Ferrocene 18-electron rule

In many respects, the successes of this model are remarkable. Iron(O) possesses a total of eight electrons in its valence shell. To satisfy the eighteen-electron rule, five two-electron donors are needed, and compounds such as [Fe(CO)5] are formed. These molecules also obey simple VSEPR precepts, and [Fe(CO)s] adopts a trigonal bipyramidal geometry. Conversely, the use of two five-electron donor ligands such as the strong r-acceptor cyclopentadienyl, Cp, gives the well-known compound ferrocene (9.3). 

Another example of a complex that obeys the 18-electron rule is ferrocene or bis(cyclopentadienyl) iron. The cyclopentadienyl anion is generated by the reaction of cydopentadiene with sodium, and ferrocene is obtained by the subsequent reaction with ferrous chloride,... 

The 18-electron rale is not obeyed as consistently by these types of oiganome-tank compounds a by the carbonyl and nitrosyl complexes and their derivatives. For example, in addition to ferrocene. M(i 5-CsHs)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 ihe 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(ip-CsH )3)4 , which reflects much of the thermal stability of ferrocene. Mixed cyclopentadienyl carbonyl complexes are common K -CjHjMCO) ]. [(if-CjH )-Cr(CO), . [( -CjHOMnCCOjJ, [(>r-C,H,>Fe(CO ,, . [fo -CjiyCoCoy. and (ip-CsH,)Ni(CO) 2. Of interest is the fact that among these compounds, the odd-atomic-number elements (V. Mn, and Co) form monomers and the even-atomic-number elements (Cr. Fe. and Ni) Ibrm dimers, which is in direct contrast to the behavior shown by the simple carbonyl complexes. Cyclopentadienyl derivatives are now known for every main group and transition metal of the periodic table and for most of the -block metals.89... 

Ferrocene has an even more interesting structure. A central iron is ir-bonded to two cyclopentadienyl ligands in what is aptly described as a sandwich. It, too, obeys the 18-electron rule. Each cyclopentadienyl ligand contributes five electrons for a total of ten and iron, with an electron configuration of [Arj Sd6 contributes eight. Alternatively, ferrocene can be viewed as being derived from Fe2+ (six valence electrons) and two aromatic cyclopentadienide rings (six electrons each). 

The best-known cyclopentadienyl complex is the sandwich compound ferrocene, (r -Cp)2Fe it is a diamagnetic, orange solid (mp 393 K) which obeys the 18-electron rule (structure 23.25). In the gas phase, the two cyclopentadienyl rings are eclipsed (23.57) but the solid exists in several phases in which the rings are co-parallel but in different orientations. 

In ferrocene, each cyclopentadienyl ring is considered to donate five electrons to the iron atom, atomic number = 26, giving a total of 10 -I- 26 = 36 electrons, the atomic number of krypton. Similarly, bonding in the compound [Mn(CO)3(C5H4-CH3)] obeys the FAN eighteen-electron rule. 

In organometalfic compounds, most p-electrons of transition metals conform to an empirical rule called the 18-electron rule. This rule assumes that the metal atom accepts from its ligands the number of electrons needed in order for it to attain the electronic configuration of the next noble gas. It assumes that the valence shells of the metal atom will contain 18 electrons. Thus, the sum of the number of d electrons plus the number of electrons suppfied by the figands will be 18. Ferrocene, for example, has 6 d electrons from Fe(II), plus 2x6 electrons from the two 5-membered rings, for a total of 18. (There are exceptions to this rule, however.)... 

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