chemistry opened : noble gases-noble metals compounds . Compounds with linear structure of type NgMF and MNgF have been computationally characterised , including an argon compound , AgArF . The type MNgF have been found to have the strongest nobel gas – metal interaction , with CuXeF the most stable and probably the easiest one yet to be synthesised .



Conclusions

With the chemistry of heavier noble gases , krypton and xenon , well established , the chemistry of the lighter ones , argon and helium , is still at an early stage , while a neon compound is still yet to be identified .


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