Organometallic chemistry 18-electron rule

The course of modern organometallic chemistry has been greatly influenced by three simple generalizations the Dewar-Chatt-Duncanson synergic bonding model for metal-olefin complexes (40, 72) Pauling s electroneutrality principle (174), and the 18-electron or inert gas rule (202). In this section the impact of recent theoretical calculations on these important generalizations will be evaluated. 

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. 

Cyclopentadienide (Cp) 1 is well known as one of the most frequently used ligands in organometallic chemistry. In addition, the cyclopentadienide anion 1 has always been quoted as a classic example of Hiickel aromaticity, to demonstrate along with benzene and the cydoheptatrienyl cation the validity of the (4n + 2) -electron rule. In contrast, a simple and stable cyclopentadienyl cation of the type 1+ remains to be elusive [5]. With the highly unstable neutral cyclobutadiene and the cydoheptatrienyl anion, 1+ shares the character-... 

Tolman CA. The 16 and 18 electron rule in organometallic chemistry and homogeneous catalysis. Chem Soc Rev 1972 1 337-353. 

The 16- and 18-electron rule in 17 organometallic chemistry and (90) homogeneous catalysis... 

This complex has an electron count matching a nido structure, but it adapts the butterfly structure expected for arachno. This is one of the many examples in which the structure of metal clusters is not predicted accurately by Wade s rules. Limitations of Wade s rules are discussed in R. N. Grimes, Metal-lacarboranes and Metallaboranes, in G. Wilkinson, F. G. A. Stone, and W. Abel, eds.. Comprehensive Organometallic Chemistry, Vol. 1, Pergamon Press, Elmsford, NY, 1982, p. 473. 

This rule is applicable only for those elements that have d electrons, and so is less important in organic chemistry (though it is important in organometallic chemistry). 

---