Organometallic compounds electron rules

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 number of complexes for which the 18-electron rule is an overriding bonding condition is very large. It is an essential tool for interpreting the bonding in organometallic compounds and it will be considered many times in the chapters to follow. 

It can easily be demonstrated that the rule of 18 electrons is effective and valid in stable 7t-complexes of both aromatic and heteroaromatic compounds (Sec. 2.2.4.1). Examples of the organometallic compounds which obey this rule are... 

Transition metal (TM) chemistry stands in contrast to this. Many compounds involve metal centres with partially filled d shells, and/or with one or several unpaired electrons. Therefore, it is not always straightforward to predict the orbital occupation pattern of a given stable compound. For intermediates on a reactive pathway, this is an even greater problem. This is also true for organometallic chemistry, despite the fact that many compounds obey the 18-electron rule and have closed-shell singlet ground states. Thus, there are many 16- or even 14-electron intermediates, odd-electron species [1], and polymetallic clusters and complexes for which the spin state is not readily predicted. 

The 18-electron rule can be a useful guide to stable organometallic compounds, especially when p-acceptor ligands are present, although it has the limitations referred to in Topic H9. Compounds 3, 5 and 6 obey this rule, but 1 without p bonding ligands has an electron count of only 12. Metallocenes [M(h5 C5H5)2 ] are known for the 3d series elements V-Ni, with 15-20 valence... 

As a general rule, the abundance is reduced when the number of associated molecules increases. However, some specific aggregates, resulting from particularly important interactions, are present at particularly high abundance. This occurs often with organometallic compounds, as the metal tries to complete its electronic shell. 

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. 

The 18-electron rule is not obeyed as consistently by these types of organometallic compounds as by the carbonyl and nitrosyl complexes and their derivatives. For example, in addition to ferrocene. M(i7 -C5H5)2 compounds are known for most of the other elements of the first transition series (M = V, Cr. Mn. Co, Ni) and these cannot obey the 18-electron rule. However, only ferrocene shows exceptional thermal stability (stable to 500 °C) and is not oxidized by air. Furthermore, cobaltocene, a 19-electron species, is readily oxidized to the 18-electron cobaltocenium ion, [Co(t7 -CjH5)2J, which reflects much of the thermal stability of ferrocene. Mixed cyclopentadienyl carbonyl complexes are common [(- j -C3H5)V(CO)J, [(tj -CjHj)-Cr(CO)3J2, [(77 -CjH5)Mn(CO)3], [(77- -C3H5)Fe(CO)2)2. [(t, -CjH5)Co(CO)2],... 

A cyclopentadienyl phosphorus compound, (MesC.dlBu lPCI, has been ideniified spec-iroscopically.- Is this an organometallic compound Explain your answer. By electron counting, draw conclusions about the hapticity of the cyclopentadienyl group. How many electrons does phosphorus need in order to obey the efl ective atomic number rule ... 

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