Ferrocene, bonding structure

The pattern shown is only one of a large number of distinct ways of choosing the L/X labels in (4.122) and (4.123), corresponding to the large number of equivalent resonance structures that could be written. However, such an exercise adds little value to the simple picture of ferrocene bonding provided by the localized bonding units. 

Stannocene, bis-Tj5-cyclopentadienyltin(II), and plumbocene, bis-Tj5-cy-clopentadienyllead(II), were first reported by Fischer and Gruber in 1956 (12). They observed the compounds to be monomeric in benzene and to have nonzero dipole moments and proposed the a-bonded structure shown in Fig. 1A. Wilkenson and co-workers reported the similarity between the IR spectrum of stannocene, plumbocene, and ferrocene and suggested the presently accepted angular sandwich structure for stannocene and plumbocene shown in Fig. IB (13). 

Figure 6.1 Structure of simple metallocenes. Ferrocene was the first metallocene (discovered in 1951), but the correct ii-bonded structure was not identified until 1952 (JP Collman, LS Hegebus, JR Norton and RG Finke, Principles and Applications of Organotransition Metal Chemistry, University Science Books, Sausalito, CA, p 9 (1987).

Both groups noted the unexpectedly great stability of this compound to heat and to air. They both proposed the a-bonded structure shown in Fig. 1. The actual structure was first proposed by Wilkinson et al. 280) and confirmed by Fischer and Pfab 90), who also reported the existence of (C5Hg)aCo . X-Ray diffraction studies on ferrocene were also done by Dunitz and Orgel (79) and Eiland and Pepinsky 82). These three papers were received by the respective journals on June 20, July 5, and August 12, 1952 For their work in this area, Fischer and Wilkinson shared the 1973 Nobel Prize in chemistry 239a). 

Although it was originally thought that there was some special feature of the bonding that gave the parallel ring, or ferrocene-like, structure, this view had to be modified to account for the monocyclopentadienyl compounds and the subsequent observation that in several di-/j5-cyclopentadienyl compounds the rings are not parallel. 

An X-ray structural study of the ferrocenylphosphonium salt (147) shows a significant shortening of the phosphorus-ferrocene bond, lending support to an earlier suggestion by McEwen that there may be overlap of the filled hog MO of the ferrocenyl group with an empty 3d orbital of phosphorus. The possible existence of an equilibrium between the substituted methylphosphonium salts (148) and the cationic phosphorane system (149) has been ruled out following spectroscopic and structural studies of a series of such salts. ... 

Section 14 14 Transition metal complexes that contain one or more organic ligands offer a rich variety of structural types and reactivity Organic ligands can be bonded to a metal by a ct bond or through its it system Metallocenes are transition metal complexes m which one or more of the ligands is a cyclopentadienyl ring Ferrocene was the first metallocene synthesized Its electrostatic potential map opens this chapter... 

The structure of ferrocene and an MO description of its bonding have already been given (p. 937). The rings are virtually eclipsed as they are in the analogous ruthenocene (light-yellow, mp 199°C) and osmocene (white, mp 229°C). 

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]. 

Due to the pronounced electron donating character of ferrocene, ot-ferrocenyl carbocations 3 possess a remarkable stability and can therefore be isolated as salts [16]. They can also be described by a fulvene-type resonance structure 3 (Fig. 4) in which the Fe center and the ot-center are significantly shifted toward each other as revealed by crystal stmcture analysis, indicating a bonding interaction [17]. 

Togni and co-workers have used the convergent methodology to link phosphine-containing chiral ferrocene ligands on the cyclophosphazene core to obtain dendrimeric structures of the type 37 (Fig. 21) (201). The reaction with the cyclophosphazene end occurs by the replacement of the P-Cl bond and by the formation of the P-0 bond. The dendrimers contain twelve and sixteen ferrocene moieties respectively. The phosphine units present can coordinate to Rh(I) to afford metallic dendrimers, which have been shown to be excellent catalysts for the enantioselective hydrogenation of dimethyl itaconate. The product... 

---