Effective atomic number Rule exceptions

For the transition metals it is often impossible to reach a noble gas structure except in covalent compounds (see effective atomic number rule) and it is found that relative stability is given by having the sub-shells (d or f) filled, half-filled or empty. 

Effective atomic number Rule, 24 226 and cluster compounds, 24 230-231 exceptions, 24 226-227, 235 and mononuclear species, 24 234-235 Effective g values, calculation, 38 195-197 EF hand domains, calcium-binding proteins, 46 442, 443 45... 

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. 

There are (3n—6) modes of vibration in a polyatomic molecule except that for a linear molecule one mode is doubly degenerate so that effectively the number is (3n-5) n is the number of atoms in the molecule. Symmetry rules may restrict the kinds of transitions which can occur during an electronic transition but often the number of possible transitions is very large. 

All the mononuclear species effect V(Co)6 in the above table satisfy the 18-electron rule. The bi and tri-nuclear species do also if (i) the two electrons in a metal-metal bond are counted as contributing to the valence shells of both metal atoms concerned, and (ii) a bridging CO contributes one electron to each metal. Monomeric Mn and Co carbonyls would have an odd number of electrons and dimerise in consequence. V(CO)6 is exceptional as a stable radical with 17 valence-shell electrons, presumably because it is satirically impossible for it to dimerise without losing one CO ligand. It does, however, readily form the 18-electron anion [V(CO)6]-. 

As a general rule, vitamins and minerals are equally potent (effective) whether they come from natural sources or synthetic procedures. Vitamin E is an exception to that general rule. Synthetic forms of the vitamin are generally less potent than natural forms. The reason for this fact is that natural vitamin E exists in only one stereo isomeric form. Stereo isomers are two or more forms of a compound with the same kind and number of atoms. But the atoms in stereoisomers have some... 

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