Bonding in Ferrocene

The five wave functions for the cyclopentadienyl anion. Shaded lobes indicate positive signs and open lobes indicate negative 

The overlap of orbitals on Fe with the molecular orbitals on the cyclopentadienyl ion. 

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We have seen how a substituent on one cyclopentadienyl ring in ferrocene markedly affects subsequent ring substitution reactions. While certain transformations of this type are at least partially determined by steric factors, it seems certain that resonance and inductive interactions also play a very important role in determining orientation. Electronic transmissions both within and across the cyclopentadienyl rings are not fully understood, since the precise nature of the bonding in ferrocene is still uncertain. 

The basic qualitative features of the bonding in ferrocene are well understood, and will serve to illustrate the basic principles for all (t7-C H )M bonding. The discussion of bonding does not depend critically on whether the preferred rotational orientation of the rings (see Fig. 16-30) in an (i7-C5H5)2M compound is staggered (Did) or eclipsed (DSh) in any event, the barriers to ring rotation in all types of arene-metal complex are very low, ca. 10-20 kJ mol-1. 

The bonding in ferrocene may be looked upon in simplified molecular-orbital terms as follows.Each of the cyclopentadienide rings has five molecular orbitals three filled bonding and two empty antibonding orbitals (p. 62). The outer... 

The overall bonding in ferrocene can now be summarized. The occupied orbitals of the cyclopentadienyl ligands—the zero-node and one-node group orbitals—are stabilized by their interactions with iron. In addition, six electrons occupy molecular orbitals that are largely derived from iron d orbitals (as one would expect for d6 iron(II)), but these occupied orbitals, displayed in Figure 5-10, have significant ligand character too. The molecular orbital picture in this... 

Many sandwich molecules have been studied during the past decade. However, there is still uncertainty about the nature of the bonds in ferrocene and related substances. 

The overall bonding in ferrocene can now be summarized. The occupied orbitals of the ligands are stabilized by their interactions with iron. Note the stabilization of... 

Fig. 2. Bonding orbitals in ferrocene. a e. Symmetries of the molecular orbitals derived from atomic n orbitals. F, A d n-hond in ferrocene, g, The d -zl-bonds in ferrocene (taken from Craig, Maccoll. Nyholm, Orgei and Sutton, ref. 272).

The mean dissociation energies, derived in this way, show the bonding in ferrocene to be some 15 kcal/mole stronger than in the corresponding nickel compound. This is in keejiing with the conclusion that the antibonding orbitals are occupied in the nickel compound, an idea which is also supported by structural studies. 

The bonding in ferrocene involves sharing of the 6 electrons from each cyclopentadienyl ring with the iron atom. Based on the electronic configuration of the iron species in the compound, show that a favorable 18-electron inert... 

A very readable account of the bonding in ferrocene is given by M. M. Rohmer and A. Veillard in Chem. Phys. (1975) 11, 349. Also of interest is The Transition Metal-Carbon Bond , by E. R. Davidson, K. L. Kunze, F. B. C. Machado and S. J. Chakravorty, in Acc. Chem. Res. (1993) 26, 628. This contains an account of the bonding in Cr(CO) which is both a bit more advanced than and different from that in the text (whilst being readable and consistent with that in the text). 

The C—C bonds in ferrocene possess similar properties as C—C bonds in benzene. Hence, the reactions shown by benzene, due to aromatic character, are also exhibited by ferrocene. Ferrocene is represented in the following reactions as Fe(77 -C5H5)2/Fe(Cp)2-... 

FIGURE 14.7 Stick models of the staggered arrangement of five-equivalent d-orbital bond directions. The application to the bonding in ferrocene seems obvious (From Powell, R.E., J. Chem. Educ., 45, 45, 1968. With permission.)... 

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