Effective atomic number rule organometallic compounds

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 ... 

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). 

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. 

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. 

Just as organic compounds follow the octet or eight valence electron rule, typical organometallic compounds tend to follow the 18e rule. This is also known as the noble-gas or effective atomic number (EAN) rule because the metals in an 18e complex achieve the noble-gas configuration— for example, in the Werner complexes, the cobalt has the same EAN as Kr, meaning it has the same number of electrons as the rare gas. We first discuss the covalent model that is the most appropriate one for counting compounds with predominant covalency, such as most organometallics. 

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