Effective atomic number rule,

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. 

Many other TT-organometallic compounds have been prepared. In the most stable of these, the total number of electrons contributed by the ligands (e.g., four for allyl anions and six for cyclopentadiene anion) plus the valence electrons on the metal atom or ion is usually 18, to satisfy the effective atomic number rule.31 ... 

This mode of calculation has been called the EAN rule (effective atomic number rule). It is valid for arbitrary metal clusters (closo and others) if the number of electrons is sufficient to assign one electron pair for every M-M connecting line between adjacent atoms, and if the octet rule or the 18-electron rule is fulfilled for main group elements or for transition group elements, respectively. The number of bonds b calculated in this way is a limiting value the number of polyhedron edges in the cluster can be greater than or equal to b, but never smaller. If it is equal, the cluster is electron precise. 

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. 

Effective atomic number rule, 76 60-61 Effective attenuation length (EAL), 24 88-89... 

A third type of cluster, [FePt2(CO)s P(OPh)3 3], was isolated from the reaction between Fe2(CO)g and [Pt P(OPh)3 4], for which the structure (73a) was proposed.These clusters join a growing number of such compounds which do not obey the effective atomic number rule. 

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... 

Although IE valence electrons are tound in lFe(H20)k]2+, the effective atomic number rule is violated. Explain. 

Nitric oxide gas provides the most direct route to many nitrosyl complexes. It will frequently displace CO groups, three of which must be substituted by two NO groups in order that the effective atomic number rule is obeyed. 

The complex (56) does not react with neutral ligands even under forcing conditions. This apparent coordinative saturation suggested strong bonding in the M2N2 ring and involvement of 10 electrons which is required if the effective atomic number rule is to be satisfied. 

Each 15-electron V(CO)4(dppe) fragment requires three electrons to satisfy the effective atomic number rule, which clearly requires a degree of multiple bonding between the tellurium and vanadium centers. Two alternative descriptions, A and B, were proposed as representations for the bonding. 

A symmetry-based molecular orbital description of the unusual four-coordinate C3v W(RC=CR)3(CO) series of molecules was presented by King in 1968 (32). The tt orbitals of the three alkynes yield linear combinations of A2 and E symmetry. Since there is no metal orbital of A2 symmetry only the degenerate E combination of n orbitals finds metal orbital mates for bonding and antibonding combinations. The three alkyne 7T orbitals serve as er donors [(Ax + E) symmetry] as does the fourth ligand (Al symmetry). Thus the total metal electron count adheres to the effective atomic number rule [W(0)(6) + 37T j(6) + 2ir (4) + lo-(2) = 18 electrons]. 

The cht ligand bonds in different ways in the complexes Ni(CO)3(cht), Fe(CO)3(cht), and Cr(CO)3(cht). Nickel has 28 electrons and gains six from the three CO ligands. Thus, it needs only two additional electrons from cht to obey the effective atomic number rule, and only one double bond in cht will be coordinated in Ni(CO)3(cht). 

Uncharged mononuclear and polynnclear metal carbonyls are known for the group 5 to group 10 metals. The EAN rule see Effective Atomic Number Rule) is normally obeyed, so elements with an even atomic number form mononuclear compounds, for example, Cr(CO)6, Fe(CO)5, and Ni(CO)4, while elements with an odd atomic number form dinuclear compounds, for example, Mn2(CO)io and Co2(CO)g, containing metal-metal bonds. 

Teo s topological approach also deserves mention in this section. It combines Euler s theorem with the idea of an effective atomic number rule for each cluster vertex, but involves a rather arbitrary parameter whose purpose is to correct for the formation of multicenter bonds. 

The DuPont ADN process involves hydrocyana-tion of butadiene (equations 5-8), catalyzed by air-and moisture-sensitive triarylphosphite-nickel(O) complexes [Ni(P(OAr)3)4]. The nickel is zero valent (see Oxidation Number) because it has its full complement of 10 electrons beyond the preceding inert gas (Ar) configuration, and the catalysts can actually be made directly from nickel metal and the phosphite ligands (see P-donor Ligands). The four phosphorus atoms each contribute an electron pair to give a total of 18 electrons, corresponding to the next inert gas (Kr), (see Effective Atomic Number Rule and Electronic Structure of Organometallic Compounds). 

The starting point for most molybdenum-based carbonyl chemistry is the white crystalline air stable hexacarbonyl [Mo(CO)6], which is commercially available. Low valent compounds that are derived from this starting material tend to give products that obey the Effective Atomic Number Rule (EAN Rule). Since Mo° has a d configuration, six two-electron ligands are required to obey the EAN rule. 

Organometallic complexes containing a single metal center usually obey the so-called 18-electron rule see Effective Atomic Number Rule), whereas the bonding in bulk metals is best described according to band theory see Band Theory). Understandably, bonding descriptions of cluster compounds may be expected to become increasingly complex as the size of the cluster increases. 

A soluble dimer [Ru(cod)Cl2]2 (71), having the same formula as the insoluble polymer, has been isolated as a product from the reductive complexation of cod to ruthenium by Zn in EtOH/THF. The compound has been structurally characterized, and the Ru-Ru distance of 2.791 A in this compound is in the range found for Ru-Ru single bonds, whereas a double bond would be required to satisly the effective atomic number rule see Effective Atomic Number Rule) ... 

It should be noted that zirconium and hafnium usually form electron deficient complexes (see Electron Deficient Compound), which do not obey the Effective Atomic Number Rule (EAN mle) (see Effective Atomic Number Rule) and have a maximum of only 16 electrons. Monomers MX4 with monodentate ligands are rather strong Lewis acids and readily yield adducts with two, three, or four donor ligands. In MX4 monomers with tt-donor ligands X (see n-Base), there is a noticeable shortening of the M X distances in comparison with the sum of the covalent radii (0.26 A for X = F 0.15 A for X = Cl, NMe2), thus pointing to some multiple bond character in these compounds. 

M.Nt ring and involvement of 10 electrons, required if the effective atomic number rule is satified. Treatment of diphenyl derivatives with iodine yielded [(77-C5H,)3M3l304] (M = Mo, W) (188) but [t7-C5H5M(CO)2 N CRR ] (R = Ph, tert-Bu R = tert-Bu) gave the oxidation products... 

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