Fluxional behaviour of metallocenes

Key point: Many metallocenes exhibit fluxionality and undergo internal rotation because the barrier to the interconversion of the various forms is low.

One of the most remarkable aspects of many cyclic polyene complexes is their stereo chemical nonrigidity (their fluxionality). For example, at room temperature the two rings in ferrocene rotate rapidly relative to each other as there is only a low staggered-eclipsed conversion barrier. This type of fluxional process is called internal rotation, and is similar to the process by which the two CH₃ groups rotate relative to each other in ethane. We have already noted how, in the gas phase, the eclipsed conformation of ferrocene is slightly more stable than the staggered one; however, the steric bulk of substituents on the metallocene rings can change the barrier and can make staggered the preferred conformation. The rings of metallocenes are often drawn in a staggered conformation simply because there is then a little more space to illustrate substitutions. Of greater interest is the stereochemical nonrigidity that is often seen when a conjugated cyclic polyene is attached to a metal atom through some, but not all, of its C atoms. In such complexes the metal ligand bonding may hop around the ring; in the informal jargon of organometallic chemists this internal rotation is called ‘ring whizzing’. A simple example is found in [Ge( 1-Cp)(CH₃)₃], in which the single site of attachment of the Ge atom to the cyclopentadiene ring hops around the ring in a series of 1,2-shifts, a motion in which a CM bond is replaced by a CM bond to the next C atom around the ring; this motion is known as a 1,2-shift, because the bond starts on atom 1 and ends up on the adjacent atom 2 (Fig. 22.14). The great majority of fluxional conjugated polyene complexes that have been investigated migrate by 1,2-shifts, but it is not known whether these shifts are controlled by a principle of least motion or by some aspect of orbital symmetry.

Nuclear magnetic resonance provides the primary evidence for the existence and mechanism of these fluxional processes, as they occur on a timescale of 10 2 to 10 4 s, and can be studied by 1H- and 13C-NMR. The compound [Ru(4-C₈H₈) (CO)₃] (71) provides a good illustration of the approach. At room temperature, its 1H-NMR spectrum consists of a single, sharp line that could be interpreted as arising from a symmetrical 8-C8 H8 ligand. However, X-ray diffraction studies of single crystals show unambiguously that the ligand is tetrahapto. This conflict is resolved by 1H-NMR spectra at lower temperatures because as the sample is cooled the signal broadens and then separates into four peaks. These peaks are expected for the four pairs of protons of an 4-C8 H8 ligand. The interpretation is that at room temperature the ring is ‘whizzing’ around the metal atom rapidly compared with the timescale of the NMR experiment, so an averaged signal is observed. At lower temperatures the motion of the ring is slower, and the distinct conformations exist long enough to be resolved. A detailed analysis of the line shape of the NMR spectra can be used to measure the activation energy of the migration.

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مواضيع ذات صلة

Isolobal analogies

Structure of clusters

Bent metallocene complexes

Bonding in bis(cyclopentadienyl)metal complexes

Synthesis and reactivity of cyclopentadienyl compounds

Spectroscopic properties of carbonyl compounds

Reactions of the CO ligand

Metal carbonyl basicity

Oxidation and reduction of carbonyls

Properties of homoleptic carbonyls

Synthesis of homoleptic carbonyls

Homoleptic carbonyls