Sidgwick effective atomic number

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. 

Langmuir (32) suggested that the metal carbonyls represent a class of compounds in which the metal acquires the number of electrons necessary to give it the atomic number of the next inert gas. Sidgwick (33) called this total number of electrons, shared and unshared, which became associated with an atom its Effective Atomic Number (E.A.N.). 

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. 

In this work, we will adopt the notation 18e to indicate a coordination center with 18 valence electrons and 6 coordination bonds. The notation is similar to that of Sidgwick s in the presentation of effective atomic number [2] however, instead of considering both core and valence electrons (for the determination of an effective atomic number )> the 18 here only includes the valence electrons rather than all electrons. Because of the abundance of examples that follow 18 , we will pictorially indicate this by drawing a circle on the metal center, and the number of M-L bonds present automatically denotes the number n in the notation, as seen in the example shown in Fig. 5. Note that because we are considering the contribution of the electrcMis, it will be convenient for us to consider only the electron densities based on the corresponding orbital contributions. To make all density contributions from different orbitals comparable, in this work we only consider cases where orbitals are either empty or doubly filled. 

For the transition metal complexes possessing the coordination number w, the number of bonding and nonbonding orbitals equals nine and, in the case of the main group compounds, the number of these orbitals is four. This leads to the effective atomic number rule (Sidgwick s rule) and the octet rule, respectively. In and EL ... 

In 1934 Nevil Vincent Sidgwick published a paper in which he established the well-known Effective Atomic Number rule for the bonding and stoichiometry of transition metal carbonyl compounds (2), For the metals of even atomic number Z, he observed that if each carbonyl group bonded to the metal atom is considered to "donate two electrons to the metal atom (in its oxidation state of zero), then the sum of the atomic number, Z, of the metal plus two times the number of CO groups attached to it equals the atomic number of the next heavier inert gas. This worked beautifully for the 3d metals, i.e., Cr in Cr(CO)5, Fe in Fe(CO)5, and Ni in Ni(CO)4. All had an Effective Atomic Number of 36, Z of krypton. This idea was quicldy extended to deal with transition metals with odd atomic numbers and to groups that donated one or three electrons, e,g, Mn in Mn2(CO)iQ (to give a Mn—Mn covalent bond) and Co in Co(CO)3(NO) and Fe in Fe(CO)2(NO)2 in which the NO molecule contributed 3 electrons. 

In the above discussion, no mention has been made of the effective atomic number rule proposed by Sidgwick Bailey (1934). This states that the sum of the number of metallic electrons and those donated by the carbonyls (or other ligands or bonded metals) equals the number of electrons possessed by the next inert gas, in this case the 36 electrons of krypton. This rule, which has proved to be very useful in predicting the electronic and molecular structures of a variety of organometallic compounds, indicates that there should be iron-iron bonds in Fej(CO)i2, shown by the dashed bonds in Figure 3.1, and in both the isomers of [(// -C5H5)Fe(CO)2]2- The nature of these metal-metal bonds has been the subject of extensive experimental and theoretical work. 

Sidgwick and Powell were first to correlate the number of electron pairs in the valence shell of a central atom and its bond configuration [106], Then Gillespie and Nyholm introduced allowances for the difference between the effects of bonding pairs and lone pairs, and applied the model to large classes of inorganic compounds [107],... 

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