Fluxional organometallic compounds

The mechanisms of geometrical and optical isomerization already discussed also apply to organometallic compounds, but these show some additional unique rearrangement processes. This area has been reviewed by Mann.  

Thus far, the equatorial-axial exchange has proven to be too fast to measiure by NMR, even down to -120°C. This conclusion assumes that there is a significant chemical shift difference between the axial and equatorial nuclei this has been questioned by Mahnke et al. A solid- 

State NMR study down to 100 K indicated some fluxional behavior with an energy barrier of 1 kcal mol. More recent solid state C NMR results indicate a shift difference of 182 Hz at 22.53 MHz and an interchange rate of 10 s at -38°C. A study of the polarized UV spectrum in a CO matrix at 20 K shows that there is slow interconversion at that temperature. Theoretical studies of the fluxionality of Fe(CO)5 are consistent in predicting a low barrier of 2 kcal mol for the Berry pseudorotation pathway. This seems consistent with the observation that even hydrogen bonding to benzene is capable of converting the structure to one similar to the pseudorotation intermediate. 

The term ring whizzers was coined by Cotton to describe the phenomenon that was first studied quantitatively by Bennett et al. in the compound Fe(Ti -C5Hj)(T -C5H5)(CO)2, which has the following structure  

The H NMR for this compound at 30 C in C 2 shows only two peaks, one typical of the Ti -CjHj hydrogens and one for all of the n -CjHj hydrogens. If the latter is static, then it should show two peaks (due to a and b protons in the stmcture) plus the unique H on the same C as the Fe. At -100°C, the two peaks (+ coupling features) expected for the a and b sets become resolved. In addition, it was observed that the peak at lower field collapses more rapidly than the other one as the temperature is increased from -100°C. 

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The octahedral complexes, in general, are not fluxional. Organometallic compounds which show fluxional behaviour are as... 

Evidently, we should be wary of drawing conclusions from what may be a deceptively-simple NMR spectrum measured at room temperature wherever fluxionality seems plausible, it must be suspected. Organometallic compounds involving the cyclopentadienide ligand... 

Fluxionality is characteristic of certain classes of organometallic compounds and is found occasionally in others. The phenomenon is seen characteristically in compounds containing conjugated cyclic polyolefins such as cyclopentadienyl, cyclohep-tatrienyl, or cyclooctatetraene, to name the three most common ones, attached to a metal atom through at least one but less than all of their carbon atoms, as illustrated by the following partial structures. 

Chemical shifts have been reported for [Me3CCH2HgX] 203) (see Table XII), tr-allylic derivatives (see Table XIII), o-cyclopentadienyl derivatives (see Table XIV), and miscellaneous compounds (see Table XV). In the case of fluxional a-cyclopentadienyl derivatives, it has been shown that chemical shifts can be used to differentiate between fluxional o-cyclopentadienyl derivatives and w-cyclopentadienyl derivatives 103). The NMR spectrum of [Sb(cyclopropyl)s] has only two carbon resonances, showing that the molecule is fluxional (56) and the NMR spectrum of the naturally occurring organometallic compound, 5 -deoxyadenosylcobalamin, has been reported (65). For a number of norbornane and allylic derivatives of palladium and platinum, a number of linear relationships have been found between chemical shifts of various carbon atoms. It was suggested that a term due to paramagnetic shielding by the metal was dominant 52b). 

Carbon-13-NMR is a routine means of characterizing organometallic compounds, as well as a mechanistic probe [52-56]. The sensitivity of C-NMR is reflected in the wide range of chemical shifts and scalar coupling constants, Jc-x that allows for evaluation of subtle changes in the organometallic species at the M—C bond. This sensitivity has allowed organometallic chemists to evaluate fluxional processes and electron delocalization in... 

This has a low energy barrier ( 25 kJ mol-1) and occurs rapidly ( 2 x 1010 s-1). However, for pyramidal PR3 compounds, the activation barrier has climbed to over 100 kJ mol-1, so inversion is very slow, sufficiently so to even allow enantiomer separation in the case of chiral phosphines. Coordination of the lone pair freezes the configuration. Examples of fluxional complexes tend to be found mostly, but not exclusively, amidst organometallic compounds. Spectroscopic methods, particularly NMR spectroscopy, assist in defining the fluxional behaviour. 

Fluxional behavior is characteristic of certain classes of organometallic compound.32 A few examples have already been considered in Section 1-9 as illustrations a more organized account will be given here,... 

Examples of fluxional behaviour may also be found in organometallic compounds containing more than one metal atom in the molecule, for example (C5H5)(CO)2Mo(C7H,)Fe(CO)3 " and the cluster compound Ru3(C8H8)(CO)4, whose limited solubility unfortunately precludes satisfactory determination of kinetic parameters. " ... 

In connection with investigations on the catalytic activity of organometallic compounds of the rare earths, anionic homoleptic allyl derivatives of Ce, Nd, Sm, Gd and Dy have been prepared according to eq. (47). Allyl lithium was generated in situ from Sn(C3Hj)4 and another lithium alkyl LiL. The compounds, isolated as their dioxane complexes after precipitation from ethereal solution by addition of dioxane, are described as containing fluxional allyl groups from their infrared and H NMR spectra (Mazzei, 1979) ... 

Fluxional Organometallic and Coordination Compounds M. Gielen, R. Willem and B. Wrackmeyer 2004 John Wiley Sons, Ltd ISBN 0-470-85839-7... 

Fluxional organometallic and coordination compounds / edited by Marcel Gielen, Rudolph Willem, Bernd Wrackmeyer. 

Schroder was alert enough to understand what had happened, and went on to a highly successful career that included an examination of the properties of bullvalene and substituted versions of this molecule. Doering s predictions were completely confirmed by Schroder s synthesis. Other neutral, completely fluxional organic molecules have not appeared, although the phenomenon of fluxionality appears to be rather more common in organic cations and organometallic compounds. 

Larrabee, R. B., Fluxional Main Group IV Organometallic Compounds, The Implications for Orbital Symmetry Rules, J. Organometal. Chem. 74 [1974] 313/64. 

Fluxional and Nonrigid Behavior of Transition Metal Organometallic ir-Complexes, 16, 211 Free Radicals in Organometallic Chemistry, 14, 345 Functionally Substituted Cyclopentadienyl Metal Compounds, 21,1... 

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