Ferrocene symmetry

Metallocenes (but only with unsubstituted cyclopentadienyl rings) also form thiourea inclusion compounds. X-ray diffraction and Mdssbauer data on the ferrocene inclusion compound show that at 295 K the guest molecules are orientationally disordered at the sites of 32 symmetry. A phase change at 162 K generates a more ordered structure78). 

Table 12. Symmetry Characteristics of Vibronic d-d Bands of Ferrocene.

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... 

The Fe(Cp)2 and Ni(Cp)2 structures in Fig. 4.92 illustrate the important variation in hapticity that often distinguishes M(Cp)2 complexes for metal atoms of different acceptor characters. Thus, high-symmetry ferrocene (Sio) corresponds to highest r 5 hapticity, whereas singlet nickelocene (lowered C2v symmetry) exhibits lower r 3 coordination to each Cp ring. (The crystallographic structure of triplet nickelocene... 

While Josiphos 41 also possessed an element of atom-centered chirality in the side chain, Reetz reported a new class of ferrocene-derived diphosphines which had planar chirality only ligands 42 and 43, which have C2- and C -symmetry, respectively.87 Rhodium(i)-complexes of ligands (—)-42 and (—)-43 were used in situ as catalysts (0.75 mol%) for the hydroboration of styrene with catecholborane 1 for 12 h in toluene at — 50 °C. The rhodium/ i-symmetric (—)-43 catalyst system was the more enantioselective of the two - ( -l-phenylethanol was afforded with 52% and 77% ee with diphosphines (—)-42 and (—)-43, respectively. In both cases, the regioselectivity was excellent (>99 1). With the same reaction time but using DME as solvent at lower temperature (—60 °C), the rhodium complex of 43 afforded the alcohol product with an optimum 84% ee. 

The ligands discussed so far all contained C2 symmetry. An important new class of ligands having Ci symmetry was introduced by Togni [22] (see Figure 4.19). They can be easily made from an enantiomeric amine as the precursor in a few steps. Different substituents can be introduced at the phosphorus atoms. In addition to the chiral carbon atom the molecule has planar chirality as well. The chiral carbon atom is used to introduce the planar chirality, i.e. lithiation of the ferrocenyl amine takes place at a specific side of the amine at the ferrocene moiety. 

Both in acetonitrile and in other non-aqueous solvents, a major problem arises in terms of the manner in which the potential values are reported by various investigators. Koepp, Wendt, and Strehlow [6] noted that hydrogen ion is the poorest reference material on which to base nonaqueous potentials because of the extreme differences in its solvation in various solvents. On the basis of an investigation of the solvent dependence of 18 redox couples, these investigators concluded that ferrocene/ferrocenium ion (i.e. bis(cyclopentadienyl)iron(III/II), abbreviated as Fc+ /Fc°) and/or cobal-tocene/cobalticenium ion represented optimal potential reference materials for nonaqueous studies. On the basis of their minimal charge (+1, 0) and their symmetry (treated as though they were roughly spherical), the potentials of these two redox couples are presumed to be relatively independent of solvent properties. 

In view of the complex nature of those molecules, it is not surprising that ab initio calculations are lacking. However, in the case of ferrocene with its very high D5d symmetry, attempts have been made using the simplified SCF-LCAO-MO theory of Roothaan (68). Yamazaki (69) reported on such calculations but so briefly that it is difficult to identify the ordering of the... 

There are now no planes of symmetry. Hence, the point group is D5. Thus, as one ring in ferrocene is rotated relative to the other the symmetry... 

The energy difference between the staggered (Dw) and eclipsed Dm,) rotomers of ferrocene is apparently very small (si Real/mol), with the latter being perhaps the more stable [cf. R. K. Bohn and A. Haaland, J. Organomet. Chem., 5, 470 (1966)). The two symmetries are equally suitable and convenient for a discussion of bonding. Since most of the research literature has used Z)w, we make the same choice here. 

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