Inert gas structure

Anions formed from single atoms all have the inert gas structures Is or wp . On the other hand, cations are formed when a metal loses one, two, three or even four electrons. Various electron configurations occur (see Table 10). 

Earliest ideas on covalence were based largely on the view that the atom under consideration attained an inert gas structure by sharing electrons. Faced with such compounds as PCI5 and SF, in which sulphur and phosphorus share 10 and 12 electrons respectively, Sidgwick added the suggestion that every element had a certain maximum covalency which depended on atomic number (see Table 12). 

These elements all have a singly occupied s orbital outside a closed shell of inert gas structure. Owing to the screening of the nucleus by the closed shell the valence electron is very weakly bound. The elements therefore exist as metals, in which the valence electrons are pooled (p. 112), and, in combination with electronegative elements, tend to form ionic solids. Only the unipositive or M ion occurs. 

The first two elements of Gp.III, although they have the configuration ns npi, are effectively tervalent since promotion to ns np occurs very readily. They form cations with an inert-gas structure much less readily than the elements of Gp.II which precede them, and their bonding is predominantly covalent. The covalent and ionic radii are given in Table 49. 

Positively charged atoms like Na+ are known as cations, while negatively charged ions like Cl are called anions. Thus, metals whose atoms have one, two or three electrons more than an inert gas structure form monovalent (e.g. potassium, K+), divalent (e.g. calcium, Ca2+) or trivalent (e.g. aluminium, Al3+) cations. 

It will be remembered that ionic compounds are formed by the electrovalent combination of elements, the atoms of which have a few outer-layer electrons in excess of an inert gas structure, with elements whose atoms have outer layers which need corresponding numbers of electrons in order to acquire a stable inert gas structure. The most stable ionic compounds are therefore formed between the strongly metallic elements on the left-hand side of the periodic table and th( definitely non-metallic elements of the extreme right. There are, however, on the right-hand side of the4 periodic table a number of elements with atoms which are deficient in outer-layer electrons but, which ere not strongly non-metallic in character, e.g. sulphur, selenium and tellurium ((Jroup VI) phosphorus,... 

The reader will already be familiar with the structure of the periodic table and with how it is interpreted in terms of adding electrons successively into the various orbitals. The carbon diagram was shown in Figure 1.9, When a total of 10 electrons has been added, the Is, 2s, and 2p orbitals are completely filled, and we have the inert gas structure of neon. An additional 8 electrons, making a total of 18 electrons, gives the inert gas structure of argon. Later, 3d and 4/ levels are filled. The reader is referred to textbooks of general chemistry for further details. 

This difference in behaviour of the shift of ultra-violet absorption on melting is borne out by other characteristics of the ultraviolet absorption bands. For melts that appear to contain association complexes, these bands tend to be narrower and may have absorption maxima of intensity comparable with that in the crystals. This contrasts with melts of ions with inert gas structures, but conforms with expectations for association complexes in which the packing, though of lower symmetry, is somewhat tighter than in the crystals. 

The bicyclo[4,2,0]octa-2,4,7-trienyl iron carbonyl complexes (285) and (286) are stable, even in refluxing benzene, in marked contrast to the rapid valence isomerization of free bicyclo[4,2,0]octa-2,4,7-triene to cyclo-octatetraene. This stability is due to the loss of the inert gas configuration on rearrangement to a diTiapto-cyclo-octatetra-ene-Fe(CO)3 complex. The inert gas structure could be retained if the cyclobutene ring of the complexes opened via a forbidden disrotatory opening, but this does not occur because the metal is not in a position to help sterically. ... 

Hock and Mills, in 1958, established that the best representation of the complex obtained from 2-butyne was as shown in formula (CIV) (6, 118). One iron atom and two of its three carbonyl ligands are very nearly coplanar with the four-carbon chain and its two methyl substituents. The other iron atom is approximately equidistant from the four ring carbon atoms and is apparently analogous to that in the butadiene-Fe(CO)3 complexes. An Fe—Fe bond, which would preserve the inert gas structure of the ring Fe atom, is assumed, the distance being 2.49 A. The molecular orbital structure of this system has been discussed 119). 

If the iron atoms attain the inert gas structure, spin-pairing by an Fe-Fe interaction seems probable p-BrC6H4C CH forms similar complexes [52a]. Yet another product formed from acetylene and Fe3(CO)i2 in light petroleum is the isomeric complex (C2H2)3Fe2(CO) [74], which... 

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