Sidgwick rule

The first attempts to interpret Werner s views on an electronic basis were made in 1923 by Nevil Vincent Sidgwick (1873—1952) and Thomas Martin Lowry (1874—1936).103 Sidgwick s initial concern was to explain Werner s coordination number in terms of the sizes of the sub-groups of electrons in the Bohr atom.104 He soon developed the attempt to systematize coordination numbers into his concept of the effective atomic number (EAN).105 He considered ligands to be Lewis bases which donated electrons (usually one pair per ligand) to the metal ion, which thus behaves as a Lewis acid. Ions tend to add electrons by this process until the EAN (the sum of the electrons on the metal ion plus the electrons donated by the ligand) of the next noble gas is achieved. Today the EAN rule is of little theoretical importance. Although a number of elements obey it, there are many important stable exceptions. Nevertheless, it is extremely useful as a predictive rule in one area of coordination chemistry, that of metal carbonyls and nitrosyls. 

Transferability of ionic domains The Lewis S4)-Sidgwick-Powell 66)-Gillespie-Nyholm S6> rule... 

Much important information about the directional character of bonds may be summarized in two statements, the so-called Sidgwick-Powell rules ... 

The nature of metal carbonyls has already been indicated (p. 159). The hexacarbonyls formed by the Group Via elements are white solids whose boiling points lie between 145° C (for Cr(CO)6) and 175° C (for W(CO)e) each of the molecules is octahedral. Since each of the metals precedes a rare gas in the Periodic Table by twelve spaces, these carbonyls are in accord with the Sidgwick-Bailey rule (p. 158). The failure of all attempts to prepare uranium hexacarbonyl, U(CO)e, was taken as an early indication that the period following radon was not an 18-membered period (a good example of reaching a correct conclusion through a somewhat questionable assumption). 

Turning now to the theorem first postulated by Fajans and elaborated much further by Sidgwick and Pauling according to which the heat of formation of organic compounds from free atoms can be represented as the sum of constant contributions characteristic of each chemical link, we note first that the validity of this theorem constitutes no evidence for the existence of a constant heat of formation of the bonds covered by its scope. Take two kinds of bonds formed, say, by carbon with the two different atoms X and Y. Whatever the variation may be in the energies of the C—X and C—Y bonds with the position of the C-atom, no deviation from the additivity rule would result, so long as the variations are equal for both kinds of bonds the additivity rule merely expresses the constancy of the substitution heat — AH, in the reaction... 

Diagrams purporting to show the origin of the electrons required for the various bonds are often given in elementary texts but only for (finite) molecules and complex ions-not for solids. It might help if Sidgwick-type formulae were given for solids such as SnS (in which each atom forms three bonds), if only to show that the rules which apply to finite systems also apply to some at least of the infinite arrays of atoms in crystals ... 

Molecules are also examples of the special stability of filled shells. The simple molecular orbital rule is that a molecule is thermodynamically stable if all the bonding MOs are filled, and all the anti-bonding ones are empty. Other examples are the Octet Rule of Lewis and Langmuir and the 18-Electron Rule of Sidgwick. These relate hardness to stability just as Figure 4.1 does. So do the famous Hiickel rule (4 + 2), and the Wadc Mingos rule (2 2) ... 

The empirical inert gas rule which was first proposed by Sidgwick... 

Since the type of binding has such an influence upon the physical and chemical properties, rules to help us to anticipate whether a given compound will be ionic or covalent will be extremely useful. Such rules, though only qualitative, are extremely valuable in helping to correlate the properties of elements with their valence numbers and position in the periodic table. There are four covalency rules, the first three being due to Fajans and the fourth to Sidgwick. 

In 1937 English chemist Nevil V. Sidgwick suggested a rule (the octet rule for first-row p-block elements) for complex formation tmder which a metal can acquire ligands until the total number of electrons around it is equal to the number surrounding the next noble gas. This rule was later expanded as the eighteen-electron rule under which a d-block transition metal atom has eighteen electrons in its nine valence orbitals [five n d one (n -I-... 

A variety of metal carbonyls are known. The eighteen-electron rule and Sidgwick s effective atomic number rule (Section 2.3) are very successful in explaining their stoichiometries. Simple monomeric carbonyls are expected for transition metals with even atomic numbers Cr(CO)6, Fe(CO)s, Ni(CO)4. The heavier members of the Cr and Fe families also form monomeric carbonyls of the predicted stoichiometry. 

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