Noble gases stable states

An excimer is a dimer which is stable only in an excited electronic state but dissociates readily in the ground state. Examples of these are the noble gas dimers such as Hc2, discussed in Section 7.2.5.6. This molecule has a repulsive ground state but a bound... 

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. 

The heavier nonmetals may be expected to make some use of the less stable orbitals of the outermost shell (3d for P, S, Cl 4d for As, Se, Br etc-), as is indicated by the existence of compounds such as PCI and SFe, in which the central atom forms more bonds than permitted by the use of orbitals occupied by electron pairs in the adjacent noble gas. In our earlier discussion of the structure of PCI it was pointed out that a rough quantum-mechanical treatment leads to the conclusion that the structure in which the phosphorus atom forms five covalent bonds, with use of one 3d orbital in addition to the 3 and three 3d orbitals, makes a significant contribution to the normal state of the molecule (about 8 percent). 

In 1915, G. N. Lewis proposed several new theories describing how atoms bond together to form molecules. One of these theories states that a filled shell of electrons is especially stable, and atoms transfer or share electrons in such a way as to attain a filled shell of electrons. A filled shell of electrons is simply the electron configuration of a noble gas, such as He, Ne, or Ar. This principle has come to be called the octet rule... 

Exciplex lasers (also called excimer lasers) use reactive halogen atoms to form excited pseudo-molecules with noble-gas atoms. Molecules such as XeF are stable on ly in excited electronic states and quickly dissociate after transition to the ground state. This makes possible a large population inversion and produces laser action in the ultraviolet region. A simple prototype for such behavior is the Hcj excimer, which is an entry in Table 11.1. 

This has allowed us to identify, for the first time in solution at room temperature, organometallic noble gas complexes which are formed following irradiation of metal carbonyls in supercritical noble gas solution. We have found that these complexes are surprisingly stable and have reactivity comparable to organometallic alkane complexes. In addition, we have studied the co-ordination of COj to metal centres in supercritical CO2 (scCOj) and shown that v(C-O) bands provide a very sensitive probe for the oxidation state of the metal centre. We found evidence, albeit circumstantial, for the formation and reactivity of ri -O bound metal COj complexes in solution at or above room temperature and found these highly reactive COj complexes have similar reactivity to the analogous Xe complexes [11-12]. We have also used TRIR to examine the reactivity of CpMofCO), radicals in scCOj and found evidence for an interaction, possibly Lewis Acid/Base, between CpMo(CO), and scCO [13]. 

Note that the sodium ion has the same electron configuration as neon (ls 2s 2p ), a noble gas. This observation leads to one of the most important principles in chemistry, the octet rule. The octet rule states that atoms tend to gain, lose, or share electrons in order to acquire a full set of eight valence electrons. This reinforces what you learned earlier that the electron configuration of filled s and p orbitals of the same energy level (consisting of eight valence electrons) is unusually stable. Note that the first period elements are an exception to the rule, as they are complete with only two valence electrons. 

The octet rule states that atoms gain, lose, or share electrons in order to acquire the stable electron configuration of a noble gas. 

---