Cyclopentadienyl complexes 18-electron rule

Beryllium forms a series of cyclopentadienyl complexes [Beftj -CsHiY] with Y = H, Cl, Br, Me, —C=CH and BH4, all of which show the expected C5, symmetry (Fig. 5.10a). If the pe/ifo/topfo-cyclopentadienyl group (p. 937) contributes 5 electrons to the bonding, then these are all 8-electron Be complexes consistent with the octet rule for elements of the first short... 

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,... 

A complex with an Fe—Fe bond for which the electron density is available is bis(dicarbonyl-7i-cyclopentadienyl iron), [C5H5Fe(CO)2]2 (Fig. 10.13) (Mitschler et al. 1978). The 18-electron rule again requires only a single bond to explain the observed diamagnetism. The combined X-ray and neutron study indicates a complete absence of density accumulation in the metal-metal bonding region of the standard deformation density, in agreement with an SCF theoretical density,... 

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... 

In both the examples given above, there is concomitant loss of one or more neutral ligands. Elimination of CO is the rule in reactions of mononuclear metal carbonyls (e.g., entry 12) and cyclopentadienyl metal carbonyls (e.g., entry 4), but not those of polynuclear carbonyls (e.g., entry 16) or carbonyl halides (e.g., entry 33). Elimination of tertiary phosphines often occurs, especially when more than two molecules are present in the initial complex however, this is not always the case (see entry 24). Clearly, steric requirements and the dictates of the 18-electron rule determine the composition of the product, and normally act in concert when they conflict, as in the case of R3SiRuH3(PR3) (n = 2 or 3 entry 22), variable stoichiometry may result. Chelating diphosphines, with somewhat reduced steric requirements, are usually retained (e.g., entry 19), while complexed olefins are invariably lost the bulky ligand P(cyclohexyl)3 is associated with unusual products (entries 47 and 48). Particular mention may be made of the 17-electron species Cl3SiVH(Cp)2 and (Cl3Si)2V(Cp)2 shown... 

Although carbonyl and cyclopentadienyl complexes are well known for Tc(I) and Tc(III), none appear to have been reported for Tc(II). This may be ascribed to the tendency to follow the 18-electron rule, which, due to the odd number of electrons, would require dimer formation for compliance. Similarly, no Tc(II) cyano or isonitrile complexes appear to have been isolated. 

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. 

Uranium forms tris and tetrakis-cyclopentadienyls CpjU and Cp U. In the latter there are four fy -cyclopentadienyl groups so that the metal atom is formally 12-coordinate. This high coordination number is consistent with essentially ionic (electrostatic) bonding. This may also apply to the complexes MCp (M = Ti, Zr) where the structures apparently depend on the size of the metal atom and, for Cp Zr, the 18-electron rule does not apply. 

A very wide variety of w-cyclopentadienyl metal carbonyl complexes is known. The structures of some are illustrated in Figure 43. With very few exceptions these compounds obey the 18-electron rule. This is well exemplified by the series of complexes 7t-C5H5Co(CO)2, ir-CsH5Mn(CO)3 and 77-CsH5V(CO)4. The simplest derivatives of the elements Cr, Fe and Ni are binuclear and held together either by metal-metal bonds or by bridging carbonyl groups and metal-metal bonds. The formation of such binuclear systems by alternate members of the transition series should be compared with the similar formation of dimeric metal carbonyls in the series Cr(CO)e, Mn2(CO)io, Fe(CO)s, Co2(CO)8 and Ni(CO)4. 

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