Ferrocene molecular orbitals

Ferrocene Figure 2-47) provides a prime ex.ample of multi-haptic bonds, i.e, a situation where the electrons that coordinate the cyclopentadienyl rings with the iron atom arc contained in molecular orbitals delocalised over the iron atom and the 10 carbon atoms of the cyclopentadienyl rings [82. ... 

Ferrocene (Figure 2-61a) has already been mentioned as a prime example of multi-haptic bonds, i.c, the electrons tlrat coordinate tire cyclopcntadicnyl rings with the iron atom are contained in a molecular orbital delocalized over all 11 atom centers [811, for w hich representation by a connection table having bonds between the iron atom and the five carbon atoms of cither cyclopcntadicnyl ring is totally inadequate. 

Figure B A qualitative molecular orbital diagram for ferrocene. The subscripts g and u refer to the parity of the orbitals g (German gerade, even) indicates that the orbital (or orbital combination) is symmetric with respect to inversion, whereas the subscript u (ungerade, odd) indicates that it is antisymmetric with respect to inversion. Only orbitals with the same parity can combine.

As a consequence of the molecular orbital interactions, ferrocene adopts an axially symmetrical sandwich structure with two parallel Cp ligands with a distance of 3.32 A (eclipsed conformation) and ten identical Fe-C distances of 2.06 A as well as ten identical C-C distances of 1.43 A [12]. Deviation of the parallel Cp arrangement results in a loss of binding energy owing to a less efficient orbital overlap [8]. All ten C-H bonds are slightly tilted toward the Fe center, as judged from neutron-diffraction studies [13]. 

However, more quantitative molecular orbital treatments still leave scope for improvement, and in particular a systematic treatment of the whole metallocene series, rather than of isolated members such as ferrocene, is much to be desired. Moreover, theoretical studies have generally neglected the bis-arene and mixed sandwich series, and in both cases such treatments would be of value in indicating the extent to which the mainly metal n level is or is not less anti-bonding than in the metallocenes. In this connection it is interesting to note that in the (Ch) Cr(Cp)- anion the mainly metal n dxz, dyz ) level may still lie relatively high since esr measurements 1 78) indicate the odd electron to... 

On a more qualitative level, the bonding in the more stable isomer lb can be explained on the basis of the general molecular orbital scheme for bent (C2v) metallocenes containing 14 valence electrons, as shown in Fig. 5. The localization of three electron pairs in bonding orbitals (lal, 2 i, 2b2) is primarily responsible for the Si-Cp interaction the absence of a silicon orbital of a2 symmetry imposes the presence of a ligand-based non-bonding orbital. Structural adjustment from D5d (ferrocene type) to C2v... 

Generally, the atomic, outer, and interatomic sphere radii are chosen so as to minimize the volume between the spheres. Other physical considerations, such as empirical atomic or ionic radii, can also influence the initial choice of sphere sizes. It has also been recently shown that overlapping spheres may be used since the various expansion theorems are still valid (192). This technique has been used to calculate the molecular orbitals of Zeise s salt and ferrocene discussed later in this review (191, 193). 

Determine the qualitative form of the molecular orbitals for the eclipsed conformation of ferrocene. 

Fig. 15.33 Qualitative molecular orbital diagram fora metallocene. Occupation of the orbitals enclosed in the box depends on the identity of the metal, for ferrocene it is e L-[From Lauher, J. W. Hoffmann. R. J. Am. Cfie/n. Soc. 1976. 98, 1729-1742. Reproduced with permission.)...

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